SWIG:Examples:go:class

`SWIG/Examples/go/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++ -go example.i

## A sample Go script

See example.go for a program that calls the C++ functions from Go.

## Key points

- To create a new object, you call a constructor like this:

  >     c := example.NewCircle(10.0)

  The name of the constructor is `New` followed by the name of the class, capitalized.

- The constructor returns a value of interface type. The methods of the interface will be the methods of the C++ class, plus member accessor functions.

- To access member data, a pair of accessor methods are used. For example:

  >     c.SetX(15)          # Set member data
  >     x := c.GetX()       # Get member data.

  These are methods on the type returned by the constructor. The getter is named `Get` followed by the name of the member, capitalized. The setter is similar but uses `Set`.

- To invoke a member function, you simply do this

  >     fmt.Println("The area is", example.c.Area())

- To invoke a destructor, simply do this

  >     example.DeleteShape(c)     # Deletes a shape

  The name of the destructor is `Delete` followed by the name of the class, capitalized. (Note: destructors are currently not inherited. This might change later).

- Static member variables are wrapped much like C global variables. For example:

  >     n := GetShapeNshapes()     # Get a static data member
  >     SetShapeNshapes(13)        # Set a static data member

  The name is `Get` or `Set`, followed by the name of the class, capitalized, followed by the name of the member, capitalized.

## General Comments

- This low-level interface is not the only way to handle C++ code. Director classes provide a much higher-level interface.

- Because C++ and Go implement inheritance quite differently, you can not simply upcast an object in Go code when using multiple inheritance. When using only single inheritance, you can simply pass a class to a function expecting a parent class. When using multiple inheritance, you have to call an automatically generated getter function named `Get` followed by the capitalized name of the immediate parent. This will return the same object converted to the parent class.

- Overloaded methods should normally work. However, when calling an overloaded method you must explicitly convert constants to the expected type when it is not `int` or `float`. In particular, a floating point constant will default to type `float`, but C++ functions typically expect the C++ type `double` which is equivalent to the Go type `float64` So calling an overloaded method with a floating point constant typically requires an explicit conversion to `float64`.

- Namespaces are not supported in any very coherent way.