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<p>Functions are C++ entities that associate a sequence of <a href="statements.html" title="cpp/language/statements">statements</a> (a <i>function body</i>) with a <i>name</i> and a list of zero or more <i>function parameters</i>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// function name: "isodd"</span>
<span class="co1">// parameter list has one parameter, with name "n" and type int</span>
<span class="co1">// the return type is bool</span>
<span class="kw4">bool</span> isodd<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span>
<span class="br0">{</span> <span class="co1">// the body of the function begins</span>
<span class="kw1">return</span> n <span class="sy2">%</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="br0">}</span> <span class="co1">// the body of the function ends</span></pre></div></div>
<p>When a function is invoked, e.g. in a <a href="operator_other.html#Built-in_function_call_operator" title="cpp/language/operator other">function-call expression</a>, the parameters are initialized from the arguments (either provided at the place of call or <a href="default_arguments.html" title="cpp/language/default arguments">defaulted</a>) and the statements in the function body are executed.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> arg <span class="sy4">:</span> <span class="br0">{</span><span class="sy2">-</span><span class="nu0">3</span>, <span class="sy2">-</span><span class="nu0">2</span>, <span class="sy2">-</span><span class="nu0">1</span>, <span class="nu0">0</span>, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><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> isodd<span class="br0">(</span>arg<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="co1">// isodd called 7 times, each</span>
<span class="co1">// time n is copy-initialized from arg</span>
<span class="br0">}</span></pre></div></div>
<p>A function can terminate by <a href="return.html" title="cpp/language/return">returning</a> or by <a href="throw.html" title="cpp/language/throw">throwing</a> an <a href="exceptions.html" title="cpp/language/exceptions">exception</a>.
</p><p>A <a href="function.html" title="cpp/language/function">function declaration</a> may appear in any scope, but a <a href="function.html" title="cpp/language/function">function definition</a> may only appear in namespace scope or, for <a href="member_functions.html" title="cpp/language/member functions">member</a> and <a href="friend.html" title="cpp/language/friend">friend</a> functions, in class scope. A function that is declared in a class body without a friend specifier is a class member function. Such functions have many additional properties, see <a href="member_functions.html" title="cpp/language/member functions">member functions</a> for details.
</p><p>Functions are not objects: there are no arrays of functions and functions cannot be passed by value or returned from other functions. Pointers and references to functions are allowed, and may be used where functions themselves cannot.
</p><p>Each function has a type, which consists of the function's return type, the types of all parameters (after array-to-pointer and function-to-pointer transformations, see <a href="function.html#Parameter_list" title="cpp/language/function">parameter list</a>) <span class="t-rev-inl t-since-cxx17"><span>, whether the function is noexcept or not</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>, and, for member functions, cv-qualification and ref-qualification. Function types also have <a href="language_linkage.html" title="cpp/language/language linkage">language linkage</a>. There are no cv-qualified function types (not to be confused with the types of <a href="member_functions.html" title="cpp/language/member functions">cv-qualified functions</a> such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></span> or functions returning <a href="cv.html" title="cpp/language/cv">cv-qualified types</a>, such as <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> <span class="kw4">const</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span>)
</p><p>Unnamed functions can be generated by <a href="lambda.html" title="cpp/language/lambda">lambda-expressions</a>.
</p><p>Multiple functions in the same scope may have the same name, as long as their parameter lists and, for member functions, cv/ref qualifications are different. This is known as <a href="overload_resolution.html" title="cpp/language/overload resolution">function overloading</a>. Function declarations that differ only in the return type <span class="t-rev-inl t-since-cxx17"><span>and the noexcept specification</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> cannot be overloaded.
</p>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: links to other Function subpages </td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/functions&action=edit&section=1" title="Edit section: Function objects">edit</a>]</span> <span class="mw-headline" id="Function_objects">Function objects</span></h3>
<p>Besides function lvalues, the function call expression supports pointers to functions, dereferenced pointers to member functions, <a href="lambda.html" title="cpp/language/lambda">lambda-expressions</a>, and any variable of class type that overloads the function-call operator. Together, these types are known as <a href="../concept/FunctionObject.html" title="cpp/concept/FunctionObject"><code>FunctionObject</code>s</a>, and they are used ubiquitously through the C++ standard library, see for example, usages of <a href="../concept/BinaryPredicate.html" title="cpp/concept/BinaryPredicate"><code>BinaryPredicate</code></a> and <a href="../concept/Compare.html" title="cpp/concept/Compare"><code>Compare</code></a>.
</p><p>The standard library also provides a number of pre-defined <a href="../utility/functional.html" title="cpp/utility/functional">function object templates</a> as well as the methods to compose new ones (including <span class="t-lc"><a href="../utility/functional/mem_fn.html" title="cpp/utility/functional/mem fn">std::mem_fn</a></span>, <span class="t-lc"><a href="../utility/functional/bind.html" title="cpp/utility/functional/bind">std::bind</a></span>, and <span class="t-lc"><a href="../utility/functional/function.html" title="cpp/utility/functional/function">std::function</a></span>).
</p>
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