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<li class="toctree-l2"><a class="reference internal" href="#std-unique-ptr">std::unique_ptr</a></li>
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<div class="section" id="smart-pointers">
<h1>Smart pointers<a class="headerlink" href="#smart-pointers" title="Permalink to this headline">¶</a></h1>
<div class="section" id="std-unique-ptr">
<h2>std::unique_ptr<a class="headerlink" href="#std-unique-ptr" title="Permalink to this headline">¶</a></h2>
<p>Given a class <code class="docutils literal"><span class="pre">Example</span></code> with Python bindings, it’s possible to return
instances wrapped in C++11 unique pointers, like so</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o"><</span><span class="n">Example</span><span class="o">></span> <span class="n">create_example</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o"><</span><span class="n">Example</span><span class="o">></span><span class="p">(</span><span class="k">new</span> <span class="n">Example</span><span class="p">());</span> <span class="p">}</span>
</pre></div>
</div>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">m</span><span class="p">.</span><span class="n">def</span><span class="p">(</span><span class="s">"create_example"</span><span class="p">,</span> <span class="o">&</span><span class="n">create_example</span><span class="p">);</span>
</pre></div>
</div>
<p>In other words, there is nothing special that needs to be done. While returning
unique pointers in this way is allowed, it is <em>illegal</em> to use them as function
arguments. For instance, the following function signature cannot be processed
by pybind11.</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">do_something_with_example</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o"><</span><span class="n">Example</span><span class="o">></span> <span class="n">ex</span><span class="p">)</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span>
</pre></div>
</div>
<p>The above signature would imply that Python needs to give up ownership of an
object that is passed to this function, which is generally not possible (for
instance, the object might be referenced elsewhere).</p>
</div>
<div class="section" id="std-shared-ptr">
<h2>std::shared_ptr<a class="headerlink" href="#std-shared-ptr" title="Permalink to this headline">¶</a></h2>
<p>The binding generator for classes, <code class="xref cpp cpp-class docutils literal"><span class="pre">class_</span></code>, can be passed a template
type that denotes a special <em>holder</em> type that is used to manage references to
the object. If no such holder type template argument is given, the default for
a type named <code class="docutils literal"><span class="pre">Type</span></code> is <code class="docutils literal"><span class="pre">std::unique_ptr<Type></span></code>, which means that the object
is deallocated when Python’s reference count goes to zero.</p>
<p>It is possible to switch to other types of reference counting wrappers or smart
pointers, which is useful in codebases that rely on them. For instance, the
following snippet causes <code class="docutils literal"><span class="pre">std::shared_ptr</span></code> to be used 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">Example</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Example</span><span class="o">></span> <span class="cm">/* <- holder type */</span><span class="o">></span> <span class="n">obj</span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Example"</span><span class="p">);</span>
</pre></div>
</div>
<p>Note that any particular class can only be associated with a single holder type.</p>
<p>One potential stumbling block when using holder types is that they need to be
applied consistently. Can you guess what’s broken about the following binding
code?</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Child</span> <span class="p">{</span> <span class="p">};</span>
<span class="k">class</span> <span class="nc">Parent</span> <span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="n">Parent</span><span class="p">()</span> <span class="o">:</span> <span class="n">child</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o"><</span><span class="n">Child</span><span class="o">></span><span class="p">())</span> <span class="p">{</span> <span class="p">}</span>
<span class="n">Child</span> <span class="o">*</span><span class="n">get_child</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="n">child</span><span class="p">.</span><span class="n">get</span><span class="p">();</span> <span class="p">}</span> <span class="cm">/* Hint: ** DON'T DO THIS ** */</span>
<span class="k">private</span><span class="o">:</span>
<span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Child</span><span class="o">></span> <span class="n">child</span><span class="p">;</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="n">py</span><span class="o">::</span><span class="n">class_</span><span class="o"><</span><span class="n">Child</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Child</span><span class="o">>></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Child"</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">Parent</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Parent</span><span class="o">>></span><span class="p">(</span><span class="n">m</span><span class="p">,</span> <span class="s">"Parent"</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</span><span class="p">(</span><span class="s">"get_child"</span><span class="p">,</span> <span class="o">&</span><span class="n">Parent</span><span class="o">::</span><span class="n">get_child</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>The following Python code will cause undefined behavior (and likely a
segmentation fault).</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">example</span> <span class="kn">import</span> <span class="n">Parent</span>
<span class="k">print</span><span class="p">(</span><span class="n">Parent</span><span class="p">()</span><span class="o">.</span><span class="n">get_child</span><span class="p">())</span>
</pre></div>
</div>
<p>The problem is that <code class="docutils literal"><span class="pre">Parent::get_child()</span></code> returns a pointer to an instance of
<code class="docutils literal"><span class="pre">Child</span></code>, but the fact that this instance is already managed by
<code class="docutils literal"><span class="pre">std::shared_ptr<...></span></code> is lost when passing raw pointers. In this case,
pybind11 will create a second independent <code class="docutils literal"><span class="pre">std::shared_ptr<...></span></code> that also
claims ownership of the pointer. In the end, the object will be freed <strong>twice</strong>
since these shared pointers have no way of knowing about each other.</p>
<p>There are two ways to resolve this issue:</p>
<ol class="arabic simple">
<li>For types that are managed by a smart pointer class, never use raw pointers
in function arguments or return values. In other words: always consistently
wrap pointers into their designated holder types (such as
<code class="docutils literal"><span class="pre">std::shared_ptr<...></span></code>). In this case, the signature of <code class="docutils literal"><span class="pre">get_child()</span></code>
should be modified as follows:</li>
</ol>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Child</span><span class="o">></span> <span class="n">get_child</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="n">child</span><span class="p">;</span> <span class="p">}</span>
</pre></div>
</div>
<ol class="arabic simple" start="2">
<li>Adjust the definition of <code class="docutils literal"><span class="pre">Child</span></code> by specifying
<code class="docutils literal"><span class="pre">std::enable_shared_from_this<T></span></code> (see <a class="reference external" href="http://en.cppreference.com/w/cpp/memory/enable_shared_from_this">cppreference</a> for details) as a
base class. This adds a small bit of information to <code class="docutils literal"><span class="pre">Child</span></code> that allows
pybind11 to realize that there is already an existing
<code class="docutils literal"><span class="pre">std::shared_ptr<...></span></code> and communicate with it. In this case, the
declaration of <code class="docutils literal"><span class="pre">Child</span></code> should look as follows:</li>
</ol>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Child</span> <span class="o">:</span> <span class="k">public</span> <span class="n">std</span><span class="o">::</span><span class="n">enable_shared_from_this</span><span class="o"><</span><span class="n">Child</span><span class="o">></span> <span class="p">{</span> <span class="p">};</span>
</pre></div>
</div>
</div>
<div class="section" id="custom-smart-pointers">
<span id="id1"></span><h2>Custom smart pointers<a class="headerlink" href="#custom-smart-pointers" title="Permalink to this headline">¶</a></h2>
<p>pybind11 supports <code class="docutils literal"><span class="pre">std::unique_ptr</span></code> and <code class="docutils literal"><span class="pre">std::shared_ptr</span></code> right out of the
box. For any other custom smart pointer, transparent conversions can be enabled
using a macro invocation similar to the following. It must be declared at the
top namespace level before any binding code:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">PYBIND11_DECLARE_HOLDER_TYPE</span><span class="p">(</span><span class="n">T</span><span class="p">,</span> <span class="n">SmartPtr</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">);</span>
</pre></div>
</div>
<p>The first argument of <code class="xref cpp cpp-func docutils literal"><span class="pre">PYBIND11_DECLARE_HOLDER_TYPE()</span></code> should be a
placeholder name that is used as a template parameter of the second argument.
Thus, feel free to use any identifier, but use it consistently on both sides;
also, don’t use the name of a type that already exists in your codebase.</p>
<p>The macro also accepts a third optional boolean parameter that is set to false
by default. Specify</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">PYBIND11_DECLARE_HOLDER_TYPE</span><span class="p">(</span><span class="n">T</span><span class="p">,</span> <span class="n">SmartPtr</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">,</span> <span class="nb">true</span><span class="p">);</span>
</pre></div>
</div>
<p>if <code class="docutils literal"><span class="pre">SmartPtr<T></span></code> can always be initialized from a <code class="docutils literal"><span class="pre">T*</span></code> pointer without the
risk of inconsistencies (such as multiple independent <code class="docutils literal"><span class="pre">SmartPtr</span></code> instances
believing that they are the sole owner of the <code class="docutils literal"><span class="pre">T*</span></code> pointer). A common
situation where <code class="docutils literal"><span class="pre">true</span></code> should be passed is when the <code class="docutils literal"><span class="pre">T</span></code> instances use
<em>intrusive</em> reference counting.</p>
<p>Please take a look at the <a class="reference internal" href="misc.html#macro-notes"><span class="std std-ref">General notes regarding convenience macros</span></a> before using this feature.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last">The file <code class="file docutils literal"><span class="pre">tests/test_smart_ptr.cpp</span></code> contains a complete example
that demonstrates how to work with custom reference-counting holder types
in more detail.</p>
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