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<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">default member initializer</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"><code>friend</code> specifier</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">converting constructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect">copy assignment operator</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect">move assignment operator</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">destructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Inheritance </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">base and derived classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">virtual member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override">override</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final">final</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">pure virtual functions and abstract classes</a></td></tr>
</table></div>
</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/classes/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>The friend declaration appears in a <a href="class.html" title="cpp/language/class">class body</a> and grants a function or another class access to private and protected members of the class where the friend declaration appears.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="friend.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="friend.html#Description"><span class="tocnumber">2</span> <span class="toctext">Description</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="friend.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="friend.html#Template_friends"><span class="tocnumber">4</span> <span class="toctext">Template friends</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="friend.html#Template_friend_operators"><span class="tocnumber">5</span> <span class="toctext">Template friend operators</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="friend.html#Example"><span class="tocnumber">6</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="friend.html#References"><span class="tocnumber">7</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="friend.html#See_Also"><span class="tocnumber">8</span> <span class="toctext">See Also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td> <code><b>friend</b></code> <span class="t-spar">function-declaration</span>
</td>
<td> (1)
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<td> <code><b>friend</b></code> <span class="t-spar">function-definition</span>
</td>
<td> (2)
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<td> <code><b>friend</b></code> <span class="t-spar">elaborated-class-name</span> <code><b>;</b></code>
</td>
<td> (3)
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<td> <code><b>friend</b></code> <span class="t-spar">simple-type-specifier</span> <code><b>;</b></code>
<p><code><b>friend</b></code> <span class="t-spar">typename-specifier</span> <code><b>;</b></code>
</p>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=2" title="Edit section: Description">edit</a>]</span> <span class="mw-headline" id="Description">Description</span></h3>
<div class="t-li1"><span class="t-li">1)</span> Designates a function or several functions as friends of this class
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Y <span class="br0">{</span>
<span class="kw4">int</span> data<span class="sy4">;</span> <span class="co1">// private member</span>
<span class="co1">// the non-member function operator<< will have access to Y's private members</span>
<span class="kw1">friend</span> <a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> out, <span class="kw4">const</span> Y<span class="sy3">&</span> o<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">friend</span> <span class="kw4">char</span><span class="sy2">*</span> X<span class="sy4">::</span><span class="me2">foo</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// members of other classes can be friends too</span>
<span class="kw1">friend</span> X<span class="sy4">::</span><span class="me2">X</span><span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span>, X<span class="sy4">::</span>~X<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// constructors and destructors can be friends</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// friend declaration does not declare a member function</span>
<span class="co1">// this operator<< still needs to be defined, as a non-member</span>
<a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> out, <span class="kw4">const</span> Y<span class="sy3">&</span> y<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> out <span class="sy1"><<</span> y.<span class="me1">data</span><span class="sy4">;</span> <span class="co1">// can access private member Y::data</span>
<span class="br0">}</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> (only allowed in non-local class definitions) Defines a non-member function, and makes it a friend of this class at the same time. Such non-member function is always <a href="inline.html" title="cpp/language/inline">inline</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> X <span class="br0">{</span>
<span class="kw4">int</span> a<span class="sy4">;</span>
<span class="kw1">friend</span> <span class="kw4">void</span> friend_set<span class="br0">(</span>X<span class="sy3">&</span> p, <span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
p.<span class="me1">a</span> <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// this is a non-member function</span>
<span class="br0">}</span>
<span class="kw1">public</span><span class="sy4">:</span>
<span class="kw4">void</span> member_set<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
a <span class="sy1">=</span> i<span class="sy4">;</span> <span class="co1">// this is a member function</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">3)</span> Designates the class, struct, or union named by the <span class="t-spar">elaborated-class-name</span> as a friend of this class. This means that the friend's member declarations and definitions can access private and protected members of this class and also that the <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">nested classes</a> of the friend can inherit from private and protected members of this class. <span class="t-rev-inl t-since-cxx11"><span>The friend itself can also inherit from private and protected members of this class.</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>
If the name of the class that is used in the friend declaration is not yet declared, it is forward declared on the spot.</div>
<div class="t-li1"><span class="t-li">4)</span> Designates the type named by the <span class="t-spar">simple-type-specifier</span> or <span class="t-spar">typename-specifier</span> as a friend of this class if that type is a (possibly <a href="cv.html" title="cpp/language/cv">cv-qualified</a>) class, struct, or union; otherwise the <code>friend</code> declaration is ignored. This declaration will not forward declare new type.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> Y <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> A <span class="br0">{</span>
<span class="kw4">int</span> data<span class="sy4">;</span> <span class="co1">// private data member</span>
<span class="kw1">class</span> B <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// private nested type</span>
<span class="kw2">enum</span> <span class="br0">{</span> a <span class="sy1">=</span> <span class="nu0">100</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// private enumerator</span>
<span class="kw1">friend</span> <span class="kw1">class</span> X<span class="sy4">;</span> <span class="co1">// friend class forward declaration</span>
<span class="kw1">friend</span> Y<span class="sy4">;</span> <span class="co1">// friend class declaration (since c++11)</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">class</span> X <span class="sy4">:</span> A<span class="sy4">::</span><span class="me2">B</span> <span class="co1">// Error until C++11: the base-clause is not part of member declarations</span>
<span class="co1">// allowed in C++11</span>
<span class="br0">{</span>
A<span class="sy4">::</span><span class="me2">B</span> mx<span class="sy4">;</span> <span class="co1">// OK: A::B accessible to member of friend</span>
<span class="kw1">class</span> Y <span class="sy4">:</span> A<span class="sy4">::</span><span class="me2">B</span> <span class="br0">{</span> <span class="co1">// OK: A::B accessible to base-clause of nested member of friend</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> v<span class="br0">[</span>A<span class="sy4">::</span><span class="me2">a</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// OK: A::a accessible to member of friend</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Friendship is not transitive (a friend of your friend is not your friend)
</p><p>Friendship is not inherited (your friend's children are not your friends)
</p><p>Prior to C++11, member declarations and definitions inside the nested class of the friend of class <code>T</code> cannot access the private and protected members of class <code>T</code>, but some compilers accept it even in pre-C++11 mode.
</p><p>Storage class specifiers are not allowed in friend function declarations. A function that is defined in the friend declaration has external linkage, a function that was previously defined, keeps the linkage it was defined with.
</p><p><a href="access.html" title="cpp/language/access">Access specifiers</a> have no effect on the meaning of friend declarations (they can appear in <code>private:</code> or in <code>public:</code> sections, with no difference)
</p><p>A friend class declaration cannot define a new class (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">friend</span> <span class="kw1">class</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></span></span> is an error)
</p><p>When a local class declares an unqualified function or class as a friend, only functions and classes in the innermost non-class scope are <a href="lookup.html" title="cpp/language/lookup">looked up</a>, not the global functions:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> F <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> f<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="kw4">extern</span> <span class="kw4">int</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">class</span> Local <span class="br0">{</span> <span class="co1">// Local class in the main() function</span>
<span class="kw1">friend</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error, no such function declared in main()</span>
<span class="kw1">friend</span> <span class="kw4">int</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, there is a declaration for g in main()</span>
<span class="kw1">friend</span> <span class="kw1">class</span> F<span class="sy4">;</span> <span class="co1">// friends a local F (defined later)</span>
<span class="kw1">friend</span> <span class="kw1">class</span> <span class="sy4">::</span><span class="me2">F</span><span class="sy4">;</span> <span class="co1">// friends the global F</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> F <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// local F</span>
<span class="br0">}</span></pre></div></div>
<p>A name first declared in a friend declaration within class or class template X becomes a member of the innermost enclosing namespace of X, but is not accessible for lookup (except argument-dependent lookup that considers X) unless a matching declaration at the namespace scope is provided - see <a href="namespace.html#Namespaces" title="cpp/language/namespace">namespaces</a> for details.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=4" title="Edit section: Template friends">edit</a>]</span> <span class="mw-headline" id="Template_friends">Template friends</span></h3>
<p>Both <a href="function_template.html" title="cpp/language/function template">function template</a> and <a href="class_template.html" title="cpp/language/class template">class template</a> declarations may appear with the <code>friend</code> specifier in any non-local class or class template (although only function templates may be defined within the class or class template that is granting friendship). In this case, every specialization of the template becomes a friend, whether it is implicitly instantiated, partially specialized, or explicitly specialized.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> A <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">friend</span> <span class="kw1">class</span> B<span class="sy4">;</span> <span class="co1">// every B<T> is a friend of A</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">friend</span> <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// every f<T> is a friend of A</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Friend declarations cannot refer to partial specializations, but can refer to full specializations:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">class</span> A <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// primary</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">class</span> A<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// partial</span>
<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">class</span> A<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// full</span>
<span class="kw1">class</span> X <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">friend</span> <span class="kw1">class</span> A<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><span class="sy4">;</span> <span class="co1">// error!</span>
<span class="kw1">friend</span> <span class="kw1">class</span> A<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>When a friend declaration refers to a full specialization of a function template, the keyword <code>inline</code> and default arguments cannot be used.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1"><></span> <span class="kw4">void</span> f<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">class</span> X <span class="br0">{</span>
<span class="kw1">friend</span> <span class="kw4">void</span> f<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: default args not allowed</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If a member of a class template A is declared to be a friend of a non-template class B, the corresponding member of every specialization of A becomes a friend of B. If A is explicitly specialized, as long as there is a member of the same name, same kind (type, function, class template, function template), same parameters/signature, it will be a friend of B.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span> <span class="co1">// primary template</span>
<span class="kw1">struct</span> A
<span class="br0">{</span>
<span class="kw1">struct</span> C <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="br0">{</span>
<span class="kw4">void</span> g<span class="br0">(</span><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="kw1">template</span><span class="sy1"><></span> <span class="co1">// full specialization</span>
<span class="kw1">struct</span> A<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span>
<span class="br0">{</span>
<span class="kw1">struct</span> C <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="br0">{</span>
<span class="kw4">void</span> g<span class="br0">(</span><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="kw1">class</span> B <span class="co1">// non-template class</span>
<span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
<span class="kw1">friend</span> <span class="kw1">struct</span> A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">C</span><span class="sy4">;</span> <span class="co1">// A<int>::C is a friend, as well as all A<T>::C</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
<span class="kw1">friend</span> <span class="kw4">void</span> A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// A<int>::f() is not a friend, because the</span>
<span class="co1">// signatures do not match, but A<char>::f() is</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
<span class="kw1">friend</span> <span class="kw4">void</span> A<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">D</span><span class="sy4">::</span><span class="me2">g</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// A<int>::D::g() is not a friend: it is not a member</span>
<span class="co1">// of A, and A<int>::D is not a specialization of A<T>::D</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=5" title="Edit section: Template friend operators">edit</a>]</span> <span class="mw-headline" id="Template_friend_operators">Template friend operators</span></h3>
<p>A common use case for template friends is declaration of a non-member operator overload that acts on a class template, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span>, <span class="kw4">const</span> Foo<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span></span></span> for some user-defined <span class="t-c"><span class="mw-geshi cpp source-cpp">Foo<span class="sy1"><</span>T<span class="sy1">></span></span></span>
</p><p>Such operator can be defined in the class body, which has the effect of generating a separate non-template <code>operator<<</code> for each <code>T</code> and makes that non-template <code>operator<<</code> a friend of its <code>Foo<T></code>
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">class</span> Foo <span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
Foo<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> val<span class="br0">)</span> <span class="sy4">:</span> data<span class="br0">(</span>val<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw1">private</span><span class="sy4">:</span>
T data<span class="sy4">;</span>
 
<span class="co1">// generates a non-template operator<< for this T</span>
<span class="kw1">friend</span> <a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> Foo<span class="sy3">&</span> obj<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> os <span class="sy1"><<</span> obj.<span class="me1">data</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>
Foo<span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span> obj<span class="br0">(</span><span class="nu16">1.23</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> obj <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1.23</pre></div></div>
</div>
<p>or the function template has to be declared as a template before the class body, in which case the friend declaration within <code>Foo<T></code> can refer to the full specialization of <code>operator<<</code> for its <code>T</code>:
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">class</span> Foo<span class="sy4">;</span> <span class="co1">// forward declare to make function declaration possible</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span> <span class="co1">// declaration</span>
<a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span>, <span class="kw4">const</span> Foo<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">class</span> Foo <span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
Foo<span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> val<span class="br0">)</span> <span class="sy4">:</span> data<span class="br0">(</span>val<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw1">private</span><span class="sy4">:</span>
T data<span class="sy4">;</span>
 
<span class="co1">// refers to a full specialization for this particular T </span>
<span class="kw1">friend</span> <a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span> <span class="sy1"><></span> <span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span>, <span class="kw4">const</span> Foo<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// note: this relies on template argument deduction in declarations</span>
<span class="co1">// can also specify the template argument with operator<< <T>"</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// definition</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <span class="kw4">const</span> Foo<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> obj<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> os <span class="sy1"><<</span> obj.<span class="me1">data</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
Foo<span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span> obj<span class="br0">(</span><span class="nu16">1.23</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> obj <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=6" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><p> stream insertion and extraction operators are often declared as non-member friends
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
<span class="co2">#include <sstream></span>
 
<span class="kw1">class</span> MyClass <span class="br0">{</span>
<span class="kw4">int</span> i<span class="sy4">;</span>
 
<span class="kw1">friend</span> <a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> out, <span class="kw4">const</span> MyClass<span class="sy3">&</span> o<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">friend</span> <a href="../io/basic_istream.html"><span class="kw1450">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> operator<span class="sy1">>></span><span class="br0">(</span><a href="../io/basic_istream.html"><span class="kw1450">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> in, MyClass<span class="sy3">&</span> o<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
MyClass<span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy4">:</span> i<span class="br0">(</span>i<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> out, <span class="kw4">const</span> MyClass<span class="sy3">&</span> mc<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> out <span class="sy1"><<</span> mc.<span class="me1">i</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<a href="../io/basic_istream.html"><span class="kw1450">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> operator<span class="sy1">>></span><span class="br0">(</span><a href="../io/basic_istream.html"><span class="kw1450">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> in, MyClass<span class="sy3">&</span> mc<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> in <span class="sy1">>></span> mc.<span class="me1">i</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
MyClass mc<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> mc <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<a href="../io/basic_istringstream.html"><span class="kw1458">std::<span class="me2">istringstream</span></span></a><span class="br0">(</span><span class="st0">"100"</span><span class="br0">)</span> <span class="sy1">>></span> mc<span class="sy4">;</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> mc <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">7
100</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=7" 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> 11.3 Friends [class.friend]
</li></ul>
</dd></dl>
<dl><dd><ul><li> 14.5.4 Friends [temp.friend]
</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> 11.3 Friends [class.friend]
</li></ul>
</dd></dl>
<dl><dd><ul><li> 14.5.3 Friends [temp.friend]
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/friend&action=edit&section=8" title="Edit section: See Also">edit</a>]</span> <span class="mw-headline" id="See_Also">See Also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <a href="class.html" title="cpp/language/class"> Class declaration </a>
</td></tr>
<tr class="t-dsc">
<td colspan="2"> <a href="access.html" title="cpp/language/access"> Access specifiers </a>
</td></tr>
</table>
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