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SWIG/Examples/tcl/class/
Wrapping a simple C++ class
$Header: /cvs/projects/SWIG/Examples/tcl/class/index.html,v 1.1.4.1 2001/08/30 04:18:46 beazley Exp $
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++ -tcl example.i
Some sample Tcl scripts
SWIG performs two forms of C++ wrapping-- a low level interface and a high level widget-like interface.
-
Click here to see a script that calls the C++ functions using the
low-level interface.
-
Click here to see a the same script written with the high-level
interface.
Key points
- The low-level C++ interface works like this:
- To create a new object, you call a constructor like this:
set c [new_Circle 10.0]
- To access member data, a pair of accessor functions are used.
For example:
Shape_x_set $c 15 ;# Set member data
set x [Shape_x_get $c] ;# Get member data
Note: when accessing member data, the name of the base class must
be used such as Shape_x_get
- To invoke a member function, you simply do this
puts "The area is [Shape_area $c]"
- Type checking knows about the inheritance structure of C++. For example:
Shape_area $c # Works (c is a Shape)
Circle_area $c # Works (c is a Circle)
Square_area $c # Fails (c is definitely not a Square)
- To invoke a destructor, simply do this
delete_Shape $c # Deletes a shape
- Static member variables are wrapped as C global variables. For example:
set n $Shape_nshapes # Get a static data member
set Shapes_nshapes 13 # Set a static data member
- The high-level interface works like a Tk widget
- To create a new object, you call a constructor like this:
Circle c 10 # c becomes a name for the Circle object
- To access member data, use cget and configure methods.
For example:
c configure -x 15 ;# Set member data
set x [c cget -x] ;# Get member data
- To invoke a member function, you simply do this
puts "The area is [c area]"
- To invoke a destructor, simply destroy the object name like this:
rename c "" # c goes away
- Static member variables are wrapped as C global variables. For example:
set n $Shape_nshapes # Get a static data member
set Shapes_nshapes 13 # Set a static data member
General Comments
- The low-level function interface is much faster than the high-level interface.
In fact, all the higher level interface does is call functions in the low-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).
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