SWIG and C++

# 5 SWIG and C++

- Comments on C++ Wrapping
- Approach
- Supported C++ features
- Command line options and compilation
- Simple C++ wrapping
  - Constructors and destructors
  - Default constructors
  - When constructor wrappers aren't created
  - Copy constructors
  - Member functions
  - Static members
  - Member functions and default arguments
  - Member data
- Protection
- Enums and constants
- Friends
- References and pointers
- Pass and return by value
- Inheritance
- A brief discussion of multiple inheritance, pointers, and type checking
- Renaming
- Wrapping Overloaded Functions and Methods
  - Dispatch function generation
  - Ambiguity in Overloading
  - Ambiguity resolution and renaming
  - Comments on overloading
- Wrapping overloaded operators
- Class extension
- Templates
- Namespaces
- Exception specifiers
- Pointers to Members
- Smart pointers and operator->()
- Using declarations and inheritance
- Partial class definitions
- A brief rant about const-correctness
- Proxy classes
  - Construction of proxy classes
  - Resource management in proxies
  - Language specific details
- Where to go for more information

This chapter describes SWIG's support for wrapping C++. As a prerequisite, you should first read the chapter SWIG Basics to see how SWIG wraps ANSI C. Support for C++ builds upon ANSI C wrapping and that material will be useful in understanding this chapter.

## 5.1 Comments on C++ Wrapping

Because of its complexity and the fact that C++ can be difficult to integrate with itself let alone other languages, SWIG only provides support for a subset of C++ features. Fortunately, this is now a rather large subset.

In part, the problem with C++ wrapping is that there is no semantically obvious (or automatic ) way to map many of its advanced features into other languages. As a simple example, consider the problem of wrapping C++ multiple inheritance to a target language with no such support. Similarly, the use of overloaded operators and overloaded functions can be problematic when no such capability exists in a target language.

A more subtle issue with C++ has to do with the way that some C++ programmers think about programming libraries. In the world of SWIG, you are really trying to create binary-level software components for use in other languages. In order for this to work, a "component" has to contain real executable instructions and there has to be some kind of binary linking mechanism for accessing its functionality. In contrast, C++ has increasingly relied upon generic programming and templates for much of its functionality. Although templates are a powerful feature, they are largely orthogonal to the whole notion of binary components and libraries. For example, an STL `vector` does not define any kind of binary object for which SWIG can just create a wrapper. To further complicate matters, these libraries often utilize a lot of behind the scenes magic in which the semantics of seemingly basic operations (e.g., pointer dereferencing, procedure call, etc.) can be changed in dramatic and sometimes non-obvious ways. Although this "magic" may present few problems in a C++-only universe, it greatly complicates the problem of crossing language boundaries and provides many opportunities to shoot yourself in the foot. You will just have to be careful.

## 5.2 Approach

To wrap C++, SWIG uses a layered approach to code generation. At the lowest level, SWIG generates a collection of procedural ANSI-C style wrappers. These wrappers take care of basic type conversion, type checking, error handling, and other low-level details of the C++ binding. These wrappers are also sufficient to bind C++ into any target language that supports built-in procedures. In some sense, you might view this layer of wrapping as providing a C library interface to C++. Optionally, SWIG can also generate proxy classes that provide a natural OO interface to the underlying code. These proxies are built on top of the low-level procedural wrappers and are typically written in the target language itself. For instance, in Python, a real Python class is used to provide a wrapper around the underlying C++ object.

It is important to emphasize that SWIG takes a deliberately conservative and non-intrusive approach to C++ wrapping. SWIG does not encapsulate C++ classes inside special C++ adaptor or proxy classes, it does not rely upon templates, nor does it use C++ inheritance when generating wrappers. The last thing that most C++ programs need is even more compiler magic. Therefore, SWIG tries to maintain a very strict and clean separation between the implementation of your C++ application and the resulting wrapper code. You might say that SWIG has been written to follow the principle of least surprise--it does not play sneaky tricks with the C++ type system, it doesn't mess with your class hierarchies, and it doesn't introduce new semantics. Although this approach might not provide the most seamless integration with C++, it is safe, simple, portable, and debuggable.

Most of this chapter focuses on the low-level procedural interface to C++ that is used as the foundation for all language modules. Keep in mind that most target languages also provide a high-level OO interface via proxy classes. A few general details about proxies can be found at the end of this chapter. However, more detailed coverage can be found in the documentation for each target language.

## 5.3 Supported C++ features

SWIG's currently supports the following C++ features :

- Classes.
- Constructors and destructors
- Virtual functions
- Public inheritance (including multiple inheritance)
- Static functions
- Function and method overloading.
- Operator overloading for many standard operators
- References
- Templates (including specialization and member templates).
- Pointers to members
- Namespaces

The following C++ features are not currently supported :

- Nested classes
- Overloaded versions of certain operators (new, delete, etc.)

SWIG's C++ support is an ongoing project so some of these limitations may be lifted in future releases. However, we make no promises. Also, submitting a bug report is a very good way to get problems fixed (wink).

## 5.4 Command line options and compilation

When wrapping C++ code, it is critical that SWIG be called with the ``-c++`' option. This changes the way a number of critical features such as memory management are handled. It also enables the recognition of C++ keywords. Without the `-c++` flag, SWIG will either issue a warning or a large number of syntax errors if it encounters C++ code in an interface file.

When compiling and linking the resulting wrapper file, it is normal to use the C++ compiler. For example:

>     $ swig -c++ -tcl example.i
>     $ c++ -c example_wrap.cxx
>     $ c++ example_wrap.o $(OBJS) -o example.so

Unfortunately, the process varies slightly on each machine. Make sure you refer to the documentation on each target language for further details. The SWIG Wiki also has further details.

## 5.5 Simple C++ wrapping

The following code shows a SWIG interface file for a simple C++ class.

>     %module list
>     %{
>     #include "list.h"
>     %}

>     // Very simple C++ example for linked list

>     class List {
>     public:
>       List();
>       ~List();
>       int  search(char *value);
>       void insert(char *);
>       void remove(char *);
>       char *get(int n);
>       int  length;
>     static void print(List *l);
>     };

To generate wrappers for this class, SWIG first reduces the class to a collection of low-level C-style accessor functions. The next few sections describe this process. Later parts of the chapter decribe a higher level interface based on proxy classes.

### 5.5.1 Constructors and destructors

C++ constructors and destructors are translated into accessor functions such as the following :

>     List * new_List(void) {
>     	return new List;
>     }
>     void delete_List(List *l) {
>     	delete l;
>     }

### 5.5.2 Default constructors

If a C++ class does not define any public constructors or destructors, SWIG will automatically create a default constructor or destructor. However, there are a few rules that define this behavior:

- A default constructor is not created if a class already defines a constructor with arguments.

- Default constructors are not generated for classes with pure virtual methods or for classes that inherit from an abstract class, but don't provide definitions for all of the pure methods.

- A default constructor is not created unless all bases classes support a default constructor.

- Default constructors and destructors are not created if a class defines constructors or destructors in a `private` or `protected` section.

- Default constructors and destructors are not created if any base class defines a private default constructor or a private destructor.

SWIG should never generate a constructor or destructor for a class in which it is illegal to do so. However, if it is necessary to disable the default constructor/destructor creation, the `%nodefault` directive can be used:

>     %nodefault;   // Disable creation of constructor/destructor
>     class Foo {
>     ...
>     };
>     %makedefault;

`%nodefault` can also take a class name. For example:

>     %nodefault Foo;     // Disable for class Foo only.

Compatibility Note: The generation of default constructors/destructors was made the default behavior in SWIG 1.3.7. This may break certain older modules, but the old behavior can be easily restored using `%nodefault` or the `-nodefault` command line option. Furthermore, in order for SWIG to properly generate (or not generate) default constructors, it must be able to gather information from both the `private` and `protected` sections (specifically, it needs to know if a private or protected constructor/destructor is defined). In older versions of SWIG, it was fairly common to simply remove or comment out the private and protected sections of a class due to parser limitations. However, this removal may now cause SWIG to erroneously generate constructors for classes that define a constructor in those sections. Consider restoring those sections in the interface or using `%nodefault` to fix the problem.

### 5.5.3 When constructor wrappers aren't created

If a class defines a constructor, SWIG normally tries to generate a wrapper for it. However, SWIG will not generate a constructor wrapper if it thinks that it will result in illegal wrapper code. There are really two cases where this might show up.

First, SWIG won't generate wrappers for protected or private constructors. For example:

>     class Foo {
>     protected:
>          Foo();         // Not wrapped.
>     public:
>           ...
>     };

Next, SWIG won't generate wrappers for a class if it appears to be abstract--that is, it has undefined pure virtual methods. Here are some examples:

>     class Bar {
>     public:
>          Bar();               // Not wrappped.  Bar is abstract.
>          virtual void spam(void) = 0;
>     };

>     class Grok : public Bar {
>     public:
>           Grok();            // Not wrapped. No implementation of abstract spam().
>     };

Some users are surprised (or confused) to find missing constructor wrappers in their interfaces. In almost all cases, this is caused when classes are determined to be abstract. To see if this is the case, run SWIG with all of its warnings turned on:

>     % swig -Wall -python module.i

In this mode, SWIG will issue a warning for all abstract classes. It is possible to force a class to be non-abstract using this:

>     %feature("notabstract") Foo;

>     class Foo : public Bar {
>     public:
>          Foo();    // Generated no matter what---not abstract.
>          ...
>     };

More information about `%feature` can be found in the Customization features chapter.

### 5.5.4 Copy constructors

If a class defines more than one constructor, its behavior depends on the capabilities of the target language. If overloading is supported, the copy constructor is accessible using the normal constructor function. For example, if you have this:

>     class List {
>     public:
>         List();
>         List(const List &);      // Copy constructor
>         ...
>     };

then the copy constructor can be used as follows:

>     x = new_List()               # Create a list
>     y = new_List(x)              # Copy list x

If the target language does not support overloading, then the copy constructor is available through a special function like this:

>     List *copy_List(List *f) {
>         return new List(*f);
>     }

Note: For a class `X`, SWIG only treats a constructor as a copy constructor if it can be applied to an object of type `X` or `X *`. If more than one copy constructor is defined, only the first definition that appears is used as the copy constructor--other definitions will result in a name-clash. Constructors such as `X(const X &)`, `X(X &)`, and `X(X *)` are handled as copy constructors in SWIG.

Note: SWIG does not generate a copy constructor wrapper unless one is explicitly declared in the class. This differs from the treatment of default constructors and destructors.

Compatibility note: Special support for copy constructors was not added until SWIG-1.3.12. In previous versions, copy constructors could be wrapped, but they had to be renamed. For example:

>     class Foo {
>     public:
>         Foo();
>       %name(CopyFoo) Foo(const Foo &);
>         ...
>     };

For backwards compatibility, SWIG does not perform any special copy-constructor handling if the constructor has been manually renamed. For instance, in the above example, the name of the constructor is set to `new_CopyFoo()`. This is the same as in older versions.

### 5.5.5 Member functions

All member functions are roughly translated into accessor functions like this :

>     int List_search(List *obj, char *value) {
>     	return obj->search(value);
>     }

This translation is the same even if the member function has been declared as `virtual`.

It should be noted that SWIG does not actually create a C accessor function in the code it generates. Instead, member access such as `obj->search(value)` is directly inlined into the generated wrapper functions. However, the name and calling convention of the wrappers match the accessor function prototype described above.

### 5.5.6 Static members

Static member functions are called directly without making any special transformations. For example, the static member function `print(List *l)` directly invokes `List::print(List *l)` in the generated wrapper code.

Usually, static members are accessed as functions with names in which the class name has been prepended with an underscore. For example, `List_print`.

### 5.5.7 Member functions and default arguments

SWIG allows member functions to accept default arguments. For example:

>     class Foo {
>     public:
>         void bar(int x, int y = 3);
>     };

However, the implementation restricts the use of default arguments to values that are public. The following example illustrates a very subtle semantic incompatibility between SWIG and C++:

>     class Foo {
>     private:
>        int  spam;
>     public:
>        void bar(int x, int y = spam);   // Illegal in SWIG. Private default value
>     };

When this occurs, you will get a couple of warning messages like this:

>     example.i:15. Warning 'spam' is private in this context.
>     example.i:15. Warning. Can't set default argument (ignored)

This incompatibility arises because default values in C++ are evaluated in the same scope as the member function whereas SWIG evaluates them in the scope of a wrapper function (meaning that the values have to be public). The full set of arguments are needed in the wrappers in order to support a number of advanced customization features and the use of default arguments in ANSI C (which is not part of the ANSI standard).

There are several somewhat clumsy ways to work around this problem (which are not discussed here). However, a simpler solution may be to reconsider your design--is it really that critical to make the default argument private?

### 5.5.8 Member data

Member data is handled in exactly the same manner as for C structures. A pair of accessor functions are created. For example :

>     int List_length_get(List *obj) {
>     	return obj->length;
>     }
>     int List_length_set(List *obj, int value) {
>     	obj->length = value;
>     	return value;
>     }

A read-only member can be created using the `%immutable` and `%mutable` directives. For example, we probably wouldn't want the user to change the length of a list so we could do the following to make the value available, but read-only.

>     class List {
>     public:
>     ...
>     %immutable;
>     	int length;
>     %mutable;
>     ...
>     };

Alternatively, you can specify an immutable member in advance like this:

>     %immutable List::length;
>     ...
>     class List {
>        ...
>        int length;         // Immutable by above directive
>        ...
>     };

Similarly, all data attributes declared as `const` are wrapped as read-only members.

There are some subtle issues when wrapping data members that are themselves classes. For instance, if you had another class like this,

>     class Foo {
>     public:
>         List items;
>         ...

then access to the `items` member actually uses pointers. For example:

>     List *Foo_items_get(Foo *self) {
>         return &self->items;
>     }
>     void Foo_items_set(Foo *self, List *value) {
>         self->items = *value;
>     }

More information about this can be found in the "Structure data members" section of the SWIG Basics chapter.

Compatibility note: Read-only access used to be controlled by a pair of directives `%readonly` and `%readwrite`. Although these directives still work, they generate a warning message. Simply change the directives to `%immutable;` and `%mutable;` to silence the warning. Don't forget the extra semicolon!

Compatibility note: Prior to SWIG-1.3.12, all members of unknown type were wrapped into accessor functions using pointers. For example, if you had a structure like this

>     struct Foo {
>        size_t  len;
>     };

and nothing was known about `size_t`, then accessors would be written to work with `size_t *`. Starting in SWIG-1.3.12, this behavior has been modified. Specifically, pointers will only be used if SWIG knows that a datatype corresponds to a structure or class. Therefore, the above code would be wrapped into accessors involving `size_t`. This change is subtle, but it smooths over a few problems related to structure wrapping and some of SWIG's customization features.

## 5.6 Protection

SWIG can only wrap class members that are declared public. Anything specified in a private or protected section will simply be ignored (although the internal code generator sometimes looks at the contents of the private and protected sections so that it can properly generate code for default constructors and destructors).

By default, members of a class definition are assumed to be private until you explicitly give a ``public:`' declaration (This is the same convention used by C++).

A subtle access problem relates to default values of member functions. Specifically, default values must be public. Please go back to the section on default arguments for further details.

## 5.7 Enums and constants

Enumerations and constants placed in a class definition are mapped into constants with the classname as a prefix. For example :

>     class Swig {
>     public:
>     	enum {ALE, LAGER, PORTER, STOUT};
>     };

Generates the following set of constants in the target scripting language :

>     Swig_ALE = Swig::ALE
>     Swig_LAGER = Swig::LAGER
>     Swig_PORTER = Swig::PORTER
>     Swig_STOUT = Swig::STOUT

Members declared as `const` are wrapped as read-only members and do not create constants.

## 5.8 Friends

Friend declarations are ignored by SWIG. For example, if you have this code:

>     class Foo {
>     public:
>          ...
>          friend void blah(Foo *f);
>          ...
>     };

then the `friend` declaration does not result in any wrapper code. On the other hand, a declaration of the function itself will work fine. For instance:

>     class Foo {
>     public:
>         ...
>         friend void blah(Foo *f);       // Ignored
>         ...
>     };

>     void blah(Foo *f);       // Generates wrappers

Unlike normal member functions or static member functions, a friend declaration does not define a method that operates on an instance of an object nor does it define a declaration in the scope of the class. Therefore, it would make no sense for SWIG to create wrappers as such.

## 5.9 References and pointers

C++ references are supported, but SWIG transforms them back into pointers. For example, a declaration like this :

>     class Foo {
>     public:
>     	double bar(double &a);
>     }

is accessed using a function similar to this:

>     double Foo_bar(Foo *obj, double *a) {
>     	obj->bar(*a);
>     }

As a special case, most language modules pass `const` references to primitive datatypes (`int`, `short`, `float`, etc.) by value instead of pointers. For example, if you have a function like this,

>     void foo(const int &x);

it is called from a script as follows:

>     foo(3)              # Notice pass by value

Functions that return a reference are remapped to return a pointer instead. For example:

>     class Bar {
>     public:
>          Foo &spam();
>     };

Generates code like this:

>     Foo *Bar_spam(Bar *obj) {
>        Foo &result = obj->spam();
>        return &result;
>     }

However, functions that return `const` references to primitive datatypes (`int`, `short`, etc.) normally return the result as a value rather than a pointer. For example, a function like this,

>     const int &bar();

will return integers such as 37 or 42 in the target scripting language rather than a pointer to an integer.

Don't return references to objects allocated as local variables on the stack. SWIG doesn't make a copy of the objects so this will probably cause your program to crash.

Note: The special treatment for references to primitive datatypes is necessary to provide more seamless integration with more advanced C++ wrapping applications---especially related to templates and the STL. This was first added in SWIG-1.3.12.

## 5.10 Pass and return by value

Occasionally, a C++ program will pass and return class objects by value. For example, a function like this might appear:

>     Vector cross_product(Vector a, Vector b);

If no information is supplied about `Vector`, SWIG creates a wrapper function similar to the following:

>     Vector *wrap_cross_product(Vector *a, Vector *b) {
>        Vector x = *a;
>        Vector y = *b;
>        Vector r = cross_product(x,y);
>        return new Vector(r);
>     }

In order for the wrapper code to compile, `Vector` must define a copy constructor and a default constructor.

If `Vector` is defined as class in the interface, but it does not support a default constructor, SWIG changes the wrapper code by encapsulating the arguments inside a special C++ template wrapper class. This produces a wrapper that looks like this:

>     Vector cross_product(Vector *a, Vector *b) {
>        SwigValueWrapper<Vector> x = *a;
>        SwigValueWrapper<Vector> y = *b;
>        SwigValueWrapper<Vector> r = cross_product(x,y);
>        return new Vector(r);
>     }

This transformation is a little sneaky, but it provides support for pass-by-value even when a class does not provide a default constructor and it makes it possible to properly support a number of SWIG's customization options. The definition of `SwigValueWrapper` can be found by reading the SWIG wrapper code. This class is really nothing more than a thin wrapper around a pointer.

Note: this transformation has no effect on typemaps or any other part of SWIG---it should be transparent except that you may see this code when reading the SWIG output file.

Note: This template transformation is new in SWIG-1.3.11 and may be refined in future SWIG releases. In practice, it is only necessary to do this for classes that don't define a default constructor.

Note: The use of this template only occurs when objects are passed or returned by value. It is not used for C++ pointers or references.

Note: The performance of pass-by-value is especially bad for large objects and should be avoided if possible (consider using references instead).

## 5.11 Inheritance

SWIG supports C++ public inheritance of classes and allows both single and multiple inheritance. The SWIG type-checker knows about the relationship between base and derived classes and allows pointers to any object of a derived class to be used in functions of a base class. The type-checker properly casts pointer values and is safe to use with multiple inheritance.

SWIG does not support private or protected inheritance (it is parsed, but it has no effect on the generated code). Note: private and protected inheritance do not define an "isa" relationship between classes so it would have no effect on type-checking anyways.

The following example shows how SWIG handles inheritance. For clarity, the full C++ code has been omitted.

>     // shapes.i
>     %module shapes
>     %{
>     #include "shapes.h"
>     %}

>     class Shape {
>     public:
>             double x,y;
>     	virtual double area() = 0;
>     	virtual double perimeter() = 0;
>     	void    set_location(double x, double y);
>     };
>     class Circle : public Shape {
>     public:
>     	Circle(double radius);
>     	~Circle();
>     	double area();
>     	double perimeter();
>     };
>     class Square : public Shape {
>     public:
>     	Square(double size);
>     	~Square();
>     	double area();
>     	double perimeter();
>     }

When wrapped into Python, we can now perform the following operations :

>     $ python
>     >>> import shapes
>     >>> circle = shapes.new_Circle(7)
>     >>> square = shapes.new_Square(10)
>     >>> print shapes.Circle_area(circle)
>     153.93804004599999757
>     >>> print shapes.Shape_area(circle)
>     153.93804004599999757
>     >>> print shapes.Shape_area(square)
>     100.00000000000000000
>     >>> shapes.Shape_set_location(square,2,-3)
>     >>> print shapes.Shape_perimeter(square)
>     40.00000000000000000
>     >>>

In this example, Circle and Square objects have been created. Member functions can be invoked on each object by making calls to `Circle_area`, `Square_area`, and so on. However, the same results can be accomplished by simply using the `Shape_area` function on either object.

One important point concerning inheritance is that the low-level accessor functions are only generated for classes in which they are actually declared. For instance, in the above example, the method `set_location()` is only accessible as `Shape_set_location()` and not as `Circle_set_location()` or `Square_set_location()`. Of course, the `Shape_set_location()` function will accept any kind of object derived from Shape. Similarly, accessor functions for the attributes `x` and `y` are generated as `Shape_x_get()`, `Shape_x_set()`, `Shape_y_get()`, and `Shape_y_set()`. Functions such as `Circle_x_get()` are not available--instead you should use `Shape_x_get()`.

Although the low-level C-like interface is functional, most language modules also produce a higher level OO interface using proxy classes. This approach is described later and can be used to provide a more natural C++ interface.

Note: For the best results, SWIG requires all base classes to be defined in an interface. Otherwise, you may get an warning message like this:

>     example:18. Nothing known about class 'Foo'. Ignored.

If any base class is undefined, SWIG still generates correct type relationships. For instance, a function accepting a `Foo *` will accept any object derived from `Foo` regardless of whether or not SWIG actually wrapped the `Foo` class. If you really don't want to generate wrappers for the base class, but you want to silence the warning, you might consider using the `%import` directive to include the file that defines `Foo`. `%import` simply gathers type information, but doesn't generate wrappers. Alternatively, you could just define `Foo` as an empty class in the SWIG interface.

Note: `typedef`-names can be used as base classes. For example:

>     class Foo {
>     ...
>     };

>     typedef Foo FooObj;
>     class Bar : public FooObj {     // Ok.  Base class is Foo
>     ...
>     };

Similarly, `typedef` allows unnamed structures to be used as base classes. For example:

>     typedef struct {
>        ...
>     } Foo;

>     class Bar : public Foo {    // Ok.
>     ...
>     };

Compatibility Note: Starting in version 1.3.7, SWIG only generates low-level accessor wrappers for the declarations that are actually defined in each class. This differs from SWIG1.1 which used to inherit all of the declarations defined in base classes and regenerate specialized accessor functions such as `Circle_x_get()`, `Square_x_get()`, `Circle_set_location()`, and `Square_set_location()`. This behavior resulted in huge amounts of replicated code for large class hierarchies and made it awkward to build applications spread across multiple modules (since accessor functions are duplicated in every single module). It is also unnecessary to have such wrappers when advanced features like proxy classes are used. Future versions of SWIG may apply further optimizations such as not regenerating wrapper functions for virtual members that are already defined in a base class.

## 5.12 A brief discussion of multiple inheritance, pointers, and type checking

When a target scripting language refers to a C++ object, it normally uses a tagged pointer object that contains both the value of the pointer and a type string. For example, in Tcl, a C++ pointer might be encoded as a string like this:

>     _808fea88_p_Circle

A somewhat common question is whether or not the type-tag could be safely removed from the pointer. For instance, to get better performance, could you strip all type tags and just use simple integers instead?

In general, the answer to this question is no. In the wrappers, all pointers are converted into a common data representation in the target language. Typically this is the equivalent of casting a pointer to `void *`. This means that any C++ type information associated with the pointer is lost in the conversion.

The problem with losing type information is that it is needed to properly support many advanced C++ features--especially multiple inheritance. For example, suppose you had code like this:

>     class A {
>     public:
>        int x;
>     };

>     class B {
>     public:
>        int y;
>     };

>     class C : public A, public B {
>     };

>     int A_function(A *a) {
>        return a->x;
>     }

>     int B_function(B *b) {
>        return b->y;
>     }

Now, consider the following code that uses `void *`.

>     C *c = new C();
>     void *p = (void *) c;
>     ...
>     int x = A_function((A *) p);
>     int y = B_function((B *) p);

In this code, both `A_function()` and `B_function()` may legally accept an object of type `C *` (via inheritance). However, one of the functions will always return the wrong result when used as shown. The reason for this is that even though `p` points to an object of type `C`, the casting operation doesn't work like you would expect. Internally, this has to do with the data representation of `C`. With multiple inheritance, the data from each base class is stacked together. For example:

>                  ------------    <--- (C *),  (A *)
>                 |     A      |
>                 |------------|   <--- (B *)
>                 |     B      |
>                  ------------

Because of this stacking, a pointer of type `C *` may change value when it is converted to a `A *` or `B *`. However, this adjustment does not occur if you are converting from a `void *`.

The use of type tags marks all pointers with the real type of the underlying object. This extra information is then used by SWIG generated wrappers to correctly cast pointer values under inheritance (avoiding the above problem).

One might be inclined to fix this problem using some variation of `dynamic_cast<>`. The only problem is that it doesn't work with `void` pointers, it requires RTTI support, and it only works with polymorphic classes (i.e., classes that define one or more virtual functions).

The bottom line: learn to live with type-tagged pointers.

## 5.13 Renaming

C++ member functions and data can be renamed with the `%name` directive. The `%name` directive only replaces the member function name. For example :

>     class List {
>     public:
>       List();
>     %name(ListSize) List(int maxsize);
>       ~List();
>       int  search(char *value);
>     %name(find)    void insert(char *);
>     %name(delete)  void remove(char *);
>       char *get(int n);
>       int  length;
>     static void print(List *l);
>     };

This will create the functions `List_find`, `List_delete`, and a function named `new_ListSize` for the overloaded constructor.

The `%name` directive can be applied to all members including constructors, destructors, static functions, data members, and enumeration values.

The class name prefix can also be changed by specifying

>     %name(newname) class List {
>     ...
>     }

Although the `%name()` directive can be used to help deal with overloaded methods, it really doesn't work very well because it requires a lot of additional markup in your interface. Keep reading for a better solution.

## 5.14 Wrapping Overloaded Functions and Methods

In many language modules, SWIG provides partial support for overloaded functions, methods, and constructors. For example, if you supply SWIG with overloaded functions like this:

>     void foo(int x) {
>        printf("x is %d\n", x);
>     }
>     void foo(char *x) {
>        printf("x is '%s'\n", x);
>     }

The function is used in a completely natural way. For example:

>     >>> foo(3)
>     x is 3
>     >>> foo("hello")
>     x is 'hello'
>     >>>

Overloading works in a similar manner for methods and constructors. For example if you have this code,

>     class Foo {
>     public:
>          Foo();
>          Foo(const Foo &);   // Copy constructor
>          void bar(int x);
>          void bar(char *s, int y);
>     };

it might be used like this

>     >>> f = Foo()          # Create a Foo
>     >>> f.bar(3)
>     >>> g = Foo(f)         # Copy Foo
>     >>> f.bar("hello",2)

### 5.14.1 Dispatch function generation

The implementation of overloaded functions and methods is somewhat complicated due to the dynamic nature of scripting languages. Unlike C++, which binds overloaded methods at compile time, SWIG must determine the proper function as a runtime check. This check is further complicated by the typeless nature of certain scripting languages. For instance, in Tcl, all types are simply strings. Therefore, if you have two overloaded functions like this,

>     void foo(char *x);
>     void foo(int x);

the order in which the arguments are checked plays a rather critical role.

To implement overloading, SWIG generates a dispatch function that checks the number of passed arguments and their types. To create this function, SWIG first examines all of the overloaded methods and ranks them according to the following rules:

1. Number of required arguments. Methods are sorted by increasing number of required arguments.

2. Argument type precedence. All C++ datatypes are assigned a numeric type precedence value (which is determined by the language module).

   >     Type              Precedence
   >     ----------------  ----------
   >     TYPE *            0     (High)
   >     void *            20
   >     Integers          40
   >     Floating point    60
   >     char              80
   >     Strings           100   (Low)

   Using these precedence values, overloaded methods with the same number of required arguments are sorted in increased order of precedence values.

This may sound very confusing, but an example will help. Consider the following collection of overloaded methods:

>     void foo(double);
>     void foo(int);
>     void foo(Bar *);
>     void foo();
>     void foo(int x, int y, int z, int w);
>     void foo(int x, int y, int z = 3);
>     void foo(double x, double y);
>     void foo(double x, Bar *z);

The first rule simply ranks the functions by required argument count. This would produce the following list:

>     rank
>     -----
>     [0]   foo()
>     [1]   foo(double);
>     [2]   foo(int);
>     [3]   foo(Bar *);
>     [4]   foo(int x, int y, int z = 3);
>     [5]   foo(double x, double y)
>     [6]   foo(double x, Bar *z)
>     [7]   foo(int x, int y, int z, int w);

The second rule, simply refines the ranking by looking at argument type precedence values.

>     rank
>     -----
>     [0]   foo()
>     [1]   foo(Bar *);
>     [2]   foo(int);
>     [3]   foo(double);
>     [4]   foo(int x, int y, int z = 3);
>     [5]   foo(double x, Bar *z)
>     [6]   foo(double x, double y)
>     [7]   foo(int x, int y, int z, int w);

Finally, to generate the dispatch function, the arguments passed to an overloaded method are simply checked in the same order as they appear in this ranking.

If you're still confused, don't worry about it---SWIG is probably doing the right thing.

### 5.14.2 Ambiguity in Overloading

Regrettably, SWIG is not able to support every possible use of valid C++ overloading. Consider the following example:

>     void foo(int x);
>     void foo(long x);

In C++, this is perfectly legal. However, in a scripting language, there is generally only one kind of integer object. Therefore, which one of these functions do you pick? Clearly, there is no way to truly make a distinction just by looking at the value of the integer itself (`int` and `long` may even be the same precision). Therefore, when SWIG encounters this situation, it may generate a warning message like this:

>     example.i:4: Warning(509): Overloaded foo(long) is shadowed by foo(int) at example.i:3.

This means that the second overloaded function will be inaccessible from the scripting interface---SWIG does not know how to disambiguate it from an earlier method.

Ambiguity problems are known to arise in the following situations:

- Integer conversions. Datatypes such as `int`, `long`, and `short`. Shown above.

- Floating point conversion. `float` and `double` can not be disambiguated.

- Pointers and references. For example, `Foo *` and `Foo &`.

- Pointers and arrays. For example, `Foo *` and `Foo [4]`.

- Pointers and instances. For example, `Foo` and `Foo *`. Note: SWIG converts all instances to pointers.

- Qualifiers. For example, `const Foo *` and `Foo *`.

- Default vs. non default arguments. For example, `foo(int a, int b)` and `foo(int a, int b = 3)`.

When an ambiguity arises, methods are checked in the same order as they appear in the interface file. Therefore, earlier methods will shadow methods that appear later.

When wrapping an overloaded function, there is a chance that you will get an error message like this:

>     example.i:3: Warning(467): Overloaded foo(int) not supported (no type checking rule for 'int').

This error means that the target language module supports overloading, but for some reason there is no type-checking rule that can be used to generate a working dispatch function. The resulting behavior is then undefined. You should report this as a bug to `swig-dev@cs.uchicago.edu`.

If you get an error message such as the following,

>     foo.i:6. Overloaded declaration ignored.  Spam::foo(double )
>     foo.i:5. Previous declaration is Spam::foo(int )
>     foo.i:7. Overloaded declaration ignored.  Spam::foo(Bar *,Spam *,int )
>     foo.i:5. Previous declaration is Spam::foo(int )

it means that the target language module has not yet implemented support for overloaded functions and methods. The only way to fix the problem is to read the next section.

### 5.14.3 Ambiguity resolution and renaming

If an ambiguity in overload resolution occurs or if a module doesn't allow overloading, there are a few strategies for dealing with the problem. First, you can tell SWIG to ignore one of the methods. This is easy---simply use the `%ignore` directive. For example:

>     %ignore foo(long);

>     void foo(int);
>     void foo(long);       // Ignored.  Oh well.

The other alternative is to rename one of the methods. This can be done using `%rename`. For example:

>     %rename(foo_long) foo(long);

>     void foo(int);
>     void foo(long);       // Accessed as foo_long()

The `%ignore` and `%rename` directives are both rather powerful in their ability to match declarations. When used in their simple form, they apply to both global functions and methods. For example:

>     /* Forward renaming declarations */
>     %rename(foo_i) foo(int);
>     %rename(foo_d) foo(double);
>     ...
>     void foo(int);           // Becomes 'foo_i'
>     void foo(char *c);       // Stays 'foo' (not renamed)

>     class Spam {
>     public:
>        void foo(int);      // Becomes 'foo_i'
>        void foo(double);   // Becomes 'foo_d'
>        ...
>     };

If you only want the renaming to apply to a certain scope, the C++ scope resolution operator (::) can be used. For example:

>     %rename(foo_i) ::foo(int);      // Only rename foo(int) in the global scope.
>                                     // (will not rename class members)

>     %rename(foo_i) Spam::foo(int);  // Only rename foo(int) in class Spam

When a renaming operator is applied to a class as in `Spam::foo(int)`, it is applied to that class and all derived classes. This can be used to apply a consistent renaming across an entire class hierarchy with only a few declarations. For example:

>     %rename(foo_i) Spam::foo(int);
>     %rename(foo_d) Spam::foo(double);

>     class Spam {
>     public:
>        virtual void foo(int);      // Renamed to foo_i
>        virtual void foo(double);   // Renamed to foo_d
>        ...
>     };

>     class Bar : public Spam {
>     public:
>        virtual void foo(int);      // Renamed to foo_i
>        virtual void foo(double);   // Renamed to foo_d
>     ...
>     };

>     class Grok : public Bar {
>     public:
>        virtual void foo(int);      // Renamed to foo_i
>        virtual void foo(double);   // Renamed to foo_d
>     ...
>     };

It is also possible to include `%rename` specifications in the class definition itself. For example:

>     class Spam {
>        %rename(foo_i) foo(int);
>        %rename(foo_d) foo(double);
>     public:
>        virtual void foo(int);      // Renamed to foo_i
>        virtual void foo(double);   // Renamed to foo_d
>        ...
>     };

>     class Bar : public Spam {
>     public:
>        virtual void foo(int);      // Renamed to foo_i
>        virtual void foo(double);   // Renamed to foo_d
>     ...
>     };

In this case, the `%rename` directives still get applied across the entire inheritance hierarchy, but it's no longer necessary to explicitly specify the class prefix `Spam::`.

A special form of `%rename` can be used to apply a renaming just to class members (of all classes):

>     %rename(foo_i) *::foo(int);   // Only rename foo(int) if it appears in a class.

Note: the `*::` syntax is non-standard C++, but the '*' is meant to be a wildcard that matches any class name (we couldn't think of a better alternative so if you have a better idea, send email to swig-dev@cs.uchicago.edu).

Although this discussion has primarily focused on `%rename` all of the same rules also apply to `%ignore`. For example:

>     %ignore foo(double);          // Ignore all foo(double)
>     %ignore Spam::foo;            // Ignore foo in class Spam
>     %ignore Spam::foo(double);    // Ignore foo(double) in class Spam
>     %ignore *::foo(double);       // Ignore foo(double) in all classes

When applied to a base class, `%ignore` forces all definitions in derived clases to disappear. For example, `%ignore Spam::foo(double)` will eliminate `foo(double)` in `Spam` and all classes derived from `Spam`.

Notes on %rename and %ignore:

- Since, the `%rename` declaration is used to declare a renaming in advance, it can be placed at the start of an interface file. This makes it possible to apply a consistent name resolution without having to modify header files. For example:

  >     %module foo

  >     /* Rename these overloaded functions */
  >     %rename(foo_i) foo(int);
  >     %rename(foo_d) foo(double);

  >     %include "header.h"

- The scope qualifier (::) can also be used on simple names. For example:

  >     %rename(bar) ::foo;       // Rename foo to bar in global scope only
  >     %rename(bar) Spam::foo;   // Rename foo to bar in class Spam only
  >     %rename(bar) *::foo;      // Rename foo in classes only

- Name matching tries to find the most specific match that is defined. A qualified name such as `Spam::foo` always has higher precedence than an unqualified name `foo`. `Spam::foo` has higher precedence than `*::foo` and `*::foo` has higher precedence than `foo`. A parameterized name has higher precedence than an unparameterized name within the same scope level. However, an unparameterized name with a scope qualifier has higher precedence than a parameterized name in global scope (e.g., a renaming of `Spam::foo` takes precedence over a renaming of `foo(int)`).

- The order in which `%rename` directives are defined does not matter as long as they appear before the declarations to be renamed. Thus, there is no difference between saying:

  >     %rename(bar) foo;
  >     %rename(foo_i) Spam::foo(int);
  >     %rename(Foo) Spam::foo;

  and this

  >     %rename(Foo) Spam::foo;
  >     %rename(bar) foo;
  >     %rename(foo_i) Spam::foo(int);

  (the declarations are not stored in a linked list and order has no importance). Of course, a repeated `%rename` directive will change the setting for a previous `%rename` directive if exactly the same name, scope, and parameters are supplied.

- For multiple inheritance where renaming rules are defined for multiple base classes, the first renaming rule found on a depth-first traversal of the class hierarchy is used.

- The name matching rules strictly follow member qualification rules. For example, if you have a class like this:

  >     class Foo {
  >     public:
  >        ...
  >        void bar();
  >        ...
  >     };

  the declaration `%rename(name) Foo::bar()` only applies to the unqualified member `bar()`. However, an often overlooked C++ feature is that classes can define two different overloaded members that differ only in their qualifiers, like this:

  >     class Foo {
  >     public:
  >        ...
  >        void bar();         // Unqualified member
  >        void bar() const;   // Qualified member (OK)
  >        ...
  >     };

  In this case, the renaming operator would only apply to the first method. If you wanted to rename the qualified member function, use `%rename(name) Foo::bar() const` instead. Similarly, if you merely wanted to ignore one of the declarations, use `%ignore` with the full qualification. For example, the following directive would tell SWIG to ignore the `const` version of `bar()` above:

  >     %ignore Foo::bar() const;   // Ignore bar() const, but leave other bar() alone

### 5.14.4 Comments on overloading

Support for overloaded methods was first added in SWIG-1.3.14. The implementation is somewhat unusual when compared to similar tools. For instance, the order in which declarations appear is largely irrelevant in SWIG. Furthermore, SWIG does not rely upon trial execution or exception handling to figure out which method to invoke.

Internally, the overloading mechanism is completely configurable by the target language module. Therefore, the degree of overloading support may vary from language to language. As a general rule, statically typed languages like Java are able to provide more support than dynamically typed languages like Perl, Python, Ruby, and Tcl.

## 5.15 Wrapping overloaded operators

Starting in SWIG-1.3.10, C++ overloaded operator declarations can be wrapped. For example, consider a class like this:

>     class Complex {
>     private:
>       double rpart, ipart;
>     public:
>       Complex(double r = 0, double i = 0) : rpart(r), ipart(i) { }
>       Complex(const Complex &c) : rpart(c.rpart), ipart(c.ipart) { }
>       Complex &operator=(const Complex &c) {
>         rpart = c.rpart;
>         ipart = c.ipart;
>         return *this;
>       }
>       Complex operator+(const Complex &c) const {
>         return Complex(rpart+c.rpart, ipart+c.ipart);
>       }
>       Complex operator-(const Complex &c) const {
>         return Complex(rpart-c.rpart, ipart-c.ipart);
>       }
>       Complex operator*(const Complex &c) const {
>         return Complex(rpart*c.rpart - ipart*c.ipart,
>     		   rpart*c.ipart + c.rpart*ipart);
>       }
>       Complex operator-() const {
>         return Complex(-rpart, -ipart);
>       }
>       double re() const { return rpart; }
>       double im() const { return ipart; }
>     };

When operator declarations appear, they are handled in exactly the same manner as regular methods. However, the names of these methods are set to strings like "`operator +`" or "`operator -`". The problem with these names is that they are illegal identifiers in most scripting languages. For instance, you can't just create a method called "`operator +`" in Python--there won't be any way to call it.

Some language modules already know how to automatically handle certain operators (mapping them into operators in the target language). However, the underlying implementation of this is really managed in a very general way using the `%rename` directive. For example, in Python a declaration similar to this is used:

>     %rename(__add__) Complex::operator+;

This binds the + operator to a method called `__add__` (which is conveniently the same name used to implement the Python + operator). Internally, the generated wrapper code for a wrapped operator will look something like this pseudocode:

>     _wrap_Complex___add__(args) {
>        ... get args ...
>        obj->operator+(args);
>        ...
>     }

When used in the target language, it may now be possible to use the overloaded operator normally. For example:

>     >>> a = Complex(3,4)
>     >>> b = Complex(5,2)
>     >>> c = a + b           # Invokes __add__ method

It is important to realize that there is nothing magical happening here. The `%rename` directive really only picks a valid method name. If you wrote this:

>     %rename(add) operator+;

The resulting scripting interface might work like this:

>     a = Complex(3,4)
>     b = Complex(5,2)
>     c = a.add(b)      # Call a.operator+(b)

All of the techniques described to deal with overloaded functions also apply to operators. For example:

>     %ignore Complex::operator=;             // Ignore = in class Complex
>     %ignore *::operator=;                   // Ignore = in all classes
>     %ignore operator=;                      // Ignore = everywhere.

>     %rename(__sub__) Complex::operator-;
>     %rename(__neg__) Complex::operator-();  // Unary -

The last part of this example illustrates how multiple definitions of the `operator-` method might be handled.

Handling operators in this manner is mostly straightforward. However, there are a few subtle issues to keep in mind:

- In C++, it is fairly common to define different versions of the operators to account for different types. For example, a class might also include a friend function like this:

  >     class Complex {
  >     public:
  >       friend Complex operator+(Complex &, double);
  >     };
  >     Complex operator+(Complex &, double);

  SWIG simply ignores all `friend` declarations. Furthermore, it doesn't know how to associate the associated `operator+` with the class (because it's not a member of the class).

  It's still possible to make a wrapper for this operator, but you'll have to handle it like a normal function. For example:

  >     %rename(add_complex_double) operator+(Complex &, double);

- Certain operators are ignored by default. For instance, `new` and `delete` operators are ignored as well as conversion operators.

- The semantics of certain C++ operators may not match those in the target language.

## 5.16 Class extension

New methods can be added to a class using the `%extend` directive. This directive is primarily used in conjunction with proxy classes to add additional functionality to an existing class. For example :

>     %module vector
>     %{
>     #include "vector.h"
>     %}

>     class Vector {
>     public:
>     	double x,y,z;
>     	Vector();
>     	~Vector();
>     	... bunch of C++ methods ...
>     	%extend {
>     		char *__str__() {
>     			static char temp[256];
>     			sprintf(temp,"[ %g, %g, %g ]", v->x,v->y,v->z);
>     			return &temp[0];
>     		}
>     	}
>     };

This code adds a`__str__` method to our class for producing a string representation of the object. In Python, such a method would allow us to print the value of an object using the `print` command.

>     >>>
>     >>> v = Vector();
>     >>> v.x = 3
>     >>> v.y = 4
>     >>> v.z = 0
>     >>> print(v)
>     [ 3.0, 4.0, 0.0 ]
>     >>>

The`%extend` directive follows all of the same conventions as its use with C structures. Please refer to the SWIG Basics chapter for further details.

Compatibility note: The `%extend` directive is a new name for the `%addmethods` directive. Since `%addmethods` could be used to extend a structure with more than just methods, a more suitable directive name has been chosen.

## 5.17 Templates

In all versions of SWIG, template type names may appear anywhere a type is expected in an interface file. For example:

>     void foo(vector<int> *a, int n);
>     void bar(list<int,100> *x);

There are some restrictions on the use of non-type arguments. Specifically, they have to be simple literals and not expressions. For example:

>     void bar(list<int,100> *x);    // OK
>     void bar(list<int,2*50> *x);   // Illegal

The type system is smart enough to figure out clever games you might try to play with `typedef`. For instance, consider this code:

>     typedef int Integer;
>     void foo(vector<int> *x, vector<Integer> *y);

In this case, `vector<Integer>` is exactly the same type as `vector<int>`. The wrapper for `foo()` will accept either variant.

Starting with SWIG-1.3.7, simple C++ template declarations can also be wrapped. SWIG-1.3.12 greatly expands upon the earlier implementation. Before discussing this any further, there are a few things you need to know about template wrapping. First, a bare C++ template does not define any sort of runnable object-code for which SWIG can normally create a wrapper. Therefore, in order to wrap a template, you need to give SWIG information about a particular template instantiation (e.g., `vector<int>`, `array<double>`, etc.). Second, an instantiation name such as `vector<int>` is generally not a valid identifier name in most target languages. Thus, you will need to give the template instantiation a more suitable name such as `intvector` when creating a wrapper.

To illustrate, consider the following template definition:

>     template<class T> class List {
>     private:
>         T *data;
>         int nitems;
>         int maxitems;
>     public:
>         List(int max) {
>           data = new T [max];
>           nitems = 0;
>           maxitems = max;
>         }
>         ~List() {
>           delete [] data;
>         };
>         void append(T obj) {
>           if (nitems < maxitems) {
>             data[nitems++] = obj;
>           }
>         }
>         int length() {
>           return nitems;
>         }
>         T get(int n) {
>           return data[n];
>         }
>     };

By itself, this template declaration is useless--SWIG simply ignores it because it doesn't know how to generate any code until unless a definition of `T` is provided.

One way to create wrappers for a specific template instantiation is to simply provide an expanded version of the class directly like this:

>     %rename(intList) List<int>;       // Rename to a suitable identifier
>     class List<int> {
>     private:
>         int *data;
>         int nitems;
>         int maxitems;
>     public:
>         List(int max);
>         ~List();
>         void append(int obj);
>         int length();
>         int get(int n);
>     };

The `%rename` directive is needed to give the template class an appropriate identifier name in the target language (most languages would not recognize C++ template syntax as a valid class name). The rest of the code is the same as what would appear in a normal class definition.

Since manual expansion of templates gets old in a hurry, the `%template` directive can be used to create instantiations of a template class. Semantically, `%template` is simply a shortcut---it expands template code in exactly the same way as shown above. Here are some examples:

>     /* Instantiate a few different versions of the template */
>     %template(intList) List<int>;
>     %template(doubleList) List<double>;

The argument to `%template()` is the name of the instantiation in the target language. The name you choose should not conflict with any other declarations in the interface file with one exception---it is okay for the template name to match that of a typedef declaration. For example:

>     %template(intList) List<int>;
>     ...
>     typedef List<int> intList;    // OK

SWIG can also generate wrappers for function templates using a similar technique. For example:

>     // Function template
>     template<class T> T max(T a, T b) { return a > b ? a : b; }

>     // Make some different versions of this function
>     %template(maxint) max<int>;
>     %template(maxdouble) max<double>;

In this case, `maxint` and `maxdouble` become unique names for specific instantiations of the function.

The number of arguments supplied to `%template` should match that in the original template definition. Template default arguments are supported. For example:

>     template vector<typename T, int max=100> class vector {
>     ...
>     };

>     %template(intvec) vector<int>;           // OK
>     %template(vec1000) vector<int,1000>;     // OK

The `%template` directive should not be used to wrap the same template instantiation more than once in the same scope. This will generate an error. For example:

>     %template(intList) List<int>;
>     %template(Listint) List<int>;    // Error.   Template already wrapped.

This error is caused because the template expansion results in two identical classes with the same name. This generates a symbol table conflict. Besides, it probably more efficient to only wrap a specific instantiation only once in order to reduce the potential for code bloat.

Since the type system knows how to handle `typedef`, it is generally not necessary to instantiate different versions of a template for typenames that are equivalent. For instance, consider this code:

>     %template(intList) vector<int>;
>     typedef int Integer;
>     ...
>     void foo(vector<Integer> *x);

In this case, `vector<Integer>` is exactly the same type as `vector<int>`. Any use of `Vector<Integer>` is mapped back to the instantiation of `vector<int>` created earlier. Therefore, it is not necessary to instantiate a new class for the type `Integer` (doing so is redundant and will simply result in code bloat).

When a template is instantiated using `%template`, information about that class is saved by SWIG and used elsewhere in the program. For example, if you wrote code like this,

>     ...
>     %template(intList) List<int>;
>     ...
>     class UltraList : public List<int> {
>        ...
>     };

then SWIG knows that `List<int>` was already wrapped as a class called `intList` and arranges to handle the inheritance correctly. If, on the other hand, nothing is known about `List<int>`, you will get a warning message similar to this:

>     example.h:42. Nothing known about class 'List<int >' (ignored).
>     example.h:42. Maybe you forgot to instantiate 'List<int >' using %template.

If a template class inherits from another template class, you need to make sure that base classes are instantiated before derived classes. For example:

>     template<class T> class Foo {
>     ...
>     };

>     template<class T> class Bar : public Foo<T> {
>     ...
>     };

>     // Instantiate base classes first
>     %template(intFoo) Foo<int>;
>     %template(doubleFoo) Foo<double>;

>     // Now instantiate derived classes
>     %template(intBar) Bar<int>;
>     %template(doubleBar) Bar<double>;

The order is important since SWIG uses the instantiation names to properly set up the inheritance hierarchy in the resulting wrapper code (and base classes need to be wrapped before derived classes). Don't worry--if you get the order wrong, SWIG should generate an warning message.

Occassionally, you may need to tell SWIG about base classes that are defined by templates, but which aren't supposed to be wrapped. Since SWIG is not able to automatically instantiate templates for this purpose, you must do it manually. To do this, simply use `%template` with no name. For example:

>     // Instantiate traits<double,double>, but don't wrap it.
>     %template() traits<double,double>;

If you have to instantiate a lot of different classes for many different types, you might consider writing a SWIG macro. For example:

>     %define TEMPLATE_WRAP(T,prefix)
>     %template(prefix ## Foo) Foo<T>;
>     %template(prefix ## Bar) Bar<T>;
>     ...
>     %enddef

>     TEMPLATE_WRAP(int, int)
>     TEMPLATE_WRAP(double, double)
>     TEMPLATE_WRAP(char *, String)
>     ...

The SWIG template mechanism does support specialization. For instance, if you define a class like this,

>     template<> class List<int> {
>     private:
>         int *data;
>         int nitems;
>         int maxitems;
>     public:
>         List(int max);
>         ~List();
>         void append(int obj);
>         int length();
>         int get(int n);
>     };

then SWIG will use this code whenever the user expands `List<int>`. In practice, this may have very little effect on the underlying wrapper code since specialization is often used to provide slightly modified method bodies (which are ignored by SWIG). However, special SWIG directives such as `%typemap`, `%extend`, and so forth can be attached to a specialization to provide customization for specific types.

Partial template specialization is partially supported by SWIG. For example, this code defines a template that is applied when the template argument is a pointer.

>     template<class T> class List<T*> {
>     private:
>         T *data;
>         int nitems;
>         int maxitems;
>     public:
>         List(int max);
>         ~List();
>         void append(int obj);
>         int length();
>         T get(int n);
>     };

SWIG should be able to handle most simple uses of partial specialization. However, it may fail to match templates properly in more complicated cases. For example, if you have this code,

>     template<class T1, class T2> class Foo<T1, T2 *> { };

SWIG isn't able to match it properly for instantiations like `Foo<int *, int *>`. This problem is not due to parsing, but due to the fact that SWIG does not currently implement all of the C++ argument deduction rules.

Member function templates are supported. The underlying principle is the same as for normal templates--SWIG can't create a wrapper unless you provide more information about types. For example, a class with a member template might look like this:

>     class Foo {
>     public:
>          template<class T> void bar(T x, T y) { ... };
>          ...
>     };

To expand the template, simply use `%template` inside the class.

>     class Foo {
>     public:
>          template<class T> void bar(T x, T y) { ... };
>          ...
>          %template(barint)    bar<int>;
>          %template(bardouble) bar<double>;
>     };

Or, if you want to leave the original class definition alone, just do this:

>     class Foo {
>     public:
>          template<class T> void bar(T x, T y) { ... };
>          ...
>     };
>     ...
>     %extend Foo {
>          %template(barint)    bar<int>;
>          %template(bardouble) bar<double>;
>     };

Note: because of the way that templates are handled, the `%template` directive must always appear after the definition of the template to be expanded.

When used with members, the `%template` directive may be placed in another template class. Here is a slightly perverse example:

>     // A template
>     template<class T> class Foo {
>     public:
>          // A member template
>          template<class S> T bar(S x, S y) { ... };
>          ...
>     };

>     // Expand a few member templates
>     %extend Foo {
>          %template(bari) bar<int>;
>          %template(bard) bar<double>;
>     };

>     // Create some wrappers for the template
>     %template(Fooi) Foo<int>;
>     %template(Food) Foo<double>;

Miraculously, you will find that each expansion of `Foo` has member functions `bari()` and `bard()` added.

A common use of member templates is to define constructors for copies and conversions. For example:

>     template<class T1, class T2> struct pair {
>        T1 first;
>        T2 second;
>        pair() : first(T1()), second(T2()) { }
>        pair(const T1 &x, const T2 &y) : first(x), second(y) { }
>        template<class U1, class U2> pair(const pair<U1,U2> &x) : first(x.first),second(x.second) { }
>     };

This declaration is perfectly acceptable to SWIG, but the constructor template will be ignored unless you explicitly expand it. To do that, you could expand a few versions of the constructor in the template class itself. For example:

>     %extend pair {
>        %template(pair) pair<T1,T2>;        // Generate default copy constructor
>     };

When using `%extend` in this manner, notice how you can still use the template parameters in the original template definition.

Alternatively, you could expand the constructor template in selected instantiations. For example:

>     // Instantiate a few versions
>     %template(pairii) pair<int,int>;
>     %template(pairdd) pair<double,double>;

>     // Create a conversion constructor from int to double
>     %extend pair<double,double> {
>        %template(pairdd_from_pairii) pair<int,int>;   // Conversion constructor
>     };

Admittedly, this isn't very pretty or automatic. However, it's probably better than nothing--well, maybe.

If all of this isn't quite enough and you really want to make someone's head explode, SWIG directives such as `%rename`, `%extend`, and `%typemap` can be included directly in template definitions. For example:

>     // File : list.h
>     template<class T> class List {
>        ...
>     public:
>         %rename(__getitem__) get(int);
>         List(int max);
>         ~List();
>         ...
>         T get(int index);
>         %extend {
>             char *__str__() {
>                 /* Make a string representation */
>                 ...
>             }
>         }
>     };

In this example, the extra SWIG directives are propagated to every template instantiation.

It is also possible to separate these declarations from the template class. For example:

>     %rename(__getitem__) List::get;
>     %extend List {
>         char *__str__() {
>             /* Make a string representation */
>             ...
>         }
>         /* Make a copy */
>         T *__copy__() {
>            return new List<T>(*self);
>         }
>     };

>     ...
>     template<class T> class List {
>        ...
>        public:
>        List() { };
>        ...
>     };

When `%extend` is decoupled from the class definition, it is legal to use the same template parameters as provided in the class definition. These are replaced when the template is expanded. In addition, the `%extend` directive can be used to add additional methods to a specific instantiation. For example:

>     %template(intList) List<int>;

>     %extend List<int> {
>         void blah() {
>               printf("Hey, I'm an List<int>!\n");
>         }
>     };

Needless to say, SWIG's template support provides plenty of opportunities to break the universe. That said, an important final point is that SWIG does not perform extensive error checking of templates! Specifically, SWIG does not perform type checking nor does it check to see if the actual contents of the template declaration make any sense. Since the C++ compiler will hopefully check this when it compiles the resulting wrapper file, there is no practical reason for SWIG to duplicate this functionality (besides, none of the SWIG developers are masochistic enough to want to implement this right now).

Finally, there are a few limitations in SWIG's current support for templates:

- SWIG does not support overloaded versions of a template. Some C++ programs might do this to define overloaded functions. For example:

  >     template<class T> void foo(T x) { };
  >     template<class T> void foo(T x, T y) { };     // Error. foo already defined.

  This will generate a name conflict error message in SWIG. To silence the error message, use `%ignore`:

  >     %ignore foo(T,T);

  In this case, `%template` will only work with the first definition. To create a wrapper for the second definition, just do it manually:

  >     %name(foo2int) void foo<int>(int x, int y);

Compatibility Note: The first implementation of template support relied heavily on macro expansion in the preprocessor. Templates have been more tightly integrated into the parser and type system in SWIG-1.3.12 and the preprocessor is no longer used. Code that relied on preprocessing features in template expansion will no longer work. However, SWIG still allows the # operator to be used to generate a string from a template argument.

Compatibility Note: In earlier versions of SWIG, the `%template` directive introduced a new class name. This name could then be used with other directives. For example:

>     %template(vectori) vector<int>;
>     %extend vectori {
>         void somemethod() { }
>     };

This behavior is no longer supported. Instead, you should use the original template name as the class name. For example:

>     %template(vectori) vector<int>;
>     %extend vector<int> {
>         void somemethod() { }
>     };

Similar changes apply to typemaps and other customization features.

## 5.18 Namespaces

Support for C++ namespaces is a relatively late addition to SWIG, first appearing in SWIG-1.3.12. Before describing the implementation, it is worth nothing that the semantics of C++ namespaces is extremely non-trivial--especially with regard to the C++ type system and class machinery. At a most basic level, namespaces are sometimes used to encapsulate common functionality. For example:

>     namespace math {
>        double sin(double);
>        double cos(double);

>        class Complex {
>           double im,re;
>        public:
>           ...
>        };
>        ...
>     };

Members of the namespace are accessed in C++ by prepending the namespace prefix to names. For example:

>     double x = math::sin(1.0);
>     double magitude(math::Complex *c);
>     math::Complex c;
>     ...

At this level, namespaces are relatively easy to manage. However, things start to get very ugly when you throw in the other ways a namespace can be used. For example, selective symbols can be exported from a namespace with `using`.

>     using math::Complex;
>     double magnitude(Complex *c);       // Namespace prefix stripped

Similarly, the contents of an entire namespace can be made available like this:

>     using namespace math;
>     double x = sin(1.0);
>     double magnitude(Complex *c);

Alternatively, a namespace can be aliased:

>     namespace M = math;
>     double x = M::sin(1.0);
>     double magnitude(M::Complex *c);

Using combinations of these features, it is possible to write head-exploding code like this:

>     namespace A {
>       class Foo {
>       };
>     }

>     namespace B {
>        namespace C {
>           using namespace A;
>        }
>        typedef C::Foo FooClass;
>     }

>     namespace BIGB = B;

>     namespace D {
>        using BIGB::FooClass;
>        class Bar : public FooClass {
>        }
>     };

>     class Spam : public D::Bar {
>     };

>     void evil(A::Foo *a, B::FooClass *b, B::C::Foo *c, BIGB::FooClass *d,
>               BIGB::C::Foo *e, D::FooClass *f);

Given the possibility for such perversion, it's hard to imagine how every C++ programmer might want such code wrapped into the target language. Clearly this code defines three different classes. However, one of those classes is accessible under at least six different class names!

SWIG fully supports C++ namespaces in its internal type system and class handling code. If you feed SWIG the above code, it will be parsed correctly, it will generate compilable wrapper code, and it will produce a working scripting language module. However, the default wrapping behavior is to flatten namespaces in the target language. This means that the contents of all namespaces are merged together in the resulting scripting language module. For example, if you have code like this,

>     %module foo
>     namespace foo {
>        void bar(int);
>        void spam();
>     }

>     namespace bar {
>        void blah();
>     }

then SWIG simply creates three wrapper functions `bar()`, `spam()`, and `blah()` in the target language. SWIG does not prepend the names with a namespace prefix nor are the functions packaged in any kind of nested scope.

There is some rationale for taking this approach. Since C++ namespaces are often used to define modules in C++, there is a natural correlation between the likely contents of a SWIG module and the contents of a namespace. For instance, it would not be unreasonable to assume that a programmer might make a separate extension module for each C++ namespace. In this case, it would be redundant to prepend everything with an additional namespace prefix when the module itself already serves as a namespace in the target language. Or put another way, if you want SWIG to keep namespaces separate, simply wrap each namespace with its own SWIG interface.

Because namespaces are flattened, it is possible for symbols defined in different namespaces to generate a name conflict in the target language. For example:

>     namespace A {
>        void foo(int);
>     }
>     namespace B {
>        void foo(double);
>     }

When this conflict occurs, you will get an error message that resembles this:

>     example.i:26. Error. 'foo' is multiply defined in the generated module.
>     example.i:23. Previous declaration of 'foo'

To resolve this error, simply use `%rename` to disambiguate the declarations. For example:

>     %rename(B_foo) B::foo;
>     ...
>     namespace A {
>        void foo(int);
>     }
>     namespace B {
>        void foo(double);     // Gets renamed to B_foo
>     }

Similarly, `%ignore` can be used to ignore declarations.

`using` declarations do not have any effect on the generated wrapper code. They are ignored by SWIG language modules and they do not result in any code. However, these declarations are used by the internal type system to track type-names. Therefore, if you have code like this:

>     namespace A {
>        typedef int Integer;
>     }
>     using namespace A;
>     void foo(Integer x);

SWIG knows that `Integer` is the same as `A::Integer` which is the same as `int`.

Namespaces may be combined with templates. If necessary, the `%template` directive can be used to expand a template defined in a different namespace. For example:

>     namespace foo {
>         template<typename T> max(T a, T b) { return a > b ? a : b; }
>     }

>     using foo::max;

>     %template(maxint)   max<int>            // Okay.
>     %template(maxfloat) foo::max<float>;    // Okay (qualified name).

>     namespace bar {
>         using namespace foo;
>         %template(maxdouble)  max<double>;    // Okay.
>     }

The combination of namespaces and other SWIG directives may introduce subtle scope-related problems. The key thing to keep in mind is that all SWIG generated wrappers are produced in the global namespace. Symbols from other namespaces are always accessed using fully qualified names---names are never imported into the global space unless the interface happens to do so with a `using` declaration. In almost all cases, SWIG adjusts typenames and symbols to be fully qualified. However, this is not done in code fragments such as function bodies, typemaps, exception handlers, and so forth. For example, consider the following:

>     namespace foo {
>         typedef int Integer;
>         class bar {
>         public:
>            ...
>         };
>     }

>     %extend foo::bar {
>        Integer add(Integer x, Integer y) {
>            Integer r = x + y;        // Error. Integer not defined in this scope
>            return r;
>        }
>     };

In this case, SWIG correctly resolves the added method parameters and return type to `foo::Integer`. However, since function bodies aren't parsed and such code is emitted in the global namespace, this code produces a compiler error about `Integer`. To fix the problem, make sure you use fully qualified names. For example:

>     %extend foo::bar {
>        Integer add(Integer x, Integer y) {
>            foo::Integer r = x + y;        // Ok.
>            return r;
>        }
>     };

Note: SWIG does not propagate `using` declarations to the resulting wrapper code. If these declarations appear in an interface, they should also appear in any header files that might have been included in a `%{ ... %}` section. In other words, don't insert extra `using` declarations into a SWIG interface unless they also appear in the underlying C++ code.

Note: Code inclusion directives such as `%{ ... %}` or `%inline %{ ... %}` should not be placed inside a namespace declaration. The code emitted by these directives will not be enclosed in a namespace and you may get very strange results. If you need to use namespaces with these directives, consider the following:

>     // Good version
>     %inline %{
>     namespace foo {
>          void bar(int) { ... }
>          ...
>     }
>     %}

>     // Bad version.  Emitted code not placed in namespace.
>     namespace foo {
>     %inline %{
>          void bar(int) { ... }   /* I'm bad */
>          ...
>     %}
>     }

Note: When the `%extend` directive is used inside a namespace, the namespace name is included in the generated functions. For example, if you have code like this,

>     namespace foo {
>        class bar {
>        public:
>             %extend {
>                int blah(int x);
>             };
>        };
>     }

the added method `blah()` is mapped to a function `int foo_bar_blah(foo::bar *self, int x)`. This function resides in the global namespace.

Note: Although namespaces are flattened in the target language, the SWIG generated wrapper code observes the same namespace conventions as used in the input file. Thus, if there are no symbol conflicts in the input, there will be no conflicts in the generated code.

Note: Namespaces have a subtle effect on the wrapping of conversion operators. For instance, suppose you had an interface like this:

>     namespace foo {
>        class bar;
>        class spam {
>        public;
>             ...
>             operator bar();      // Conversion of spam -> bar
>             ...
>        };
>     }

To wrap the conversion function, you might be inclined to write this:

>     %rename(tofoo) foo::spam::operator bar();

The only problem is that it doesn't work. The reason it doesn't work is that `bar` is not defined in the global scope. Therefore, to make it work, do this instead:

>     %rename(tofoo) foo::spam::operator foo::bar();

Note: The flattening of namespaces is only intended to serve as a basic namespace implementation. Since namespaces are a new addition to SWIG, none of the target language modules are currently programmed with any namespace awareness. In the future, language modules may or may not provide more advanced namespace support.

## 5.19 Exception specifiers

When C++ programs utilize exceptions, exceptional behavior is sometimes specified as part of a function or method declaration. For example:

>     class Error { };

>     class Foo {
>     public:
>         ...
>         void blah() throw(Error);
>         ...
>     };

If an exception specification is used, SWIG automatically generates wrapper code for catching the indicated exception and converting it into an error in the target language. In certain language modules, wrapped exception classes themselves can be used to catch errors. For example, in Python, you can write code like this:

>     f = Foo()
>     try:
>         f.blah()
>     except Error,e:
>          # e is a wrapped instance of "Error"

Obviously, the exact details of how exceptions are handled depend on the target language module.

Since exception specifiers are sometimes only used sparingly, this alone may not be enough to properly handle C++ exceptions. To do that, a different set of special SWIG directives are used. Consult the "Customization features" chapter for details.

## 5.20 Pointers to Members

Starting with SWIG1.3.7, there is limited parsing support for pointers to C++ class members. For example:

>     double do_op(Object *o, double (Object::*callback)(double,double));
>     extern double (Object::*fooptr)(double,double);
>     %constant double (Object::*FOO)(double,double) = &Object::foo;

Although these kinds of pointers can be parsed and represented by the SWIG type system, few language modules know how to handle them due to implementation differences from standard C pointers. Readers are strongly advised to consult an advanced text such as the "The Annotated C++ Manual" for specific details.

When pointers to members are supported, the pointer value might appear as a special string like this:

>     >>> print example.FOO
>     _ff0d54a800000000_m_Object__f_double_double__double
>     >>>

In this case, the hexadecimal digits represent the entire value of the pointer which is usually the contents of a small C++ structure on most machines.

SWIG's type-checking mechanism is also more limited when working with member pointers. Normally SWIG tries to keep track of inheritance when checking types. However, no such support is currently provided for member pointers.

## 5.21 Smart pointers and operator->()

In some C++ programs, objects are often encapsulated by smart-pointers or proxy classes. This is sometimes done to implement automatic memory management (reference counting) or persistence. Typically a smart-pointer is defined by a template class where the `->` operator has been overloaded. This class is then wrapped around some other class. For example:

>     // Smart-pointer class
>     template<class T> class SmartPtr {
>         T *pointee;
>     public:
>         ...
>         T *operator->() {
>             return pointee;
>         }
>         ...
>     };

>     // Ordinary class
>     class Foo_Impl {
>     public:
>         int x;
>         virtual void bar();
>         ...
>     };

>     // Smart-pointer wrapper
>     typedef SmartPtr<Foo_Impl> Foo;

>     // Create smart pointer Foo
>     Foo make_Foo() {
>         return SmartPtr(new Foo_Impl());
>     }

>     // Do something with smart pointer Foo
>     void do_something(Foo f) {
>         printf("x = %d\n", f->x);
>         f->bar();
>     }

A key feature of this approach is that by defining `operator->` the methods and attributes of the object wrapped by a smart pointer are transparently accessible. For example, expressions such as these (from the previous example),

>     f->x
>     f->bar()

are transparently mapped to the following

>     (f.operator->())->x;
>     (f.operator->())->bar();

When generating wrappers, SWIG tries to emulate this functionality to the extent that it is possible. To do this, whenever `operator->()` is encountered in a class, SWIG looks at its returned type and uses it to generate wrappers for accessing attributes of the underlying object. For example, wrapping the above code produces wrappers like this:

>     int Foo_x_get(Foo *f) {
>        return (*f)->x;
>     }
>     void Foo_x_set(Foo *f, int value) {
>        (*f)->x = value;
>     }
>     void Foo_bar(Foo *f) {
>        (*f)->bar();
>     }

These wrappers take a smart-pointer instance as an argument, but dereference it in a way to gain access to the object returned by `operator->()`. You should carefully compare these wrappers to those in the first part of this chapter (they are slightly different).

The end result is that access looks very similar to C++. For example, you could do this in Python:

>     >>> f = make_Foo()
>     >>> print f.x
>     0
>     >>> f.bar()
>     >>>

When generating wrappers through a smart-pointer, SWIG tries to generate wrappers for all methods and attributes that might be accessible through `operator->()`. This includes any methods that might be accessible through inheritance. However, there are a number of restrictions:

- Only member variables and methods are wrapped through a smart pointer. Static members, enumerations, constructors, and destructors are not wrapped.

- If the smart-pointer class and the underlying object both define a method or variable of the same name, then the smart-pointer version has precedence. For example, if you have this code

  >     class Foo {
  >     public:
  >         int x;
  >     };

  >     class Bar {
  >     public:
  >         int x;
  >         Foo *operator->();
  >     };

  then the wrapper for `Bar::x` accesses the `x` defined in `Bar`, and not the `x` defined in `Foo`.

If your intent is to only expose the smart-pointer class in the interface, it is not necessary to wrap both the smart-pointer class and the class for the underlying object. However, you must still tell SWIG about both classes if you want the technique described in this section to work. To only generate wrappers for the smart-pointer class, you can use the %ignore directive. For example:

>     %ignore Foo;
>     class Foo {       // Ignored
>     };

>     class Bar {
>     public:
>        Foo *operator->();
>        ...
>     };

Alternatively, you can import the definition of `Foo` from a separate file using `%import`.

Note: When a class defines `operator->()`, the operator itself is wrapped as a method `__deref__()`. For example:

>     f = Foo()               # Smart-pointer
>     p = f.__deref__()       # Raw pointer from operator->

Note: To disable the smart-pointer behavior, use `%ignore` to ignore `operator->()`. For example:

>     %ignore Bar::operator->;

Note: Smart pointer support was first added in SWIG-1.3.14.

## 5.22 Using declarations and inheritance

`using` declarations are sometimes used to adjust access to members of base classes. For example:

>     class Foo {
>     public:
>           int  blah(int x);
>     };

>     class Bar {
>     public:
>           double blah(double x);
>     };

>     class FooBar : public Foo, public Bar {
>     public:
>           using Foo::blah;
>           using Bar::blah;
>           char *blah(const char *x);
>     };

In this example, the `using` declarations make different versions of the overloaded `blah()` method accessible from the derived class. For example:

>     FooBar *f;
>     f->blah(3);         // Ok. Invokes Foo::blah(int)
>     f->blah(3.5);       // Ok. Invokes Bar::blah(double)
>     f->blah("hello");   // Ok. Invokes FooBar::blah(const char *);

SWIG emulates the same functionality when creating wrappers. For example, if you wrap this code in Python, the module works just like you would expect:

>     >>> import example
>     >>> f = example.FooBar()
>     >>> f.blah(3)
>     >>> f.blah(3.5)
>     >>> f.blah("hello")

`using` declarations can also be used to change access when applicable. For example:

>     class Foo {
>     protected:
>         int x;
>         int blah(int x);
>     };

>     class Bar : public Foo {
>     public:
>         using Foo::x;       // Make x public
>         using Foo::blah;    // Make blah public
>     };

This also works in SWIG---the exposed declarations will be wrapped normally.

When `using` declarations are used as shown in these examples, declarations from the base classes are copied into the derived class and wrapped normally. When copied, the declarations retain any properties that might have been attached using `%rename`, `%ignore`, or `%feature`. Thus, if a method is ignored in a base class, it will also be ignored by a `using` declaration.

Because a `using` declaration does not provide fine-grained control over the declarations that get imported, it may be difficult to manage such declarations in applications that make heavy use of SWIG customization features. If you can't get `using` to work correctly, you can always change the interface to the following:

>     class FooBar : public Foo, public Bar {
>     public:
>     #ifndef SWIG
>           using Foo::blah;
>           using Bar::blah;
>     #else
>           int blah(int x);         // explicitly tell SWIG about other declarations
>           double blah(double x);
>     #endif

>           char *blah(const char *x);
>     };

Notes:

- If a derived class redefines a method defined in a base class, then a `using` declaration won't cause a conflict. For example:

  >     class Foo {
  >     public:
  >            int blah(int );
  >            double blah(double);
  >     };

  >     class Bar : public Foo {
  >     public:
  >            using Foo::blah;    // Only imports blah(double);
  >            int blah(int);
  >     };

- Resolving ambiguity in overloading may prevent declarations from being imported by `using`. For example:

  >     %rename(blah_long) Foo::blah(long);
  >     class Foo {
  >     public:
  >          int blah(int);
  >          long blah(long);  // Renamed to blah_long
  >     };

  >     class Bar : public Foo {
  >     public:
  >          using Foo::blah;     // Only imports blah(int)
  >          double blah(double x);
  >     };

## 5.23 Partial class definitions

Since SWIG is still limited in its support of C++, it may be necessary to use partial class information in an interface file. However, since SWIG does not need the entire class specification to work, conditional compilation can be used to comment out problematic parts. For example, if you had a nested class definition, you might do this:

>     class Foo {
>     public:
>     #ifndef SWIG
>        class Bar {
>        public:
>          ...
>        };
>     #endif
>        Foo();
>       ~Foo();
>        ...
>     };

Also, as a rule of thumb, SWIG should not be used on raw C++ source files.

## 5.24 A brief rant about const-correctness

A common issue when working with C++ programs is dealing with all possible ways in which the `const` qualifier (or lack thereof) will break your program, all programs linked against your program, and all programs linked against those programs.

Although SWIG knows how to correctly deal with `const` in its internal type system and it knows how to generate wrappers that are free of const-related warnings, SWIG does not make any attempt to preserve const-correctness in the target language. Thus, it is possible to pass `const` qualified objects to non-const methods and functions. For example, consider the following code in C++:

>     const Object * foo();
>     void bar(Object *);

>     ...
>     // C++ code
>     void blah() {
>        bar(foo());         // Error: bar discards const
>     };

Now, consider the behavior when wrapped into a Python module:

>     >>> bar(foo())         # Okay
>     >>>

Although this is clearly a violation of the C++ type-system, fixing the problem doesn't seem to be worth the added implementation complexity that would be required to support it in the SWIG run-time type system. There are no plans to change this in future releases (although we'll never rule anything out entirely).

The bottom line is that this particular issue does not appear to be a problem for most SWIG projects. Of course, you might want to consider using another tool if maintaining constness is the most important part of your project.

## 5.25 Proxy classes

In order to provide a more natural API, many of SWIG's target languages also wrap C++ classes with special proxy classes. These proxy classes are typically implemented in the target language itself. For example, if you're building a Python module, each C++ class is wrapped with Python class. Or if you're building a Java module, each C++ class is wrapped by a Java class.

### 5.25.1 Construction of proxy classes

Proxy classes are always constructed as an extra layer of wrapping that uses the low-level accessor functions described in the previous section. To illustrate, suppose you had a C++ class like this:

>     class Foo {
>     public:
>           Foo();
>          ~Foo();
>           int  bar(int x);
>           int  x;
>     };

Using C++ as pseudocode, a proxy class looks something like this:

>     class FooProxy {
>     private:
>           Foo    *self;
>     public:
>           FooProxy() {
>                 self = new_Foo();
>           }
>          ~FooProxy() {
>                 delete_Foo(self);
>           }
>           int bar(int x) {
>                 return Foo_bar(self,x);
>           }
>           int x_get() {
>                 return Foo_x_get(self);
>           }
>           void x_set(int x) {
>                 Foo_x_set(self,x);
>           }
>     };

Of course, always keep in mind that the real proxy class is written in the target language. For example, in Python, the proxy might look roughly like this:

>     class Foo:
>         def __init__(self):
>              self.this = new_Foo()
>         def __del__(self):
>              delete_Foo(self.this)
>         def bar(self,x):
>              return Foo_bar(self.this,x)
>         def __getattr__(self,name):
>              if name == 'x':
>                   return Foo_x_get(self.this)
>              ...
>         def __setattr__(self,name,value):
>              if name == 'x':
>                   Foo_x_set(self.this,value)
>              ...

Again, it's important to emphasize that the low-level accessor functions are always used to construct the proxy classes.

Whenever possible, proxies try to take advantage of language features that are similar to C++. This might include operator overloading, exception handling, and other features.

### 5.25.2 Resource management in proxies

A major issue with proxies concerns the memory management of wrapped objects. Consider the following C++ code:

>     class Foo {
>     public:
>           Foo();
>          ~Foo();
>           int bar(int x);
>           int x;
>     };

>     class Spam {
>     public:
>           Foo *value;
>           ...
>     };

Now, consider some script code that uses these classes:

>     f = Foo()               # Creates a new Foo
>     s = Spam()              # Creates a new Spam
>     s.value = f             # Stores a reference to f inside s
>     g = s.value             # Returns stored reference
>     g = 4                   # Reassign g to some other value
>     del f                   # Destroy f

Now, ponder the resulting memory management issues. When objects are created in the script, the objects are wrapped by newly created proxy classes. That is, there is both a new proxy class instance and a new instance of the underlying C++ class. In this example, both `f` and `s` are created in this way. However, the statement `s.value` is rather curious---when executed, a pointer to `f` is stored inside another object. This means that the scripting proxy class AND another C++ class share a reference to the same object. To make matters even more interesting, consider the statement `g = s.value`. When executed, this creates a new proxy class `g` that provides a wrapper around the C++ object stored in `s.value`. In general, there is no way to know where this object came from---it could have been created by the script, but it could also have been generated internally. In this particular example, the assignment of `g` results in a second proxy class for `f`. In other words, a reference to `f` is now shared by two proxy classes and a C++ class.

Finally, consider what happens when objects are destroyed. In the statement, `g=4`, the variable `g` is reassigned. In many languages, this makes the old value of `g` available for garbage collection. Therefore, this causes one of the proxy classes to be destroyed. Later on, the statement `del f` destroys the other proxy class. Of course, there is still a reference to the original object stored inside another C++ object. What happens to it? Is it the object still valid?

To deal with memory management problems, proxy classes always provide an API for controlling ownership. In C++ pseudocode, ownership control might look roughly like this:

>     class FooProxy {
>     public:
>           Foo    *self;
>           int     thisown;

>           FooProxy() {
>                 self = new_Foo();
>                 thisown = 1;       // Newly created object
>           }
>          ~FooProxy() {
>                 if (thisown) delete_Foo(self);
>           }
>           ...
>           // Ownership control API
>           void disown() {
>                thisown = 0;
>           }
>           void acquire() {
>                thisown = 1;
>           }
>     };

>     class FooPtrProxy: public FooProxy {
>     public:
>           FooPtrProxy(Foo *s) {
>               self = s;
>               thisown = 0;
>           }
>     };

>     class SpamProxy {
>          ...
>          FooProxy *value_get() {
>               return FooPtrProxy(Spam_value_get(self));
>          }
>          void value_set(FooProxy *v) {
>               Spam_value_set(self,v->self);
>               v->disown();
>          }
>          ...
>     };

Looking at this code, there are a few central features:

- Each proxy class keeps an extra flag to indicate ownership. C++ objects are only destroyed if the ownership flag is set.

- When new objects are created in the target language, the ownership flag is set.

- When a reference to an internal C++ object is returned, it is wrapped by a proxy class, but the proxy class does not have ownership.

- In certain cases, ownership is adjusted. For instance, when a value is assigned to the member of a class, ownership is lost.

- Manual ownership control is provided by special `disown()` and `acquire()` methods.

Given the tricky nature of C++ memory management, it is impossible for proxy classes to automatically handle every possible memory management problem. However, proxies do provide a mechanism for manual control that can be used (if necessary) to address some of the more tricky memory management problems.

### 5.25.3 Language specific details

Language specific details on proxy classes are contained the chapters describing each target language. This chapter has merely introduced the topic in a very general way.

## 5.26 Where to go for more information

If you're wrapping serious C++ code, you might want to pick up a copy of "The Annotated C++ Reference Manual" by Ellis and Stroustrup. This is the reference document we use to guide a lot of SWIG's C++ support.