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<tr class="t-nv-h2"><td colspan="5"> Block declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> simple-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type_alias.html" title="cpp/language/type alias"> alias declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace_alias.html" title="cpp/language/namespace alias"> namespace alias definition </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"> using-declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html#Using-directives" title="cpp/language/namespace"> using-directive</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_assert.html" title="cpp/language/static assert"> static_assert declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> asm-definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="enum.html" title="cpp/language/enum"> opaque enum declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Other declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html" title="cpp/language/namespace"> namespace definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html" title="cpp/language/function"> function declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> class template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> function template declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html#Explicit_instantiation" title="cpp/language/class template"> explicit template instantiation</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> explicit template specialization</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="language_linkage.html" title="cpp/language/language linkage"> linkage specification</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> attribute declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> empty declaration</a> </td></tr>
</table></div></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/declarations/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Declares a named variable as a reference, that is, an alias to an already-existing object or function.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="reference.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="reference.html#Lvalue_references"><span class="tocnumber">1.1</span> <span class="toctext">Lvalue references</span></a></li>
<li class="toclevel-2"><a href="reference.html#Rvalue_references"><span class="tocnumber">1.2</span> <span class="toctext">Rvalue references</span></a></li>
<li class="toclevel-2 tocsection-4"><a href="reference.html#Dangling_references"><span class="tocnumber">1.3</span> <span class="toctext">Dangling references</span></a></li>
</ul>
</li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/reference&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<p>A reference variable declaration is any simple declaration whose <a href="declarations.html" title="cpp/language/declarations">declarator</a> has the form
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>&</b></code> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">declarator</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>&&</b></code> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">declarator</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> <b>Lvalue reference declarator</b>: the declaration <span class="t-c"><span class="mw-geshi cpp source-cpp">S<span class="sy3">&</span> D<span class="sy4">;</span></span></span> declares <code>D</code> as an <i>lvalue reference</i> to the type determined by <span class="t-spar">decl-specifier-seq</span> <code>S</code>.</div>
<div class="t-li1"><span class="t-li">2)</span> <b>Rvalue reference declarator</b>: the declaration <span class="t-c"><span class="mw-geshi cpp source-cpp">S<span class="sy3">&&</span> D<span class="sy4">;</span></span></span> declares <code>D</code> as an <i>rvalue reference</i> to the type determined by <span class="t-spar">decl-specifier-seq</span> <code>S</code>.</div>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">declarator</span>
</td>
<td> -
</td>
<td> any <a href="declarations.html" title="cpp/language/declarations">declarator</a> except another reference declarator (there are no references to references)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> optional list of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr></table>
<p>A reference is required to be initialized to refer to a valid object or function: see <a href="reference_initialization.html" title="cpp/language/reference initialization">reference initialization</a>.
</p><p>There are no references to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> and no references to references.
</p><p>Reference types cannot be <a href="cv.html" title="cpp/language/cv">cv-qualified</a> at the top level; there is no syntax for that in declaration, and if a qualification is introduced through a typedef, decltype, or template type argument, it is ignored.
</p><p>References are not objects; they do not necessarily occupy storage, although the compiler may allocate storage if it is necessary to implement the desired semantics (e.g. a non-static data member of reference type usually increases the size of the class by the amount necessary to store a memory address).
</p><p>Because references are not objects, there are no arrays of references, no pointers to references, and no references to references:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span><span class="sy3">&</span> a<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// error</span>
<span class="kw4">int</span><span class="sy3">&</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// error</span>
<span class="kw4">int</span><span class="sy3">&</span> <span class="sy3">&</span>r<span class="sy4">;</span> <span class="co1">// error</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>It is permitted to form references to references through type manipulations in templates or typedefs, in which case the <i>reference collapsing</i> rules apply: rvalue reference to rvalue reference collapses to rvalue reference, all other combinations form lvalue reference:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">typedef</span> <span class="kw4">int</span><span class="sy3">&</span> lref<span class="sy4">;</span>
<span class="kw1">typedef</span> <span class="kw4">int</span><span class="sy3">&&</span> rref<span class="sy4">;</span>
<span class="kw4">int</span> n<span class="sy4">;</span>
lref<span class="sy3">&</span> r1 <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// type of r1 is int&</span>
lref<span class="sy3">&&</span> r2 <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// type of r2 is int&</span>
rref<span class="sy3">&</span> r3 <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// type of r3 is int&</span>
rref<span class="sy3">&&</span> r4 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// type of r4 is int&&</span></pre></div></div>
<p>(This, along with special rules for <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> when <code>T&&</code> is used in a function template, forms the rules that make <span class="t-lc">std::forward</span> possible.)
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/reference&action=edit&section=2" title="Edit section: Lvalue references">edit</a>]</span> <span class="mw-headline" id="Lvalue_references">Lvalue references</span></h4>
<p>Lvalue references can be used to alias an existing object (optionally with different cv-qualification):
</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="co2">#include <string></span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"Ex"</span><span class="sy4">;</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> r1 <span class="sy1">=</span> s<span class="sy4">;</span>
<span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> r2 <span class="sy1">=</span> s<span class="sy4">;</span>
 
r1 <span class="sy2">+</span><span class="sy1">=</span> <span class="st0">"ample"</span><span class="sy4">;</span> <span class="co1">// modifies s</span>
<span class="co1">// r2 += "!"; // error: cannot modify through reference to const</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> r2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints s, which now holds "Example"</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<p>They can also be used to implement pass-by-reference semantics in function calls:
</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="co2">#include <string></span>
 
<span class="kw4">void</span> double_string<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> s<span class="br0">)</span>
<span class="br0">{</span>
s <span class="sy2">+</span><span class="sy1">=</span> s<span class="sy4">;</span> <span class="co1">// 's' is the same object as main()'s 'str'</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span>
double_string<span class="br0">(</span>str<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> str <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>
<p>When a function's return type is lvalue reference, the function call expression becomes an <a href="value_category.html" title="cpp/language/value category">lvalue expression</a>:
</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="co2">#include <string></span>
 
<span class="kw4">char</span><span class="sy3">&</span> char_number<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> s, <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> s.<span class="me1">at</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// string::at() returns a reference to char</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span>
char_number<span class="br0">(</span>str, <span class="nu0">1</span><span class="br0">)</span> <span class="sy1">=</span> <span class="st0">'a'</span><span class="sy4">;</span> <span class="co1">// the function call is lvalue, can be assigned to</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> str <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>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<h4> <span class="mw-headline" id="Rvalue_references">Rvalue references</span></h4>
<p>Rvalue references can be used to <a href="reference_initialization.html" title="cpp/language/reference initialization">extend the lifetimes</a> of temporary objects (note, lvalue references to const can extend the lifetimes of temporary objects too, but they are not modifiable through them):
</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="co2">#include <string></span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s1 <span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span>
<span class="co1">// std::string&& r1 = s1; // error: can't bind to lvalue</span>
 
<span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> r2 <span class="sy1">=</span> s1 <span class="sy2">+</span> s1<span class="sy4">;</span> <span class="co1">// okay: lvalue reference to const extends lifetime</span>
<span class="co1">// r2 += "Test"; // error: can't modify through reference to const</span>
 
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&&</span> r3 <span class="sy1">=</span> s1 <span class="sy2">+</span> s1<span class="sy4">;</span> <span class="co1">// okay: rvalue reference extends lifetime</span>
r3 <span class="sy2">+</span><span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span> <span class="co1">// okay: can modify through reference to non-const</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> r3 <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>
<p>More importantly, when a function has both rvalue reference and lvalue reference <a href="overload_resolution.html" title="cpp/language/overload resolution">overloads</a>, the rvalue reference overload binds to rvalues, while the lvalue reference overload binds to lvalues:
</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="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy3">&</span> x<span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"lvalue reference overload f("</span> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&</span> x<span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"lvalue reference to const overload f("</span> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy3">&&</span> x<span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"rvalue reference overload f("</span> <span class="sy1"><<</span> x <span class="sy1"><<</span> <span class="st0">")<span class="es1">\n</span>"</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>
<span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span> ci <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
f<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(int&)</span>
f<span class="br0">(</span>ci<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(const int&)</span>
f<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(int&&)</span>
<span class="co1">// would call f(const int&) if f(int&&) overload wasn't provided</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<p>This allows <a href="move_constructor.html" title="cpp/language/move constructor">move constructors</a>, <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect">move assignment</a> operators, and other move-aware functions (e.g. <a href="../container/vector/push_back.html" title="cpp/container/vector/push back"><tt>vector::push_back()</tt></a> to be automatically selected when suitable.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/reference&action=edit&section=4" title="Edit section: Dangling references">edit</a>]</span> <span class="mw-headline" id="Dangling_references">Dangling references</span></h4>
<p>Although references, once initialized, always refer to valid objects or functions, it is possible to create a program where the <a href="lifetime.html" title="cpp/language/lifetime">lifetime</a> of the referred-to object ends, but the reference remains accessible (<i>dangling</i>). Accessing such a reference is undefined behavior.
A common example is a function returning a reference to an automatic variable:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"Example"</span><span class="sy4">;</span>
<span class="kw1">return</span> s<span class="sy4">;</span> <span class="co1">// exits the scope of s: its destructor is called and its storage deallocated</span>
<span class="br0">}</span>
 
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> r <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// dangling reference</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> r<span class="sy4">;</span> <span class="co1">// undefined behavior: reads from a dangling reference</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// undefined behavior: copy-initializes from a dangling reference</span></pre></div></div>
<p>Note that rvalue references and lvalue references to const extend the lifetimes of temporary objects (see <a href="reference_initialization.html" title="cpp/language/reference initialization">reference initialization</a> for rules and exceptions).
</p><p>If the referred-to object was destroyed (e.g. by explicit destructor call), but the storage was not deallocated, a reference to the out-of-lifetime object may be used in limited ways, and may become valid if the object is recreated in the same storage (see <a href="lifetime.html#Access_outside_of_lifetime" title="cpp/language/lifetime">Access outside of lifetime</a> for details).
</p>
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