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<tr class="t-nv"><td colspan="5"> <a href="constant_expression.html" title="cpp/language/constant expression"> constant expressions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="expressions.html#Unevaluated_expressions" title="cpp/language/expressions"> unevaluated expressions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="expressions.html#Primary_expressions" title="cpp/language/expressions"> primary expressions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lambda.html" title="cpp/language/lambda">lambda-expression</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Literals </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="integer_literal.html" title="cpp/language/integer literal">integer literals</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="floating_literal.html" title="cpp/language/floating literal">floating-point literals</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bool_literal.html" title="cpp/language/bool literal">boolean literals</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="character_literal.html" title="cpp/language/character literal">character literals</a> including <a href="escape.html" title="cpp/language/escape">escape sequences</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="string_literal.html" title="cpp/language/string literal">string literals</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nullptr.html" title="cpp/language/nullptr">null pointer literal</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="user_literal.html" title="cpp/language/user literal">user-defined literal</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Operators </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_assignment.html" title="cpp/language/operator assignment"> Assignment operators</a>:
<code>a=b</code>, <code>a+=b</code>, <code>a-=b</code>, <code>a*=b</code>, <code>a/=b</code>, <code>a%=b</code>, <code>a&=b</code>, <code>a|=b</code>, <code>a^=b</code>, <code>a<<=b</code>, <code>a>>=</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_incdec.html" title="cpp/language/operator incdec"> Increment and decrement</a>: <code>++a</code>, <code>--a</code>, <code>a++</code>, <code>a--</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_arithmetic.html" title="cpp/language/operator arithmetic"> Arithmetic operators</a>:
<code>+a</code>, <code>-a</code>, <code>a+b</code>, <code>a-b</code>, <code>a*b</code>, <code>a/b</code>, <code>a%b</code>, <code>~a</code>, <code>a&b</code>, <code>a|b</code>, <code>a^b</code>, <code>a<<b</code>, <code>a>>b</code> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_logical.html" title="cpp/language/operator logical"> Logical operators</a>: <code>a||b</code>, <code>a&&b</code>, <code>!a</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_comparison.html" title="cpp/language/operator comparison"> Comparison operators</a>: <code>a==b</code>, <code>a!=b</code>, <code>a<b</code>, <code>a>b</code>, <code>a<=b</code>, <code>a>=b</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_member_access.html" title="cpp/language/operator member access"> Member access operators</a>: <code>a[b]</code>, <code>*a</code>, <code>&a</code>, <code>a->b</code>, <code>a.b</code>, <code>a->*b</code>, <code>a.*b</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_other.html" title="cpp/language/operator other"> Other operators</a>: <code>a(...)</code>, <code>a,b</code>, <code>a?b:c</code> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_alternative.html" title="cpp/language/operator alternative">Alternative representations of operators</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_precedence.html" title="cpp/language/operator precedence">Precedence and associativity</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="fold.html" title="cpp/language/fold">Fold expression</a><span class="t-mark-rev t-since-cxx17">(C++17)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new">new-expression</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete">delete-expression</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="throw.html" title="cpp/language/throw">throw-expression</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="alignof.html" title="cpp/language/alignof">alignof</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sizeof.html" title="cpp/language/sizeof">sizeof</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sizeof....html" title="cpp/language/sizeof...">sizeof...</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="typeid.html" title="cpp/language/typeid">typeid</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="noexcept.html" title="cpp/language/noexcept">noexcept</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">Operator overloading</strong></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Conversions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit cast">Implicit conversions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast">const_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static_cast.html" title="cpp/language/static cast">static_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit_cast.html" title="cpp/language/explicit cast">Explicit conversions</a> <code>(T)a</code>, <code>T(a)</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="cast_operator.html" title="cpp/language/cast operator">User-defined conversion</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/expressions/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Customizes the C++ operators for operands of user-defined types.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="operators.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="operators.html#Overloaded_operators"><span class="tocnumber">2</span> <span class="toctext">Overloaded operators</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="operators.html#Restrictions"><span class="tocnumber">3</span> <span class="toctext">Restrictions</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="operators.html#Canonical_implementations"><span class="tocnumber">4</span> <span class="toctext">Canonical implementations</span></a>
<ul>
<li class="toclevel-2 tocsection-5"><a href="operators.html#Assignment_operator"><span class="tocnumber">4.1</span> <span class="toctext">Assignment operator</span></a></li>
<li class="toclevel-2 tocsection-6"><a href="operators.html#Stream_extraction_and_insertion"><span class="tocnumber">4.2</span> <span class="toctext">Stream extraction and insertion</span></a></li>
<li class="toclevel-2 tocsection-7"><a href="operators.html#Function_call_operator"><span class="tocnumber">4.3</span> <span class="toctext">Function call operator</span></a></li>
<li class="toclevel-2 tocsection-8"><a href="operators.html#Increment_and_decrement"><span class="tocnumber">4.4</span> <span class="toctext">Increment and decrement</span></a></li>
<li class="toclevel-2 tocsection-9"><a href="operators.html#Binary_arithmetic_operators"><span class="tocnumber">4.5</span> <span class="toctext">Binary arithmetic operators</span></a></li>
<li class="toclevel-2 tocsection-10"><a href="operators.html#Relational_operators"><span class="tocnumber">4.6</span> <span class="toctext">Relational operators</span></a></li>
<li class="toclevel-2 tocsection-11"><a href="operators.html#Array_subscript_operator"><span class="tocnumber">4.7</span> <span class="toctext">Array subscript operator</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-12"><a href="operators.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-13"><a href="operators.html#See_Also"><span class="tocnumber">6</span> <span class="toctext">See Also</span></a></li>
<li class="toclevel-1 tocsection-14"><a href="operators.html#References"><span class="tocnumber">7</span> <span class="toctext">References</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<p>Overloaded operators are <a href="functions.html" title="cpp/language/functions">functions</a> with special function names:
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>operator</b></code> <span class="t-spar">op</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>operator</b></code> <span class="t-spar">type</span>
</td>
<td> (2)
</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>operator</b></code> <code><b>new</b></code> <br /> <code><b>operator</b></code> <code><b>new []</b></code>
</td>
<td> (3)
</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>operator</b></code> <code><b>delete</b></code> <br /> <code><b>operator</b></code> <code><b>delete []</b></code>
</td>
<td> (4)
</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>operator</b></code> <code><b>""</b></code> <span class="t-spar">suffix-identifier</span>
</td>
<td> (5)
</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>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">op</span>
</td>
<td> -
</td>
<td> any of the following 38 operators:<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">ˆ</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">~</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">></span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">ˆ<span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><<</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">>></span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">>>=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><<=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">>=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&&</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">--</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">></span><span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">></span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span> <span class="br0">)</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span> <span class="br0">]</span></span></span>
<p>.
</p>
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> overloaded operator</div>
<div class="t-li1"><span class="t-li">2)</span> <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion function</a></div>
<div class="t-li1"><span class="t-li">3)</span> <a href="../memory/new/operator_new.html" title="cpp/memory/new/operator new">allocation function</a></div>
<div class="t-li1"><span class="t-li">4)</span> <a href="../memory/new/operator_delete.html" title="cpp/memory/new/operator delete">deallocation function</a></div>
<div class="t-li1"><span class="t-li">5)</span> <a href="user_literal.html" title="cpp/language/user literal">user-defined literal</a></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=2" title="Edit section: Overloaded operators">edit</a>]</span> <span class="mw-headline" id="Overloaded_operators">Overloaded operators</span></h3>
<p>When an operator appears in an <a href="expressions.html" title="cpp/language/expressions">expression</a>, and at least one of its operands has a <a href="class.html" title="cpp/language/class">class type</a> or an <a href="enum.html" title="cpp/language/enum">enumeration type</a>, then <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a> is used to determine the user-defined function to be called among all the functions whose signatures match the following:
</p>
<table class="wikitable" style="font-size:85%; text-align:left;">
<tr>
<th> Expression
</th>
<th> As member function
</th>
<th> As non-member function
</th>
<th> Example
</th></tr>
<tr>
<td> @a
</td>
<td> (a).operator@ ( )
</td>
<td> operator@ (a)
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><a href="../io/cin.html"><span class="kw1479">std::<span class="me2">cin</span></span></a></span></span> calls <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../io/cin.html"><span class="kw1479">std::<span class="me2">cin</span></span></a>.<span class="me1">operator</span><span class="sy3">!</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> a@b
</td>
<td> (a).operator@ (b)
</td>
<td> operator@ (a, b)
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="nu0">42</span></span></span> calls <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a>.<span class="me1">operator</span><span class="sy1"><<</span><span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> a=b
</td>
<td> (a).operator= (b)
</td>
<td style="background:#ff9090; color:black; vertical-align: middle; text-align: center;" class="table-no"> cannot be non-member
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span> s <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span></span></span> calls <span class="t-c"><span class="mw-geshi cpp source-cpp">s.<span class="me1">operator</span><span class="sy1">=</span><span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> a[b]
</td>
<td> (a).operator[](b)
</td>
<td style="background:#ff9090; color:black; vertical-align: middle; text-align: center;" class="table-no"> cannot be non-member
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../container/map.html"><span class="kw1100">std::<span class="me2">map</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">></span> m<span class="sy4">;</span> m<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span></span></span> calls <span class="t-c"><span class="mw-geshi cpp source-cpp">m.<span class="me1">operator</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> a->
</td>
<td> (a).operator-> ( )
</td>
<td style="background:#ff9090; color:black; vertical-align: middle; text-align: center;" class="table-no"> cannot be non-member
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../memory/unique_ptr.html"><span class="kw614">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>S<span class="sy1">></span> ptr<span class="br0">(</span>new S<span class="br0">)</span><span class="sy4">;</span> ptr<span class="sy2">-</span><span class="sy1">></span>bar<span class="br0">(</span><span class="br0">)</span></span></span> calls <span class="t-c"><span class="mw-geshi cpp source-cpp">ptr.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> a@
</td>
<td> (a).operator@ (0)
</td>
<td> operator@ (a, 0)
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../container/vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">iterator</span> i <span class="sy1">=</span> v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> i<span class="sy2">++</span></span></span> calls <span class="t-c"><span class="mw-geshi cpp source-cpp">i.<span class="me1">operator</span><span class="sy2">++</span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td colspan="4">
<p>in this table, <code><b>@</b></code> is a placeholder representing all matching operators: all prefix operators in @a, all postfix operators other than -> in a@, all infix operators other than = in a@b
</p>
</td></tr></table>
<p>Note: for overloading <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion functions</a>, <a href="user_literal.html" title="cpp/language/user literal">user-defined literals</a>, <a href="../memory/new/operator_new.html" title="cpp/memory/new/operator new">allocation</a> and <a href="../memory/new/operator_new.html" title="cpp/memory/new/operator new">deallocation</a> see their respective articles.
</p><p>Overloaded operators (but not the built-in operators) can be called using function notation:
</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> str <span class="sy1">=</span> <span class="st0">"Hello, "</span><span class="sy4">;</span>
str.<span class="me1">operator</span><span class="sy2">+</span><span class="sy1">=</span><span class="br0">(</span><span class="st0">"world"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same as str += "world";</span>
operator<span class="sy1"><<</span><span class="br0">(</span>operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a>, str<span class="br0">)</span> , <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same as std::cout << str << '\n';</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=3" title="Edit section: Restrictions">edit</a>]</span> <span class="mw-headline" id="Restrictions">Restrictions</span></h3>
<ul><li> The operators <code><b>::</b></code> (scope resolution), <code><b>.</b></code> (member access), <code><b>.*</b></code> (member access through pointer to member), and <code><b>?:</b></code> (ternary conditional) cannot be overloaded
</li><li> New operators such as <code><b>**</b></code>, <code><b><></b></code>, or <code><b>&|</b></code> cannot be created
</li><li> The overloads of operators <code><b>&&</b></code>, <code><b>||</b></code>, and <code><b>,</b></code> (comma) lose their special properties: short-circuit evaluation and <a href="eval_order.html" title="cpp/language/eval order">sequencing</a>.
</li><li> The overload of operator <code><b>-></b></code> must either return a raw pointer or return an object (by reference or by value), for which operator <code><b>-></b></code> is in turn overloaded.
</li><li> It is not possible to change the precedence, grouping, or number of operands of operators.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=4" title="Edit section: Canonical implementations">edit</a>]</span> <span class="mw-headline" id="Canonical_implementations">Canonical implementations</span></h3>
<p>Other than the restrictions above, the language puts no other constraints on what the overloaded operators do, or on the return type (it does not participate in overload resolution), but in general, overloaded operators are expected to behave as similar as possible to the built-in operators: <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> is expected to add, rather than multiply its arguments, <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">=</span></span></span> is expected to assign, etc. The related operators are expected to behave similarly (<span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span><span class="sy1">=</span></span></span> do the same addition-like operation). The return types are limited by the expressions in which the operator is expected to be used: for example, assignment operators return by reference to make it possible to write <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="sy1">=</span>b<span class="sy1">=</span>c<span class="sy1">=</span>d</span></span>, because the built-in operators allow that.
Commonly overloaded operators have the following typical, canonical forms:<sup id="cite_ref-1" class="reference"><a href="operators.html#cite_note-1">[1]</a></sup>
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=5" title="Edit section: Assignment operator">edit</a>]</span> <span class="mw-headline" id="Assignment_operator">Assignment operator</span></h4>
<p>The assignment operator (<span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">=</span></span></span>) has special properties: see <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect">copy assignment</a> and <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect">move_assignment</a> for details. To summarize, the canonical "universal assignment operator" implementation is
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1">T<span class="sy3">&</span> T<span class="sy4">::</span><span class="me2">operator</span><span class="sy1">=</span><span class="br0">(</span>T arg<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// copy/move constructor is called to construct arg</span>
swap<span class="br0">(</span>arg<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// resources exchanged between *this and arg</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
<span class="br0">}</span> <span class="co1">// destructor is called to release the resources formerly held by *this</span></pre></div></div>
<p>When there are resources that can be reused in assignment, for example, if the class owns a heap-allocated array, then copy-assignment between arrays of the same size can avoid allocation and deallocation:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1">T<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span> other<span class="br0">)</span> <span class="co1">// copy assignment</span>
<span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span>this <span class="sy3">!</span><span class="sy1">=</span> <span class="sy3">&</span>other<span class="br0">)</span> <span class="br0">{</span> <span class="co1">// self-assignment check expected</span>
<span class="kw1">if</span> <span class="br0">(</span><span class="coMULTI">/* storage cannot be reused (e.g. different sizes) */</span><span class="br0">)</span>
<span class="br0">{</span>
delete<span class="br0">[</span><span class="br0">]</span> mArray<span class="sy4">;</span> <span class="co1">// destroy storage in this</span>
<span class="coMULTI">/* reset size to zero and mArray to null, in case allocation throws */</span>
mArray <span class="sy1">=</span> new <span class="kw4">int</span><span class="br0">[</span><span class="coMULTI">/*size*/</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// create storage in this</span>
<span class="br0">}</span>
<span class="coMULTI">/* copy data from other's storage to this storage */</span>
<span class="br0">}</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
<span class="br0">}</span>
T<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>T<span class="sy3">&&</span> other<span class="br0">)</span> <span class="co1">// move assignment</span>
<span class="br0">{</span>
<a href="../error/assert.html"><span class="kw654">assert</span></a><span class="br0">(</span>this <span class="sy3">!</span><span class="sy1">=</span> <span class="sy3">&</span>other<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// self-assignment check not required</span>
delete<span class="br0">[</span><span class="br0">]</span> mArray<span class="sy4">;</span> <span class="co1">// delete this storage</span>
mArray <span class="sy1">=</span> other.<span class="me1">mArray</span><span class="sy4">;</span> <span class="co1">// move</span>
other.<span class="me1">mArray</span> <span class="sy1">=</span> nullptr<span class="sy4">;</span> <span class="co1">// leave moved-from in valid state</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=6" title="Edit section: Stream extraction and insertion">edit</a>]</span> <span class="mw-headline" id="Stream_extraction_and_insertion">Stream extraction and insertion</span></h4>
<p>The overloads of <code>operator>></code> and <code>operator<<</code> that take a <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../io/basic_istream.html"><span class="kw1450">std::<span class="me2">istream</span></span></a><span class="sy3">&</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../io/basic_ostream.html"><span class="kw1452">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span></span></span> as the left hand argument are known as insertion and extraction operators. Since they take the user-defined type as the right argument (<code>b</code> in <span class="t-spar">a@b</span>), they must be implemented as non-members.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><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> T<span class="sy3">&</span> obj<span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// write obj to stream</span>
<span class="kw1">return</span> os<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> is, T<span class="sy3">&</span> obj<span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// read obj from stream</span>
<span class="kw1">if</span><span class="br0">(</span> <span class="coMULTI">/* T could not be constructed */</span> <span class="br0">)</span>
is.<span class="me1">setstate</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ios</span><span class="sy4">::</span><span class="me2">failbit</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">return</span> is<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>These operators are sometimes implemented as <a href="friend.html" title="cpp/language/friend">friend functions</a>.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=7" title="Edit section: Function call operator">edit</a>]</span> <span class="mw-headline" id="Function_call_operator">Function call operator</span></h4>
<p>When a user-defined class overloads the function call operator, <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">(</span><span class="br0">)</span></span></span>, it becomes a <a href="../concept/FunctionObject.html" title="cpp/concept/FunctionObject"><code>FunctionObject</code></a> type. Many standard algorithms, from <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../algorithm/sort.html"><span class="kw1351">std::<span class="me2">sort</span></span></a></span></span> to <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../algorithm/accumulate.html"><span class="kw1383">std::<span class="me2">accumulate</span></span></a></span></span> accept objects of such types to customize behavior. There are no particularly notable canonical forms of <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">(</span><span class="br0">)</span></span></span>, but to illustrate the usage
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Sum <span class="br0">{</span>
<span class="kw4">int</span> sum<span class="sy4">;</span>
Sum<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> sum<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw4">void</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span> sum <span class="sy2">+</span><span class="sy1">=</span> n<span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
Sum s <span class="sy1">=</span> <a href="../algorithm/for_each.html"><span class="kw1302">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, Sum<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=8" title="Edit section: Increment and decrement">edit</a>]</span> <span class="mw-headline" id="Increment_and_decrement">Increment and decrement</span></h4>
<p>When the postfix increment and decrement appear in an expression, the corresponding user-defined function (<span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">++</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">--</span></span></span>) is called with an integer argument <code>0</code>. Typically, it is implemented as <span class="t-c"><span class="mw-geshi cpp source-cpp">T operator<span class="sy2">++</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span></span></span>, where the argument is ignored. The postfix increment and decrement operator is usually implemented in terms of the prefix version:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> X <span class="br0">{</span>
X<span class="sy3">&</span> operator<span class="sy2">++</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<span class="co1">// actual increment takes place here</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
<span class="br0">}</span>
X operator<span class="sy2">++</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span>
X tmp<span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// copy</span>
operator<span class="sy2">++</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// pre-increment</span>
<span class="kw1">return</span> tmp<span class="sy4">;</span> <span class="co1">// return old value</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Although canonical form of pre-increment/pre-decrement returns a reference, as with any operator overload, the return type is user-defined; for example the overloads of these operators for <span class="t-lc">std::atomic</span> return by value.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=9" title="Edit section: Binary arithmetic operators">edit</a>]</span> <span class="mw-headline" id="Binary_arithmetic_operators">Binary arithmetic operators</span></h4>
<p>Binary operators are typically implemented as non-members to maintain symmetry (for example, when adding a complex number and an integer, if <code>operator+</code> is a member function of the complex type, then only <span class="t-c"><span class="mw-geshi cpp source-cpp">complex<span class="sy2">+</span>integer</span></span> would compile, and not <span class="t-c"><span class="mw-geshi cpp source-cpp">integer<span class="sy2">+</span>complex</span></span>). Since for every binary arithmetic operator there exists a corresponding compound assignment operator, canonical forms of binary operators are implemented in terms of their compound assignments:
</p>
<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="kw1">public</span><span class="sy4">:</span>
X<span class="sy3">&</span> operator<span class="sy2">+</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span> <span class="co1">// compound assignment (does not need to be a member,</span>
<span class="br0">{</span> <span class="co1">// but often is, to modify the private members)</span>
<span class="coMULTI">/* addition of rhs to *this takes place here */</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span> <span class="co1">// return the result by reference</span>
<span class="br0">}</span>
 
<span class="co1">// friends defined inside class body are inline and are hidden from non-ADL lookup</span>
<span class="kw1">friend</span> X operator<span class="sy2">+</span><span class="br0">(</span>X lhs, <span class="co1">// passing lhs by value helps optimize chained a+b+c</span>
<span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span> <span class="co1">// otherwise, both parameters may be const references.</span>
<span class="br0">{</span>
lhs <span class="sy2">+</span><span class="sy1">=</span> rhs<span class="sy4">;</span> <span class="co1">// reuse compound assignment</span>
<span class="kw1">return</span> lhs<span class="sy4">;</span> <span class="co1">// return the result by value (uses move constructor)</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=10" title="Edit section: Relational operators">edit</a>]</span> <span class="mw-headline" id="Relational_operators">Relational operators</span></h4>
<p>Standard algorithms such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../algorithm/sort.html"><span class="kw1351">std::<span class="me2">sort</span></span></a></span></span> and containers such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../container/set.html"><span class="kw1098">std::<span class="me2">set</span></span></a></span></span> expect <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> to be defined, by default, for the user-provided types, and expect it to implement strict weak ordering (thus satisfying the <a href="../concept/Compare.html" title="cpp/concept/Compare"><code>Compare</code></a> concept). An idiomatic way to implement strict weak ordering for a structure is to use lexicographical comparison provided by <span class="t-lc">std::tie</span>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Record <span class="br0">{</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> name<span class="sy4">;</span>
<span class="kw4">unsigned</span> <span class="kw4">int</span> floor<span class="sy4">;</span>
<span class="kw4">double</span> weight<span class="sy4">;</span>
<span class="kw1">friend</span> <span class="kw4">bool</span> operator<span class="sy1"><</span><span class="br0">(</span><span class="kw4">const</span> Record<span class="sy3">&</span> l, <span class="kw4">const</span> Record<span class="sy3">&</span> r<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> <a href="../utility/tuple/tie.html"><span class="kw943">std::<span class="me2">tie</span></span></a><span class="br0">(</span>l.<span class="me1">name</span>, l.<span class="me1">floor</span>, l.<span class="me1">weight</span><span class="br0">)</span>
<span class="sy1"><</span> <a href="../utility/tuple/tie.html"><span class="kw943">std::<span class="me2">tie</span></span></a><span class="br0">(</span>r.<span class="me1">name</span>, r.<span class="me1">floor</span>, r.<span class="me1">weight</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// keep the same order</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Typically, once <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> is provided, the other relational operators are implemented in terms of <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">inline</span> <span class="kw4">bool</span> operator<span class="sy1"><</span> <span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> lhs, <span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span><span class="br0">{</span> <span class="coMULTI">/* do actual comparison */</span> <span class="br0">}</span>
<span class="kw1">inline</span> <span class="kw4">bool</span> operator<span class="sy1">></span> <span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> lhs, <span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span><span class="br0">{</span><span class="kw1">return</span> rhs <span class="sy1"><</span> lhs<span class="sy4">;</span><span class="br0">}</span>
<span class="kw1">inline</span> <span class="kw4">bool</span> operator<span class="sy1"><=</span><span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> lhs, <span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span><span class="br0">{</span><span class="kw1">return</span> <span class="sy3">!</span><span class="br0">(</span>lhs <span class="sy1">></span> rhs<span class="br0">)</span><span class="sy4">;</span><span class="br0">}</span>
<span class="kw1">inline</span> <span class="kw4">bool</span> operator<span class="sy1">>=</span><span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> lhs, <span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span><span class="br0">{</span><span class="kw1">return</span> <span class="sy3">!</span><span class="br0">(</span>lhs <span class="sy1"><</span> rhs<span class="br0">)</span><span class="sy4">;</span><span class="br0">}</span></pre></div></div>
<p>Likewise, the inequality operator is typically implemented in terms of <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">==</span></span></span>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">inline</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> lhs, <span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span><span class="br0">{</span> <span class="coMULTI">/* do actual comparison */</span> <span class="br0">}</span>
<span class="kw1">inline</span> <span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> X<span class="sy3">&</span> lhs, <span class="kw4">const</span> X<span class="sy3">&</span> rhs<span class="br0">)</span><span class="br0">{</span><span class="kw1">return</span> <span class="sy3">!</span><span class="br0">(</span>lhs <span class="sy1">==</span> rhs<span class="br0">)</span><span class="sy4">;</span><span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=11" title="Edit section: Array subscript operator">edit</a>]</span> <span class="mw-headline" id="Array_subscript_operator">Array subscript operator</span></h4>
<p>User-defined classes that provide array-like access that allows both reading and writing typically define two overloads for <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">[</span><span class="br0">]</span></span></span>: const and non-const variants:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> T <span class="br0">{</span>
value_t<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> idx<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> mVector<span class="br0">[</span>idx<span class="br0">]</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">const</span> value_t<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> idx<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> mVector<span class="br0">[</span>idx<span class="br0">]</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If the value type is known to be a built-in type, the const variant should return by value.
</p><p>Where direct access to the elements of the container is not wanted or not possible or distinguishing between l-value <span class="t-c"><span class="mw-geshi cpp source-cpp">c<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1">=</span> v<span class="sy4">;</span></span></span> and r-value <span class="t-c"><span class="mw-geshi cpp source-cpp">v <span class="sy1">=</span> c<span class="br0">[</span>i<span class="br0">]</span><span class="sy4">;</span></span></span> usage, operator[] may return a proxy. see for example <span class="t-lc">std::bitset::operator[]</span>.
</p><p>To provide multidimensional array access semantics, e.g. to implement a 3D array access <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">[</span>i<span class="br0">]</span><span class="br0">[</span>j<span class="br0">]</span><span class="br0">[</span>k<span class="br0">]</span> <span class="sy1">=</span> x<span class="sy4">;</span></span></span>, operator[] has to return a reference to a 2D plane, which has to have its own operator[] which returns a reference to a 1D row, which has to have operator[] which returns a reference to the element. To avoid this complexity, some libraries opt for overloading <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">(</span><span class="br0">)</span></span></span> instead, so that 3D access expressions have the Fortran-like syntax <span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">(</span>i,j,k<span class="br0">)</span> <span class="sy1">=</span> x<span class="sy4">;</span></span></span>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=12" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example"> Example </span></h3>
<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">class</span> Fraction
<span class="br0">{</span>
<span class="kw4">int</span> gcd<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b<span class="br0">)</span> <span class="br0">{</span><span class="kw1">return</span> b<span class="sy1">==</span><span class="nu0">0</span> <span class="sy4">?</span> a <span class="sy4">:</span> gcd<span class="br0">(</span>b,a<span class="sy2">%</span>b<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">int</span> n, d<span class="sy4">;</span>
<span class="kw1">public</span><span class="sy4">:</span>
Fraction<span class="br0">(</span><span class="kw4">int</span> n, <span class="kw4">int</span> d <span class="sy1">=</span> <span class="nu0">1</span><span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span>n<span class="sy2">/</span>gcd<span class="br0">(</span>n,d<span class="br0">)</span><span class="br0">)</span>, d<span class="br0">(</span>d<span class="sy2">/</span>gcd<span class="br0">(</span>n,d<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw4">int</span> num<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> n<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">int</span> den<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> d<span class="sy4">;</span> <span class="br0">}</span>
Fraction<span class="sy3">&</span> operator<span class="sy2">*</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> Fraction<span class="sy3">&</span> rhs<span class="br0">)</span> <span class="br0">{</span>
<span class="kw4">int</span> new_n <span class="sy1">=</span> n<span class="sy2">*</span>rhs.<span class="me1">n</span> <span class="sy2">/</span> gcd<span class="br0">(</span>n<span class="sy2">*</span>rhs.<span class="me1">n</span>, d<span class="sy2">*</span>rhs.<span class="me1">d</span><span class="br0">)</span><span class="sy4">;</span>
d <span class="sy1">=</span> d<span class="sy2">*</span>rhs.<span class="me1">d</span> <span class="sy2">/</span> gcd<span class="br0">(</span>n<span class="sy2">*</span>rhs.<span class="me1">n</span>, d<span class="sy2">*</span>rhs.<span class="me1">d</span><span class="br0">)</span><span class="sy4">;</span>
n <span class="sy1">=</span> new_n<span class="sy4">;</span>
<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</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> Fraction<span class="sy3">&</span> f<span class="br0">)</span><span class="br0">{</span>
<span class="kw1">return</span> out <span class="sy1"><<</span> f.<span class="me1">num</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'/'</span> <span class="sy1"><<</span> f.<span class="me1">den</span><span class="br0">(</span><span class="br0">)</span> <span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> Fraction<span class="sy3">&</span> lhs, <span class="kw4">const</span> Fraction<span class="sy3">&</span> rhs<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> lhs.<span class="me1">num</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> rhs.<span class="me1">num</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">&&</span> lhs.<span class="me1">den</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> rhs.<span class="me1">den</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> Fraction<span class="sy3">&</span> lhs, <span class="kw4">const</span> Fraction<span class="sy3">&</span> rhs<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> <span class="sy3">!</span><span class="br0">(</span>lhs <span class="sy1">==</span> rhs<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
Fraction operator<span class="sy2">*</span><span class="br0">(</span>Fraction lhs, <span class="kw4">const</span> Fraction<span class="sy3">&</span> rhs<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> lhs <span class="sy2">*</span><span class="sy1">=</span> rhs<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>
Fraction f1<span class="br0">(</span><span class="nu0">3</span>,<span class="nu0">8</span><span class="br0">)</span>, f2<span class="br0">(</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">)</span>, f3<span class="br0">(</span><span class="nu0">10</span>,<span class="nu0">2</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> f1 <span class="sy1"><<</span> <span class="st0">'*'</span> <span class="sy1"><<</span> f2 <span class="sy1"><<</span> <span class="st0">'='</span> <span class="sy1"><<</span> f1<span class="sy2">*</span>f2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> f2 <span class="sy1"><<</span> <span class="st0">'*'</span> <span class="sy1"><<</span> f3 <span class="sy1"><<</span> <span class="st0">'='</span> <span class="sy1"><<</span> f2<span class="sy2">*</span>f3 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="nu0">2</span> <span class="sy1"><<</span> <span class="st0">'*'</span> <span class="sy1"><<</span> f1 <span class="sy1"><<</span> <span class="st0">'='</span> <span class="sy1"><<</span> <span class="nu0">2</span> <span class="sy2">*</span>f1 <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">3/8*1/2=3/16
1/2*5/1=5/2
2*3/8=3/4</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=13" title="Edit section: See Also">edit</a>]</span> <span class="mw-headline" id="See_Also">See Also</span></h3>
<ul><li> <a href="operator_precedence.html" title="cpp/language/operator precedence"> Operator precedence</a>
</li><li> <a href="operator_alternative.html" title="cpp/language/operator alternative"> Alternative operator syntax</a>
</li></ul>
<table class="wikitable">
<tr style="text-align:center">
<th colspan="7"> Common operators
</th></tr>
<tr style="text-align:center">
<td> <a href="operator_assignment.html" title="cpp/language/operator assignment"> assignment</a>
</td>
<td> <a href="operator_incdec.html" title="cpp/language/operator incdec"> increment<br />decrement</a>
</td>
<td> <a href="operator_arithmetic.html" title="cpp/language/operator arithmetic"> arithmetic</a>
</td>
<td> <a href="operator_logical.html" title="cpp/language/operator logical"> logical</a>
</td>
<td> <a href="operator_comparison.html" title="cpp/language/operator comparison"> comparison</a>
</td>
<td> <a href="operator_member_access.html" title="cpp/language/operator member access"> member<br />access</a>
</td>
<td> <a href="operator_other.html" title="cpp/language/operator other"> other</a>
</td></tr>
<tr style="text-align:center">
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">=</span> b<br />
a <span class="sy2">+</span><span class="sy1">=</span> b<br />
a <span class="sy2">-</span><span class="sy1">=</span> b<br />
a <span class="sy2">*</span><span class="sy1">=</span> b<br />
a <span class="sy2">/</span><span class="sy1">=</span> b<br />
a <span class="sy2">%</span><span class="sy1">=</span> b<br />
a <span class="sy3">&</span><span class="sy1">=</span> b<br />
a <span class="sy3">|</span><span class="sy1">=</span> b<br />
a <span class="sy3">^</span><span class="sy1">=</span> b<br />
a <span class="sy1"><<=</span> b<br />
a <span class="sy1">>>=</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>a<br />
<span class="sy2">--</span>a<br />
a<span class="sy2">++</span><br />
a<span class="sy2">--</span></span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span>a<br />
<span class="sy2">-</span>a<br />
a <span class="sy2">+</span> b<br />
a <span class="sy2">-</span> b<br />
a <span class="sy2">*</span> b<br />
a <span class="sy2">/</span> b<br />
a <span class="sy2">%</span> b<br />
~a<br />
a <span class="sy3">&</span> b<br />
a <span class="sy3">|</span> b<br />
a <span class="sy3">^</span> b<br />
a <span class="sy1"><<</span> b<br />
a <span class="sy1">>></span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span>a<br />
a <span class="sy3">&&</span> b<br />
a <span class="sy3">||</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a <span class="sy1">==</span> b<br />
a <span class="sy3">!</span><span class="sy1">=</span> b<br />
a <span class="sy1"><</span> b<br />
a <span class="sy1">></span> b<br />
a <span class="sy1"><=</span> b<br />
a <span class="sy1">>=</span> b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">[</span>b<span class="br0">]</span><br />
<span class="sy2">*</span>a<br />
<span class="sy3">&</span>a<br />
a<span class="sy2">-</span><span class="sy1">></span>b<br />
a.<span class="me1">b</span><br />
a<span class="sy2">-</span><span class="sy1">></span><span class="sy2">*</span>b<br />
a.<span class="sy2">*</span>b</span></span>
</p>
</td>
<td>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a<span class="br0">(</span>...<span class="br0">)</span><br />
a, b<br />
<span class="sy4">?</span> <span class="sy4">:</span></span></span>
</p>
</td></tr>
<tr>
<th colspan="7"> Special operators
</th></tr>
<tr>
<td colspan="7">
<p><a href="static_cast.html" title="cpp/language/static cast"><tt>static_cast</tt></a> converts one type to another related type <br />
<a href="dynamic_cast.html" title="cpp/language/dynamic cast"><tt>dynamic_cast</tt></a> converts within inheritance hierarchies <br />
<a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> adds or removes <a href="cv.html" title="cpp/language/cv">cv</a> qualifiers<br />
<a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a> converts type to unrelated type<br />
<a href="explicit_cast.html" title="cpp/language/explicit cast">C-style cast</a> converts one type to another by a mix of <code>static_cast</code>, <code>const_cast</code>, and <code>reinterpret_cast</code> <br />
<a href="../memory/new/operator_new.html" title="cpp/memory/new/operator new"><tt>new</tt></a> allocates memory<br />
<a href="../memory/new/operator_delete.html" title="cpp/memory/new/operator delete"><tt>delete</tt></a> deallocates memory<br />
<a href="sizeof.html" title="cpp/language/sizeof"><tt>sizeof</tt></a> queries the size of a type<br />
<a href="sizeof....html" title="cpp/language/sizeof..."><tt>sizeof...</tt></a> queries the size of a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> <span class="t-mark-rev t-since-cxx11">(since C++11)</span><br />
<a href="typeid.html" title="cpp/language/typeid"><tt>typeid</tt></a> queries the type information of a type<br />
<a href="noexcept.html" title="cpp/language/noexcept"><tt>noexcept</tt></a> checks if an expression can throw an exception <span class="t-mark-rev t-since-cxx11">(since C++11)</span><br />
<a href="alignof.html" title="cpp/language/alignof"><tt>alignof</tt></a> queries alignment requirements of a type <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</p>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/operators&action=edit&section=14" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><a href="operators.html#cite_ref-1">↑</a></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://stackoverflow.com/questions/4421706/operator-overloading/4421708#4421708">Operator Overloading</a> on StackOverflow C++ FAQ</span>
</li>
</ol>
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