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<p>A declaration of a <a href="class.html" title="cpp/language/class">class/struct</a> or <a href="union.html" title="cpp/language/union">union</a> may appear in within another class. Such declaration declares a <i>nested class</i>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/nested_types&action=edit&section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The name of the nested class exists in the scope of the enclosing class, and name lookup from a member function of a nested class visits the scope of the enclosing class after examining the scope of the nested class. As any member of its enclosing class, nested class has access to all names (private, protected, etc) to which the enclosing class has access, but it is otherwise independent and has no special access to the <a href="this.html" title="cpp/language/this">this</a> pointer of the enclosing class.
</p>
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<tr class="t-rev t-until-cxx11"><td>Declarations in a nested class can use only type names, static members, and enumerators from the enclosing class
</td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr>
<tr class="t-rev t-since-cxx11"><td>
<p>Declarations in a nested class can use any members of the enclosing class, following the <a href="data_members.html#Usage" title="cpp/language/data members">usual usage rules</a> for the non-static members.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x,y<span class="sy4">;</span> <span class="co1">// globals</span>
<span class="kw1">class</span> enclose <span class="br0">{</span> <span class="co1">// enclosing class</span>
<span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// note: private members</span>
<span class="kw4">static</span> <span class="kw4">int</span> s<span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
<span class="kw1">struct</span> inner <span class="br0">{</span> <span class="co1">// nested class</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
x <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// Error: can't write to non-static enclose::x without instance</span>
<span class="kw4">int</span> a <span class="sy1">=</span> sizeof x<span class="sy4">;</span> <span class="co1">// Error until C++11,</span>
<span class="co1">// OK in C++11: operand of sizeof is unevaluated,</span>
<span class="co1">// this use of the non-static enclose::x is allowed.</span>
s <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// OK: can assign to the static enclose::s</span>
<span class="sy4">::</span><span class="me2">x</span> <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// OK: can assign to global x</span>
y <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// OK: can assign to global y</span>
<span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span>enclose<span class="sy2">*</span> p, <span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
p<span class="sy2">-</span><span class="sy1">></span>x <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// OK: assign to enclose::x</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>A <a href="friend.html" title="cpp/language/friend">friend</a> functions defined within a nested class has no special access to the members of the enclosing class even if lookup from the body of a name function that is defined within a nested class can find the private members of the enclosing class.
</p><p>Out-of-class definitions of the members of a nested class appear in the namespace of the enclosing class:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> enclose <span class="br0">{</span>
<span class="kw1">struct</span> inner <span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">int</span> x<span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> enclose<span class="sy4">::</span><span class="me2">inner</span><span class="sy4">::</span><span class="me2">x</span> <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// definition</span>
<span class="kw4">void</span> enclose<span class="sy4">::</span><span class="me2">inner</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// definition</span></pre></div></div>
<p>Nested classes can be forward-declared and later defined, either within the same enclosing class body, or outside of it:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> enclose <span class="br0">{</span>
<span class="kw1">class</span> nested1<span class="sy4">;</span> <span class="co1">// forward declaration</span>
<span class="kw1">class</span> nested2<span class="sy4">;</span> <span class="co1">// forward declaration</span>
<span class="kw1">class</span> nested1 <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// definition of nested class</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> enclose<span class="sy4">::</span><span class="me2">nested2</span> <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// definition of nested class</span></pre></div></div>
<p>Nested class declarations obey <a href="access.html" title="cpp/language/access">member access</a> specifiers, a private member class cannot be named outside the scope of the enclosing class, although objects of that class may be manipulated:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> enclose <span class="br0">{</span>
<span class="kw1">struct</span> nested <span class="br0">{</span> <span class="co1">// private member</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
<span class="kw4">static</span> nested f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> nested<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</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">//enclose::nested n1 = e.f(); // error: 'nested' is private</span>
 
enclose<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">g</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: does not name 'nested'</span>
<span class="kw4">auto</span> n2 <span class="sy1">=</span> enclose<span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: does not name 'nested'</span>
n2.<span class="me1">g</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/nested_types&action=edit&section=2" 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> 9.7 Nested class declarations [class.nest]
</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> 9.7 Nested class declarations [class.nest]
</li></ul>
</dd></dl>
</div>
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