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<div class="section" id="object-oriented-code">
<span id="classes"></span><h1>Object-oriented code<a class="headerlink" href="#object-oriented-code" title="Permalink to this headline">¶</a></h1>
<div class="section" id="creating-bindings-for-a-custom-type">
<h2>Creating bindings for a custom type<a class="headerlink" href="#creating-bindings-for-a-custom-type" title="Permalink to this headline">¶</a></h2>
<p>Let’s now look at a more complex example where we’ll create bindings for a
custom C++ data structure named <code class="docutils literal"><span class="pre">Pet</span></code>. Its definition is given below:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">Pet</span> <span class="p">{</span>
<span class="n">Pet</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">)</span> <span class="o">:</span> <span class="n">name</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="kt">void</span> <span class="n">setName</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name_</span><span class="p">)</span> <span class="p">{</span> <span class="n">name</span> <span class="o">=</span> <span class="n">name_</span><span class="p">;</span> <span class="p">}</span>
<span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">getName</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">name</span><span class="p">;</span> <span class="p">}</span>
<span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">name</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The binding code for <code class="docutils literal"><span class="pre">Pet</span></code> looks as follows:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf"><pybind11/pybind11.h></span><span class="cp"></span>
<span class="k">namespace</span> <span class="n">py</span> <span class="o">=</span> <span class="n">pybind11</span><span class="p">;</span>
<span class="n">PYBIND11_PLUGIN</span><span class="p">(</span><span class="n">example</span><span class="p">)</span> <span class="p">{</span>
<span class="n">py</span><span class="o">::</span><span class="n">module</span> <span class="n">m</span><span class="p">(</span><span class="s">"example"</span><span class="p">,</span> <span class="s">"pybind11 example plugin"</span><span class="p">);</span>
<span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"setName"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">setName</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"getName"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">getName</span><span class="p">);</span>
<span class="k">return</span> <span class="n">m</span><span class="p">.</span><span class="n">ptr</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<p><code class="xref cpp cpp-class docutils literal"><span class="pre">class_</span></code> creates bindings for a C++ <cite>class</cite> or <cite>struct</cite>-style data
structure. <code class="xref cpp cpp-func docutils literal"><span class="pre">init()</span></code> is a convenience function that takes the types of a
constructor’s parameters as template arguments and wraps the corresponding
constructor (see the <a class="reference internal" href="advanced/classes.html#custom-constructors"><span class="std std-ref">Custom constructors</span></a> section for details). An
interactive Python session demonstrating this example is shown below:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="go">% python</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">example</span>
<span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">example</span><span class="o">.</span><span class="n">Pet</span><span class="p">(</span><span class="s1">'Molly'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">p</span><span class="p">)</span>
<span class="go"><example.Pet object at 0x10cd98060></span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">getName</span><span class="p">()</span>
<span class="go">u'Molly'</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">setName</span><span class="p">(</span><span class="s1">'Charly'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">getName</span><span class="p">()</span>
<span class="go">u'Charly'</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last">Static member functions can be bound in the same way using
<code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_static()</span></code>.</p>
</div>
</div>
<div class="section" id="keyword-and-default-arguments">
<h2>Keyword and default arguments<a class="headerlink" href="#keyword-and-default-arguments" title="Permalink to this headline">¶</a></h2>
<p>It is possible to specify keyword and default arguments using the syntax
discussed in the previous chapter. Refer to the sections <a class="reference internal" href="basics.html#keyword-args"><span class="std std-ref">Keyword arguments</span></a>
and <a class="reference internal" href="basics.html#default-args"><span class="std std-ref">Default arguments</span></a> for details.</p>
</div>
<div class="section" id="binding-lambda-functions">
<h2>Binding lambda functions<a class="headerlink" href="#binding-lambda-functions" title="Permalink to this headline">¶</a></h2>
<p>Note how <code class="docutils literal"><span class="pre">print(p)</span></code> produced a rather useless summary of our data structure in the example above:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">p</span><span class="p">)</span>
<span class="go"><example.Pet object at 0x10cd98060></span>
</pre></div>
</div>
<p>To address this, we could bind an utility function that returns a human-readable
summary to the special method slot named <code class="docutils literal"><span class="pre">__repr__</span></code>. Unfortunately, there is no
suitable functionality in the <code class="docutils literal"><span class="pre">Pet</span></code> data structure, and it would be nice if
we did not have to change it. This can easily be accomplished by binding a
Lambda function instead:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"setName"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">setName</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"getName"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">getName</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"__repr__"</span><span class="p">,</span>
<span class="p">[](</span><span class="k">const</span> <span class="n">Pet</span> <span class="o">&</span><span class="n">a</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="s">"<example.Pet named '"</span> <span class="o">+</span> <span class="n">a</span><span class="p">.</span><span class="n">name</span> <span class="o">+</span> <span class="s">"'>"</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">);</span>
</pre></div>
</div>
<p>Both stateless <a class="footnote-reference" href="#f1" id="id1">[1]</a> and stateful lambda closures are supported by pybind11.
With the above change, the same Python code now produces the following output:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">p</span><span class="p">)</span>
<span class="go"><example.Pet named 'Molly'></span>
</pre></div>
</div>
<table class="docutils footnote" frame="void" id="f1" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Stateless closures are those with an empty pair of brackets <code class="docutils literal"><span class="pre">[]</span></code> as the capture object.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="instance-and-static-fields">
<span id="properties"></span><h2>Instance and static fields<a class="headerlink" href="#instance-and-static-fields" title="Permalink to this headline">¶</a></h2>
<p>We can also directly expose the <code class="docutils literal"><span class="pre">name</span></code> field using the
<code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_readwrite()</span></code> method. A similar <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_readonly()</span></code>
method also exists for <code class="docutils literal"><span class="pre">const</span></code> fields.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">name</span><span class="p">)</span>
<span class="c1">// ... remainder ...</span>
</pre></div>
</div>
<p>This makes it possible to write</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">example</span><span class="o">.</span><span class="n">Pet</span><span class="p">(</span><span class="s1">'Molly'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span>
<span class="go">u'Molly'</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Charly'</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span>
<span class="go">u'Charly'</span>
</pre></div>
</div>
<p>Now suppose that <code class="docutils literal"><span class="pre">Pet::name</span></code> was a private internal variable
that can only be accessed via setters and getters.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Pet</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="n">Pet</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">)</span> <span class="o">:</span> <span class="n">name</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="kt">void</span> <span class="n">setName</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name_</span><span class="p">)</span> <span class="p">{</span> <span class="n">name</span> <span class="o">=</span> <span class="n">name_</span><span class="p">;</span> <span class="p">}</span>
<span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">getName</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="n">name</span><span class="p">;</span> <span class="p">}</span>
<span class="k">private</span><span class="o">:</span>
<span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">name</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>In this case, the method <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_property()</span></code>
(<code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_property_readonly()</span></code> for read-only data) can be used to
provide a field-like interface within Python that will transparently call
the setter and getter functions:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_property</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">getName</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">setName</span><span class="p">)</span>
<span class="c1">// ... remainder ...</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last">Similar functions <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_readwrite_static()</span></code>,
<code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_readonly_static()</span></code> <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_property_static()</span></code>,
and <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_property_readonly_static()</span></code> are provided for binding
static variables and properties. Please also see the section on
<a class="reference internal" href="advanced/classes.html#static-properties"><span class="std std-ref">Static properties</span></a> in the advanced part of the documentation.</p>
</div>
</div>
<div class="section" id="dynamic-attributes">
<h2>Dynamic attributes<a class="headerlink" href="#dynamic-attributes" title="Permalink to this headline">¶</a></h2>
<p>Native Python classes can pick up new attributes dynamically:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">Pet</span><span class="p">:</span>
<span class="gp">... </span> <span class="n">name</span> <span class="o">=</span> <span class="s1">'Molly'</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">Pet</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Charly'</span> <span class="c1"># overwrite existing</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">age</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># dynamically add a new attribute</span>
</pre></div>
</div>
<p>By default, classes exported from C++ do not support this and the only writable
attributes are the ones explicitly defined using <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_readwrite()</span></code>
or <code class="xref cpp cpp-func docutils literal"><span class="pre">class_::def_property()</span></code>.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">name</span><span class="p">);</span>
</pre></div>
</div>
<p>Trying to set any other attribute results in an error:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">example</span><span class="o">.</span><span class="n">Pet</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Charly'</span> <span class="c1"># OK, attribute defined in C++</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">age</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># fail</span>
<span class="go">AttributeError: 'Pet' object has no attribute 'age'</span>
</pre></div>
</div>
<p>To enable dynamic attributes for C++ classes, the <code class="xref cpp cpp-class docutils literal"><span class="pre">py::dynamic_attr</span></code> tag
must be added to the <code class="xref cpp cpp-class docutils literal"><span class="pre">py::class_</span></code> constructor:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">dynamic_attr</span><span class="p">())</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">name</span><span class="p">);</span>
</pre></div>
</div>
<p>Now everything works as expected:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">example</span><span class="o">.</span><span class="n">Pet</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Charly'</span> <span class="c1"># OK, overwrite value in C++</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">age</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># OK, dynamically add a new attribute</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="vm">__dict__</span> <span class="c1"># just like a native Python class</span>
<span class="go">{'age': 2}</span>
</pre></div>
</div>
<p>Note that there is a small runtime cost for a class with dynamic attributes.
Not only because of the addition of a <code class="docutils literal"><span class="pre">__dict__</span></code>, but also because of more
expensive garbage collection tracking which must be activated to resolve
possible circular references. Native Python classes incur this same cost by
default, so this is not anything to worry about. By default, pybind11 classes
are more efficient than native Python classes. Enabling dynamic attributes
just brings them on par.</p>
</div>
<div class="section" id="inheritance">
<span id="id2"></span><h2>Inheritance<a class="headerlink" href="#inheritance" title="Permalink to this headline">¶</a></h2>
<p>Suppose now that the example consists of two data structures with an
inheritance relationship:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">Pet</span> <span class="p">{</span>
<span class="n">Pet</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">)</span> <span class="o">:</span> <span class="n">name</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">name</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span> <span class="nl">Dog</span> <span class="p">:</span> <span class="n">Pet</span> <span class="p">{</span>
<span class="n">Dog</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">)</span> <span class="o">:</span> <span class="n">Pet</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">bark</span><span class="p">()</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="s">"woof!"</span><span class="p">;</span> <span class="p">}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>There are two different ways of indicating a hierarchical relationship to
pybind11: the first specifies the C++ base class as an extra template
parameter of the <code class="xref cpp cpp-class docutils literal"><span class="pre">class_</span></code>:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">name</span><span class="p">);</span>
<span class="c1">// Method 1: template parameter:</span>
<span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Dog</span><span class="p">,</span> <span class="n">Pet</span> <span class="cm">/* <- specify C++ parent type */</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Dog"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"bark"</span><span class="p">,</span> <span class="o">&</span><span class="n">Dog</span><span class="o">::</span><span class="n">bark</span><span class="p">);</span>
</pre></div>
</div>
<p>Alternatively, we can also assign a name to the previously bound <code class="docutils literal"><span class="pre">Pet</span></code>
<code class="xref cpp cpp-class docutils literal"><span class="pre">class_</span></code> object and reference it when binding the <code class="docutils literal"><span class="pre">Dog</span></code> class:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span> <span class="n">pet</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">);</span>
<span class="n">pet</span><span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">name</span><span class="p">);</span>
<span class="c1">// Method 2: pass parent class_ object:</span>
<span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Dog</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Dog"</span><span class="p">,</span> <span class="n">pet</span> <span class="cm">/* <- specify Python parent type */</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">())</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"bark"</span><span class="p">,</span> <span class="o">&</span><span class="n">Dog</span><span class="o">::</span><span class="n">bark</span><span class="p">);</span>
</pre></div>
</div>
<p>Functionality-wise, both approaches are equivalent. Afterwards, instances will
expose fields and methods of both types:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">example</span><span class="o">.</span><span class="n">Dog</span><span class="p">(</span><span class="s1">'Molly'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">name</span>
<span class="go">u'Molly'</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">bark</span><span class="p">()</span>
<span class="go">u'woof!'</span>
</pre></div>
</div>
</div>
<div class="section" id="overloaded-methods">
<h2>Overloaded methods<a class="headerlink" href="#overloaded-methods" title="Permalink to this headline">¶</a></h2>
<p>Sometimes there are several overloaded C++ methods with the same name taking
different kinds of input arguments:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">Pet</span> <span class="p">{</span>
<span class="n">Pet</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">,</span> <span class="kt">int</span> <span class="n">age</span><span class="p">)</span> <span class="o">:</span> <span class="n">name</span><span class="p">(</span><span class="n">name</span><span class="p">),</span> <span class="n">age</span><span class="p">(</span><span class="n">age</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="kt">void</span> <span class="n">set</span><span class="p">(</span><span class="kt">int</span> <span class="n">age</span><span class="p">)</span> <span class="p">{</span> <span class="n">age</span> <span class="o">=</span> <span class="n">age</span><span class="p">;</span> <span class="p">}</span>
<span class="kt">void</span> <span class="n">set</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span> <span class="n">name</span> <span class="o">=</span> <span class="n">name</span><span class="p">;</span> <span class="p">}</span>
<span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">name</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">age</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>Attempting to bind <code class="docutils literal"><span class="pre">Pet::set</span></code> will cause an error since the compiler does not
know which method the user intended to select. We can disambiguate by casting
them to function pointers. Binding multiple functions to the same Python name
automatically creates a chain of function overloads that will be tried in
sequence.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="p">,</span> <span class="kt">int</span><span class="o">></span><span class="p">())</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"set"</span><span class="p">,</span> <span class="p">(</span><span class="kt">void</span> <span class="p">(</span><span class="n">Pet</span><span class="o">::*</span><span class="p">)(</span><span class="kt">int</span><span class="p">))</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">set</span><span class="p">,</span> <span class="s">"Set the pet's age"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"set"</span><span class="p">,</span> <span class="p">(</span><span class="kt">void</span> <span class="p">(</span><span class="n">Pet</span><span class="o">::*</span><span class="p">)(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="p">))</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">set</span><span class="p">,</span> <span class="s">"Set the pet's name"</span><span class="p">);</span>
</pre></div>
</div>
<p>The overload signatures are also visible in the method’s docstring:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">help</span><span class="p">(</span><span class="n">example</span><span class="o">.</span><span class="n">Pet</span><span class="p">)</span>
<span class="go">class Pet(__builtin__.object)</span>
<span class="go"> | Methods defined here:</span>
<span class="go"> |</span>
<span class="go"> | __init__(...)</span>
<span class="go"> | Signature : (Pet, str, int) -> NoneType</span>
<span class="go"> |</span>
<span class="go"> | set(...)</span>
<span class="go"> | 1. Signature : (Pet, int) -> NoneType</span>
<span class="go"> |</span>
<span class="go"> | Set the pet's age</span>
<span class="go"> |</span>
<span class="go"> | 2. Signature : (Pet, str) -> NoneType</span>
<span class="go"> |</span>
<span class="go"> | Set the pet's name</span>
</pre></div>
</div>
<p>If you have a C++14 compatible compiler <a class="footnote-reference" href="#cpp14" id="id3">[2]</a>, you can use an alternative
syntax to cast the overloaded function:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"set"</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">overload_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">set</span><span class="p">),</span> <span class="s">"Set the pet's age"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"set"</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">overload_cast</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&></span><span class="p">(</span><span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">set</span><span class="p">),</span> <span class="s">"Set the pet's name"</span><span class="p">);</span>
</pre></div>
</div>
<p>Here, <code class="docutils literal"><span class="pre">py::overload_cast</span></code> only requires the parameter types to be specified.
The return type and class are deduced. This avoids the additional noise of
<code class="docutils literal"><span class="pre">void</span> <span class="pre">(Pet::*)()</span></code> as seen in the raw cast. If a function is overloaded based
on constness, the <code class="docutils literal"><span class="pre">py::const_</span></code> tag should be used:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">Widget</span> <span class="p">{</span>
<span class="kt">int</span> <span class="n">foo</span><span class="p">(</span><span class="kt">int</span> <span class="n">x</span><span class="p">,</span> <span class="kt">float</span> <span class="n">y</span><span class="p">);</span>
<span class="kt">int</span> <span class="nf">foo</span><span class="p">(</span><span class="kt">int</span> <span class="n">x</span><span class="p">,</span> <span class="kt">float</span> <span class="n">y</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
<span class="p">};</span>
<span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Widget</span><span class="o">></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Widget"</span><span class="p">)</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"foo_mutable"</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">overload_cast</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="kt">float</span><span class="o">></span><span class="p">(</span><span class="o">&</span><span class="n">Widget</span><span class="o">::</span><span class="n">foo</span><span class="p">))</span>
<span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"foo_const"</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">overload_cast</span><span class="o"><</span><span class="kt">int</span><span class="p">,</span> <span class="kt">float</span><span class="o">></span><span class="p">(</span><span class="o">&</span><span class="n">Widget</span><span class="o">::</span><span class="n">foo</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">const_</span><span class="p">));</span>
</pre></div>
</div>
<table class="docutils footnote" frame="void" id="cpp14" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id3">[2]</a></td><td>A compiler which supports the <code class="docutils literal"><span class="pre">-std=c++14</span></code> flag
or Visual Studio 2015 Update 2 and newer.</td></tr>
</tbody>
</table>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">To define multiple overloaded constructors, simply declare one after the
other using the <code class="docutils literal"><span class="pre">.def(py::init<...>())</span></code> syntax. The existing machinery
for specifying keyword and default arguments also works.</p>
</div>
</div>
<div class="section" id="enumerations-and-internal-types">
<h2>Enumerations and internal types<a class="headerlink" href="#enumerations-and-internal-types" title="Permalink to this headline">¶</a></h2>
<p>Let’s now suppose that the example class contains an internal enumeration type,
e.g.:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="n">Pet</span> <span class="p">{</span>
<span class="k">enum</span> <span class="n">Kind</span> <span class="p">{</span>
<span class="n">Dog</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span>
<span class="n">Cat</span>
<span class="p">};</span>
<span class="n">Pet</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="n">name</span><span class="p">,</span> <span class="n">Kind</span> <span class="n">type</span><span class="p">)</span> <span class="o">:</span> <span class="n">name</span><span class="p">(</span><span class="n">name</span><span class="p">),</span> <span class="n">type</span><span class="p">(</span><span class="n">type</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="n">name</span><span class="p">;</span>
<span class="n">Kind</span> <span class="n">type</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The binding code for this example looks as follows:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Pet</span><span class="o">></span> <span class="n">pet</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Pet"</span><span class="p">);</span>
<span class="n">pet</span><span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="n">py</span><span class="o">::</span><span class="n">init</span><span class="o"><</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span> <span class="o">&</span><span class="p">,</span> <span class="n">Pet</span><span class="o">::</span><span class="n">Kind</span><span class="o">></span><span class="p">())</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"name"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">name</span><span class="p">)</span>
<span class="p">.</span><span class="n">def_readwrite</span><span class="p">(</span><span class="s">"type"</span><span class="p">,</span> <span class="o">&</span><span class="n">Pet</span><span class="o">::</span><span class="n">type</span><span class="p">);</span>
<span class="n">py</span><span class="o">::</span><span class="n">enum_</span><span class="o"><</span><span class="n">Pet</span><span class="o">::</span><span class="n">Kind</span><span class="o">></span><span class="p">(</span><span class="n">pet</span><span class="p">,</span> <span class="s">"Kind"</span><span class="p">)</span>
<span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="s">"Dog"</span><span class="p">,</span> <span class="n">Pet</span><span class="o">::</span><span class="n">Kind</span><span class="o">::</span><span class="n">Dog</span><span class="p">)</span>
<span class="p">.</span><span class="n">value</span><span class="p">(</span><span class="s">"Cat"</span><span class="p">,</span> <span class="n">Pet</span><span class="o">::</span><span class="n">Kind</span><span class="o">::</span><span class="n">Cat</span><span class="p">)</span>
<span class="p">.</span><span class="n">export_values</span><span class="p">();</span>
</pre></div>
</div>
<p>To ensure that the <code class="docutils literal"><span class="pre">Kind</span></code> type is created within the scope of <code class="docutils literal"><span class="pre">Pet</span></code>, the
<code class="docutils literal"><span class="pre">pet</span></code> <code class="xref cpp cpp-class docutils literal"><span class="pre">class_</span></code> instance must be supplied to the <code class="xref cpp cpp-class docutils literal"><span class="pre">enum_</span></code>.
constructor. The <code class="xref cpp cpp-func docutils literal"><span class="pre">enum_::export_values()</span></code> function exports the enum entries
into the parent scope, which should be skipped for newer C++11-style strongly
typed enums.</p>
<div class="highlight-pycon"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">Pet</span><span class="p">(</span><span class="s1">'Lucy'</span><span class="p">,</span> <span class="n">Pet</span><span class="o">.</span><span class="n">Cat</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">type</span>
<span class="go">Kind.Cat</span>
<span class="gp">>>> </span><span class="nb">int</span><span class="p">(</span><span class="n">p</span><span class="o">.</span><span class="n">type</span><span class="p">)</span>
<span class="go">1L</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>When the special tag <code class="docutils literal"><span class="pre">py::arithmetic()</span></code> is specified to the <code class="docutils literal"><span class="pre">enum_</span></code>
constructor, pybind11 creates an enumeration that also supports rudimentary
arithmetic and bit-level operations like comparisons, and, or, xor, negation,
etc.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">py</span><span class="o">::</span><span class="n">enum_</span><span class="o"><</span><span class="n">Pet</span><span class="o">::</span><span class="n">Kind</span><span class="o">></span><span class="p">(</span><span class="n">pet</span><span class="p">,</span> <span class="s">"Kind"</span><span class="p">,</span> <span class="n">py</span><span class="o">::</span><span class="n">arithmetic</span><span class="p">())</span>
<span class="p">...</span>
</pre></div>
</div>
<p class="last">By default, these are omitted to conserve space.</p>
</div>
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