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<p>Allows the size of an empty base subobject to be zero.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/ebo&action=edit&section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The size of any <a href="objects.html" title="cpp/language/object">object</a> or member subobject is required to be at least 1 even if the type is an empty <a href="class.html" title="cpp/language/class">class type</a> (that is, a class or struct that has no non-static data members), in order to be able to guarantee that the addresses of distinct objects of the same type are always distinct.
</p><p>However, base class subobjects are not so constrained, and can be completely optimized out from the object layout:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <cassert></span>
 
<span class="kw1">struct</span> Base <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// empty class</span>
 
<span class="kw1">struct</span> Derived1 <span class="sy4">:</span> Base <span class="br0">{</span>
<span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// the size of any object of empty class type is at least 1</span>
<a href="../error/assert.html"><span class="kw654">assert</span></a><span class="br0">(</span>sizeof<span class="br0">(</span>Base<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// empty base optimization applies</span>
<a href="../error/assert.html"><span class="kw654">assert</span></a><span class="br0">(</span>sizeof<span class="br0">(</span>Derived1<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Empty base optimization is prohibited if one of the empty base classes is also the type or the base of the type of the first non-static data member, since the two base subobjects of the same type are required to have different addresses within the object representation of the most derived type.
</p><p>A typical example of such situation is the naive implementation of <span class="t-lc">std::reverse_iterator</span> (derived from the empty base <span class="t-lc">std::iterator</span>), which holds the underlying iterator (also derived from <span class="t-lc">std::iterator</span>) as its first non-static data member.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <cassert></span>
 
<span class="kw1">struct</span> Base <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// empty class</span>
 
<span class="kw1">struct</span> Derived1 <span class="sy4">:</span> Base <span class="br0">{</span>
<span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Derived2 <span class="sy4">:</span> Base <span class="br0">{</span>
Base c<span class="sy4">;</span> <span class="co1">// Base, occupies 1 byte, followed by padding for i</span>
<span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Derived3 <span class="sy4">:</span> Base <span class="br0">{</span>
Derived1 c<span class="sy4">;</span> <span class="co1">// derived from Base, occupies sizeof(int) bytes</span>
<span class="kw4">int</span> i<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// empty base optimization does not apply,</span>
<span class="co1">// base occupies 1 byte, Base member occupies 1 byte</span>
<span class="co1">// followed by 2 bytes of padding to satisfy int alignment requirements</span>
<a href="../error/assert.html"><span class="kw654">assert</span></a><span class="br0">(</span>sizeof<span class="br0">(</span>Derived2<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">2</span><span class="sy2">*</span>sizeof<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// empty base optimization does not apply,</span>
<span class="co1">// base takes up at least 1 byte plus the padding</span>
<span class="co1">// to satisfy alignment requirement of the first member (whose</span>
<span class="co1">// alignment is the same as int)</span>
<a href="../error/assert.html"><span class="kw654">assert</span></a><span class="br0">(</span>sizeof<span class="br0">(</span>Derived3<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">3</span><span class="sy2">*</span>sizeof<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>Empty base optimization is <i>required</i> for <a href="../concept/StandardLayoutType.html" title="cpp/concept/StandardLayoutType"><code>StandardLayoutType</code>s</a> in order to maintain the requirement that the pointer to a standard-layout object, converted using <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a>, points to its initial member, which is why one of the requirements for a standard layout type is "has no base classes with non-static data members and has no base classes of the same type as the first non-static data member".
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/ebo&action=edit&section=2" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Empty base optimization is commonly used by allocator-aware standard library classes (<span class="t-lc">std::vector</span>, <span class="t-lc">std::function</span>, <span class="t-lc">std::shared_ptr</span>, etc) to avoid occupying any additional storage for its allocator member if the allocator is stateless. This is achieved by storing one of the required data members (e.g., begin, end, or capacity pointer for the vector) in an equivalent of <a rel="nofollow" class="external text" href="http://www.boost.org/doc/libs/release/libs/utility/compressed_pair.htm">boost::compressed_pair</a> with the allocator.
</p><p><br />
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/ebo&action=edit&section=3" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011):
</li></ul>
<dl><dd><ul><li> 5.10 Equality operators [expr.eq](p: 2)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 5.3.3 Sizeof [expr.sizeof](p: 2)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 9 Classes [class](p: 4,7)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 9.2 Class members [class.mem](p: 20)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++98">
<ul><li> C++98 standard (ISO/IEC 14882:1998):
</li></ul>
<dl><dd><ul><li> 5.10 Equality operators [expr.eq](p: 2)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 5.3.3 Sizeof [expr.sizeof](p: 2)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 9 Classes [class](p: 3)
</li></ul>
</dd></dl>
</div>
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