#### /trunk/Examples/ruby/value/index.html

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```  1<html>
2<head>
3<title>SWIG:Examples:ruby:value</title>
4</head>
5
6<body bgcolor="#ffffff">
7
8
9<tt>SWIG/Examples/ruby/value/</tt>
10<hr>
11
12<H2>Passing and Returning Structures by Value</H2>
13
14<p>
15Occasionally, a C program will manipulate structures by value such as shown in the
16following code:
17
18<blockquote>
19<pre>
20/* File : example.c */
21
22typedef struct Vector {
23   double x, y, z;
24} Vector;
25
26double dot_product(Vector a, Vector b) {
27  return (a.x*b.x + a.y*b.y + a.z*b.z);
28}
29
30Vector vector_add(Vector a, Vector b) {
31  Vector r;
32  r.x = a.x + b.x;
33  r.y = a.y + b.y;
34  r.z = a.z + b.z;
35  return r;
36}
37</pre>
38</blockquote>
39
40Since SWIG only knows how to manage pointers to structures (not their internal
41representation), the following translations are made when wrappers are
42created:
43
44<blockquote>
45<pre>
46double wrap_dot_product(Vector *a, Vector *b) {
47    return dot_product(*a,*b);
48}
49
50Vector *wrap_vector_add(Vector *a, Vector *b) {
51    Vector *r = (Vector *) malloc(sizeof(Vector));
52    *r = vector_add(*a,*b);
53    return r;
54}
55</pre>
56</blockquote>
57
58The functions are then called using pointers from the scripting language interface.
59It should also be noted that any function that returns a structure by value results
60in an implicit memory allocation. This will be a memory leak unless you take steps
61to free the result (see below).
62
63<h2>The SWIG interface</h2>
64
65Click <a href="example.i">here</a> to see a SWIG interface file that
66wraps these two functions.  In this file, there are a few essential features:
67
68<ul>
69<li>A wrapper for the <tt>free()</tt> function is created so that we
70can clean up the return result created by <tt>vector_add()</tt>
71function.
72
73<p>
74<li>The %inline directive is used to create a few helper functions for creating new Vector
75objects and to print out the value (for debugging purposes).
76</ul>
77
78<h2>A Ruby Script</h2>
79
80Click <a href="runme.rb">here</a> to see a script that uses these functions from Ruby.
81
82<h2>Notes</h2>
83
84<ul>
85<li>When the '<tt>-c++</tt>' option is used, the resulting wrapper code for the return value
86changes to the following:
87
88<blockquote>
89<pre>
90Vector *wrap_vector_add(Vector *a, Vector *b) {
91    Vector *r = new Vector(vector_add(*a,*b));
92    return r;
93}
94</pre>
95</blockquote>
96
97<p>
98<li>If you define C structure (or C++ class with '<tt>-c++</tt>' option)
99in the interface file, the SWIG generated wrappers can automaticallyclean
100up the result of return-by-reference by GC.
101
102<p>
103<li>Passing parameters by value like this really isn't the best C programming style.
104If possible, you might change your application to use pointers.
105
106<p>
107<li>Similar translations are made when C++ references are used.
108
109
110</ul>
111
112<hr>
113</body>
114</html>
```