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1<html> 2<head> 3<title>SWIG:Examples:python:reference</title> 4</head> 5 6<body bgcolor="#ffffff"> 7 8 9<tt>SWIG/Examples/python/reference/</tt> 10<hr> 11 12<H2>C++ Reference Handling</H2> 13 14<p> 15This example tests SWIG's handling of C++ references. Since C++ 16references are closely related to pointers (as both refer to a 17location in memory), SWIG simply collapses all references into 18pointers when creating wrappers. 19 20<h2>Some examples</h2> 21 22References are most commonly used as function parameter. For example, 23you might have an operator like this: 24 25<blockquote> 26<pre> 27Vector operator+(const Vector &a, const Vector &b) { 28 Vector result; 29 result.x = a.x + b.x; 30 result.y = a.y + b.y; 31 result.z = a.z + b.z; 32 return result; 33} 34</pre> 35</blockquote> 36 37or a function: 38 39<blockquote> 40<pre> 41Vector addv(const Vector &a, const Vector &b) { 42 Vector result; 43 result.x = a.x + b.x; 44 result.y = a.y + b.y; 45 result.z = a.z + b.z; 46 return result; 47} 48</pre> 49</blockquote> 50 51In these cases, SWIG transforms everything into a pointer and creates a wrapper 52that looks like this: 53 54<blockquote> 55<pre> 56Vector wrap_addv(Vector *a, Vector *b) { 57 return addv(*a,*b); 58} 59</pre> 60</blockquote> 61 62Occasionally, a reference is used as a return value of a function 63when the return result is to be used as an lvalue in an expression. 64The prototypical example is an operator like this: 65 66<blockquote> 67<pre> 68Vector &operator[](int index); 69</pre> 70</blockquote> 71 72or a method: 73 74<blockquote> 75<pre> 76Vector &get(int index); 77</pre> 78</blockquote> 79 80For functions returning references, a wrapper like this is created: 81 82<blockquote> 83<pre> 84Vector *wrap_Object_get(Object *self, int index) { 85 Vector &result = self->get(index); 86 return &result; 87} 88</pre> 89</blockquote> 90 91The following <a href="example.h">header file</a> contains some class 92definitions with some operators and use of references. 93 94<h2>SWIG Interface</h2> 95 96SWIG does NOT support overloaded operators so it can not directly build 97an interface to the classes in the above file. However, a number of workarounds 98can be made. For example, an overloaded operator can be stuck behind a function 99call such as the <tt>addv()</tt> function above. Array access can be handled 100with a pair of set/get functions like this: 101 102<blockquote> 103<pre> 104class VectorArray { 105public: 106 ... 107 %addmethods { 108 Vector &get(int index) { 109 return (*self)[index]; 110 } 111 void set(int index, Vector &a) { 112 (*self)[index] = a; 113 } 114 } 115 ... 116} 117</pre> 118</blockquote> 119 120Click <a href="example.i">here</a> to see a SWIG interface file with these additions. 121 122<h2>Sample Python script</h2> 123 124Click <a href="example.py">here</a> to see a script that manipulates some C++ references. 125 126<h2>Notes:</h2> 127 128<ul> 129<li>C++ references primarily provide notational convenience for C++ 130source code. However, Python only supports the 'x.a' 131notation so it doesn't much matter. 132 133<p> 134<li>When a program returns a reference, a pointer is returned. 135Unlike return by value, memory is not allocated to hold the 136return result. 137 138<p> 139<li>SWIG has particular trouble handling various combinations of references 140and pointers. This is side effect of an old parsing scheme and 141type representation that will be replaced in future versions. 142 143</ul> 144 145<hr> 146</body> 147</html>