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<title>Is return_internal_reference efficient?</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="faq.is_return_internal_reference_eff"></a><a class="link" href="is_return_internal_reference_eff.html" title="Is return_internal_reference efficient?">Is <code class="computeroutput"><span class="identifier">return_internal_reference</span></code> efficient?</a>
</h3></div></div></div>
<p>
        <span class="bold"><strong>Q:</strong></span> /I have an object composed of 12 doubles.
        A <code class="computeroutput"><span class="keyword">const</span><span class="special">&amp;</span></code>
        to this object is returned by a member function of another class. From the
        viewpoint of using the returned object in Python I do not care if I get a
        copy or a reference to the returned object. In Boost.Python I have the choice
        of using <code class="computeroutput"><span class="identifier">copy_const_reference</span></code>
        or <code class="computeroutput"><span class="identifier">return_internal_reference</span></code>.
        Are there considerations that would lead me to prefer one over the other,
        such as size of generated code or memory overhead?/
      </p>
<p>
        <span class="bold"><strong>A:</strong></span> <code class="computeroutput"><span class="identifier">copy_const_reference</span></code>
        will make an instance with storage for one of your objects, <code class="computeroutput"><span class="identifier">size</span> <span class="special">=</span> <span class="identifier">base_size</span> <span class="special">+</span> <span class="number">12</span> <span class="special">*</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="keyword">double</span><span class="special">)</span></code>.
        <code class="computeroutput"><span class="identifier">return_internal_reference</span></code>
        will make an instance with storage for a pointer to one of your objects,
        <code class="computeroutput"><span class="identifier">size</span> <span class="special">=</span>
        <span class="identifier">base_size</span> <span class="special">+</span>
        <span class="keyword">sizeof</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>. However,
        it will also create a weak reference object which goes in the source object's
        weakreflist and a special callback object to manage the lifetime of the internally-referenced
        object. My guess? <code class="computeroutput"><span class="identifier">copy_const_reference</span></code>
        is your friend here, resulting in less overall memory use and less fragmentation,
        also probably fewer total cycles.
      </p>
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