SWIG:Examples:python:class

SWIG/Examples/python/class/

Wrapping a simple C++ class

This example illustrates the most primitive form of C++ class wrapping performed by SWIG. In this case, C++ classes are simply transformed into a collection of C-style functions that provide access to class members.

The C++ Code

Suppose you have some C++ classes described by the following (and admittedly lame) header file:

/* File : example.h */

class Shape {
public:
  Shape() {
    nshapes++;
  }
  virtual ~Shape() {
    nshapes--;
  };
  double  x, y;   
  void    move(double dx, double dy);
  virtual double area() = 0;
  virtual double perimeter() = 0;
  static  int nshapes;
};

class Circle : public Shape {
private:
  double radius;
public:
  Circle(double r) : radius(r) { };
  virtual double area();
  virtual double perimeter();
};

class Square : public Shape {
private:
  double width;
public:
  Square(double w) : width(w) { };
  virtual double area();
  virtual double perimeter();
};

The SWIG interface

A simple SWIG interface for this can be built by simply grabbing the header file like this:

/* File : example.i */
%module example

%{
#include "example.h"
%}

/* Let's just grab the original header file here */
%include "example.h"

Note: when creating a C++ extension, you must run SWIG with the -c++ option like this:

% swig -c++ -python example.i

A sample Python script

Click here to see a script that calls the C++ functions from Python.

Key points

To create a new object, you call a constructor like this:
```
c = example.new_Circle(10.0)
```
To access member data, a pair of accessor functions are used. For example:
```
example.Shape_x_set(c,15)    # Set member data
x = example.Shape_x_get(c)    # Get member data
```
Note: when accessing member data, the name of the class in which the member data was must be used. In this case, Shape_x_get() and Shape_x_set() are used since 'x' was defined in Shape.
To invoke a member function, you simply do this
```
print "The area is ", example.Shape_area(c)
```

Type checking knows about the inheritance structure of C++. For example:

example.Shape_area(c)       # Works (c is a Shape)
example.Circle_area(c)      # Works (c is a Circle)
example.Square_area(c)      # Fails (c is definitely not a Square)

To invoke a destructor, simply do this
```
example.delete_Shape(c)     # Deletes a shape
```
(Note: destructors are currently not inherited. This might change later).

Static member variables are wrapped as C global variables. For example:

n = example.cvar.Shape_nshapes     # Get a static data member
example.cvar.Shapes_nshapes = 13   # Set a static data member

General Comments

This low-level interface is not the only way to handle C++ code. Proxy classes provide a much higher-level interface.
SWIG *does* know how to properly perform upcasting of objects in an inheritance hierarchy (including multiple inheritance). Therefore it is perfectly safe to pass an object of a derived class to any function involving a base class.
A wide variety of C++ features are not currently supported by SWIG. Here is the short and incomplete list:
- Overloaded methods and functions. SWIG wrappers don't know how to resolve name conflicts so you must give an alternative name to any overloaded method name using the %name directive like this:
```
void foo(int a);  
%name(foo2) void foo(double a, double b);
```
- Overloaded operators. Not supported at all. The only workaround for this is to write a helper function. For example:
```
%inline %{
    Vector *vector_add(Vector *a, Vector *b) {
          ... whatever ...
    }
%}
```
- Namespaces. Not supported at all. Won't be supported until SWIG2.0 (if at all).
- Dave's snide remark: Like a large bottle of strong Tequilla, it's better to use C++ in moderation.