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<html>
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<title>Constructors</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Constructors</b></font>
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<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=special>{} </span><span class=comment>// added 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=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=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=special>{ </span><span class=keyword>return </span><span class=identifier>msg</span><span class=special>; </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=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=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=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"greet"</span><span class=special>, </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=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"set"</span><span class=special>, </span><span class=special>&</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
</span><span class=special>;
</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=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=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"greet"</span><span class=special>, </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=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"set"</span><span class=special>, </span><span class=special>&</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
</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>
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<hr size="1"><p class="copyright">Copyright © 2002 David Abrahams<br>Copyright © 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
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