1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
|
<html>
<head>
<title>SWIG:Examples:perl5:reference</title>
</head>
<body bgcolor="#ffffff">
<tt>SWIG/Examples/perl5/reference/</tt>
<hr>
<H2>C++ Reference Handling</H2>
<tt>$Header$</tt><br>
<p>
This example tests SWIG's handling of C++ references. Since C++
references are closely related to pointers (as both refer to a
location in memory), SWIG simply collapses all references into
pointers when creating wrappers.
<h2>Some examples</h2>
References are most commonly used as function parameter. For example,
you might have an operator like this:
<blockquote>
<pre>
Vector operator+(const Vector &a, const Vector &b) {
Vector result;
result.x = a.x + b.x;
result.y = a.y + b.y;
result.z = a.z + b.z;
return result;
}
</pre>
</blockquote>
or a function:
<blockquote>
<pre>
Vector addv(const Vector &a, const Vector &b) {
Vector result;
result.x = a.x + b.x;
result.y = a.y + b.y;
result.z = a.z + b.z;
return result;
}
</pre>
</blockquote>
In these cases, SWIG transforms everything into a pointer and creates a wrapper
that looks like this:
<blockquote>
<pre>
Vector wrap_addv(Vector *a, Vector *b) {
return addv(*a,*b);
}
</pre>
</blockquote>
Occasionally, a reference is used as a return value of a function
when the return result is to be used as an lvalue in an expression.
The prototypical example is an operator like this:
<blockquote>
<pre>
Vector &operator[](int index);
</pre>
</blockquote>
or a method:
<blockquote>
<pre>
Vector &get(int index);
</pre>
</blockquote>
For functions returning references, a wrapper like this is created:
<blockquote>
<pre>
Vector *wrap_Object_get(Object *self, int index) {
Vector &result = self->get(index);
return &result;
}
</pre>
</blockquote>
The following <a href="example.h">header file</a> contains some class
definitions with some operators and use of references.
<h2>SWIG Interface</h2>
SWIG does NOT support overloaded operators so it can not directly build
an interface to the classes in the above file. However, a number of workarounds
can be made. For example, an overloaded operator can be stuck behind a function
call such as the <tt>addv()</tt> function above. Array access can be handled
with a pair of set/get functions like this:
<blockquote>
<pre>
class VectorArray {
public:
...
%addmethods {
Vector &get(int index) {
return (*self)[index];
}
void set(int index, Vector &a) {
(*self)[index] = a;
}
}
...
}
</pre>
</blockquote>
Click <a href="example.i">here</a> to see a SWIG interface file with these additions.
<h2>Sample Perl script</h2>
Click <a href="runme.pl">here</a> to see a script that manipulates some C++ references.
<h2>Notes:</h2>
<ul>
<li>C++ references primarily provide notational convenience for C++
source code. However, it doesn't much matter to Perl.
<p>
<li>When a program returns a reference, a pointer is returned.
Unlike return by value, memory is not allocated to hold the
return result.
<p>
<li>SWIG has particular trouble handling various combinations of references
and pointers. This is side effect of an old parsing scheme and
type representation that will be replaced in future versions.
</ul>
<hr>
</body>
</html>
|
