mirror of
https://github.com/boostorg/python.git
synced 2026-01-19 16:32:16 +00:00
103 lines
7.6 KiB
HTML
103 lines
7.6 KiB
HTML
<html>
|
|
<head>
|
|
<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
|
|
<title>Constructors</title>
|
|
<link rel="stylesheet" href="theme/style.css" type="text/css">
|
|
<link rel="prev" href="exposing_classes.html">
|
|
<link rel="next" href="class_data_members.html">
|
|
</head>
|
|
<body>
|
|
<table width="100%" height="48" border="0" cellspacing="2">
|
|
<tr>
|
|
<td><img src="theme/c%2B%2Bboost.gif">
|
|
</td>
|
|
<td width="85%">
|
|
<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Constructors</b></font>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
<br>
|
|
<table border="0">
|
|
<tr>
|
|
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
|
|
<td width="30"><a href="exposing_classes.html"><img src="theme/l_arr.gif" border="0"></a></td>
|
|
<td width="20"><a href="class_data_members.html"><img src="theme/r_arr.gif" border="0"></a></td>
|
|
</tr>
|
|
</table>
|
|
<p>
|
|
Our previous example didn't have any explicit constructors.
|
|
Since <tt>World</tt> is declared as a plain struct, it has an implicit default
|
|
constructor. Boost.Python exposes the default constructor by default,
|
|
which is why we were able to write</p>
|
|
<code><pre>
|
|
<span class=special>>>> </span><span class=identifier>planet </span><span class=special>= </span><span class=identifier>hello</span><span class=special>.</span><span class=identifier>World</span><span class=special>()
|
|
</span></pre></code>
|
|
<p>
|
|
We may wish to wrap a class with a non-default constructor. Let us
|
|
build on our previous example:</p>
|
|
<code><pre>
|
|
<span class=keyword>struct </span><span class=identifier>World
|
|
</span><span class=special>{
|
|
</span><span class=identifier>World</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>): </span><span class=identifier>msg</span><span class=special>(</span><span class=identifier>msg</span><span class=special>) {} // </span><span class=identifier>added </span><span class=identifier>constructor
|
|
</span><span class=keyword>void </span><span class=identifier>set</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>) { </span><span class=keyword>this</span><span class=special>-></span><span class=identifier>msg </span><span class=special>= </span><span class=identifier>msg</span><span class=special>; }
|
|
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>greet</span><span class=special>() { </span><span class=keyword>return </span><span class=identifier>msg</span><span class=special>; }
|
|
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>;
|
|
};
|
|
</span></pre></code>
|
|
<p>
|
|
This time <tt>World</tt> has no default constructor; our previous
|
|
wrapping code would fail to compile when the library tried to expose
|
|
it. We have to tell <tt>class_<World></tt> about the constructor we want to
|
|
expose instead.</p>
|
|
<code><pre>
|
|
<span class=preprocessor>#include </span><span class=special><</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>python</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>>
|
|
</span><span class=keyword>using </span><span class=keyword>namespace </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>python</span><span class=special>;
|
|
|
|
</span><span class=identifier>BOOST_PYTHON_MODULE</span><span class=special>(</span><span class=identifier>hello</span><span class=special>)
|
|
{
|
|
</span><span class=identifier>class_</span><span class=special><</span><span class=identifier>World</span><span class=special>>(</span><span class=string>"World"</span><span class=special>, </span><span class=identifier>init</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>>())
|
|
.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"greet"</span><span class=special>, &</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>greet</span><span class=special>)
|
|
.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"set"</span><span class=special>, &</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
|
|
;
|
|
}
|
|
</span></pre></code>
|
|
<p>
|
|
<tt>init<std::string>()</tt> exposes the constructor taking in a
|
|
<tt>std::string</tt> (in Python, constructors are spelled
|
|
"<tt>"__init__"</tt>").</p>
|
|
<p>
|
|
We can expose additional constructors by passing more <tt>init<...></tt>s to
|
|
the <tt>def()</tt> member function. Say for example we have another World
|
|
constructor taking in two doubles:</p>
|
|
<code><pre>
|
|
<span class=identifier>class_</span><span class=special><</span><span class=identifier>World</span><span class=special>>(</span><span class=string>"World"</span><span class=special>, </span><span class=identifier>init</span><span class=special><</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>>())
|
|
.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>init</span><span class=special><</span><span class=keyword>double</span><span class=special>, </span><span class=keyword>double</span><span class=special>>())
|
|
.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"greet"</span><span class=special>, &</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>greet</span><span class=special>)
|
|
.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"set"</span><span class=special>, &</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
|
|
;
|
|
</span></pre></code>
|
|
<p>
|
|
On the other hand, if we do not wish to expose any constructors at
|
|
all, we may use <tt>no_init</tt> instead:</p>
|
|
<code><pre>
|
|
<span class=identifier>class_</span><span class=special><</span><span class=identifier>Abstract</span><span class=special>>(</span><span class=string>"Abstract"</span><span class=special>, </span><span class=identifier>no_init</span><span class=special>)
|
|
</span></pre></code>
|
|
<p>
|
|
This actually adds an <tt>__init__</tt> method which always raises a
|
|
Python RuntimeError exception.</p>
|
|
<table border="0">
|
|
<tr>
|
|
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
|
|
<td width="30"><a href="exposing_classes.html"><img src="theme/l_arr.gif" border="0"></a></td>
|
|
<td width="20"><a href="class_data_members.html"><img src="theme/r_arr.gif" border="0"></a></td>
|
|
</tr>
|
|
</table>
|
|
<br>
|
|
<hr size="1"><p class="copyright">Copyright © 2002-2003 David Abrahams<br>Copyright © 2002-2003 Joel de Guzman<br><br>
|
|
<font size="2">Permission to copy, use, modify, sell and distribute this document
|
|
is granted provided this copyright notice appears in all copies. This document
|
|
is provided "as is" without express or implied warranty, and with
|
|
no claim as to its suitability for any purpose. </font> </p>
|
|
</body>
|
|
</html>
|