Add support for the Go programming language.
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Ian Lance Taylor
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Examples/go/class/index.html
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Examples/go/class/index.html
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<html>
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<head>
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<title>SWIG:Examples:go:class</title>
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</head>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/go/class/</tt>
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<hr>
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<H2>Wrapping a simple C++ class</H2>
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<p>
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This example illustrates the most primitive form of C++ class wrapping
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performed by SWIG. In this case, C++ classes are simply transformed
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into a collection of C-style functions that provide access to class
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members.
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<h2>The C++ Code</h2>
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Suppose you have some C++ classes described by the following (and
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admittedly lame) header file:
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<blockquote>
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<pre>
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/* File : example.h */
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class Shape {
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public:
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Shape() {
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nshapes++;
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}
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virtual ~Shape() {
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nshapes--;
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};
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double x, y;
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void move(double dx, double dy);
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virtual double area() = 0;
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virtual double perimeter() = 0;
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static int nshapes;
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};
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class Circle : public Shape {
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private:
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double radius;
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public:
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Circle(double r) : radius(r) { };
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virtual double area();
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virtual double perimeter();
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};
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class Square : public Shape {
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private:
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double width;
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public:
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Square(double w) : width(w) { };
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virtual double area();
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virtual double perimeter();
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};
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</pre>
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</blockquote>
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<h2>The SWIG interface</h2>
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A simple SWIG interface for this can be built by simply grabbing the
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header file like this:
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<blockquote>
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<pre>
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/* File : example.i */
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%module example
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%{
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#include "example.h"
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%}
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/* Let's just grab the original header file here */
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%include "example.h"
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</pre>
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</blockquote>
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Note: when creating a C++ extension, you must run SWIG with
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the <tt>-c++</tt> option like this:
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<blockquote>
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<pre>
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% swig -c++ -go example.i
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</pre>
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</blockquote>
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<h2>A sample Go script</h2>
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See <a href="example.go">example.go</a> for a program that calls the
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C++ functions from Go.
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<h2>Key points</h2>
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<ul>
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<li>To create a new object, you call a constructor like this:
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<blockquote>
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<pre>
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c := example.NewCircle(10.0)
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</pre>
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</blockquote>
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The name of the constructor is <tt>New</tt> followed by the name of
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the class, capitalized.
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<p>
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<li>
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The constructor returns a value of interface type. The methods of the
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interface will be the methods of the C++ class, plus member accessor
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functions.
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<p>
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<li>To access member data, a pair of accessor methods are used. For
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example:
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<blockquote>
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<pre>
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c.SetX(15) # Set member data
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x := c.GetX() # Get member data.
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</pre>
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</blockquote>
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These are methods on the type returned by the constructor. The getter
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is named <tt>Get</tt> followed by the name of the member,
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capitalized. The setter is similar but uses <tt>Set</tt>.
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<p>
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<li>To invoke a member function, you simply do this
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<blockquote>
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<pre>
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fmt.Println("The area is", example.c.Area())
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</pre>
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</blockquote>
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<li>To invoke a destructor, simply do this
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<blockquote>
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<pre>
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example.DeleteShape(c) # Deletes a shape
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</pre>
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</blockquote>
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The name of the destructor is <tt>Delete</tt> followed by the name of
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the class, capitalized. (Note: destructors are currently not
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inherited. This might change later).
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<p>
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<li>Static member variables are wrapped much like C global variables.
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For example:
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<blockquote>
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<pre>
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n := GetShapeNshapes() # Get a static data member
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SetShapeNshapes(13) # Set a static data member
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</pre>
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</blockquote>
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The name is <tt>Get</tt> or <tt>Set</tt>, followed by the name of the
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class, capitalized, followed by the name of the member, capitalized.
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</ul>
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<h2>General Comments</h2>
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<ul>
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<li>This low-level interface is not the only way to handle C++ code.
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Director classes provide a much higher-level interface.
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<p>
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<li>Because C++ and Go implement inheritance quite differently, you
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can not simply upcast an object in Go code when using multiple
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inheritance. When using only single inheritance, you can simply pass
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a class to a function expecting a parent class. When using multiple
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inheritance, you have to call an automatically generated getter
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function named <tt>Get</tt> followed by the capitalized name of the
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immediate parent. This will return the same object converted to the
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parent class.
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<p>
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<li>
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Overloaded methods should normally work. However, when calling an
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overloaded method you must explicitly convert constants to the
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expected type when it is not <tt>int</tt> or <tt>float</tt>. In
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particular, a floating point constant will default to
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type <tt>float</tt>, but C++ functions typically expect the C++
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type <tt>double</tt> which is equivalent to the Go
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type <tt>float64</tt> So calling an overloaded method with a floating
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point constant typically requires an explicit conversion
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to <tt>float64</tt>.
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<p>
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<li>Namespaces are not supported in any very coherent way.
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</ul>
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<hr>
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</body>
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</html>
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