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<h1 id="firstHeading" class="firstHeading">Argument-dependent lookup</h1>
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<p>Argument-dependent lookup, also known as ADL, or Koenig lookup, is the set of rules for looking up the unqualified function names in <a href="operator_other.html" title="cpp/language/operator other">function-call expressions</a>, including implicit function calls to <a href="operators.html" title="cpp/language/operators">overloaded operators</a>. These function names are looked up in the namespaces of their arguments in addition to the scopes and namespaces considered by the usual <a href="lookup.html" title="cpp/language/lookup">unqualified name lookup</a>.
</p>
<div class="t-example"><p>
<p>Argument-dependent lookup makes it possible to use operators defined in a different namespace
</p>
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1451">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Test<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// There is no operator<< in global namespace, but ADL</span>
<span class="co1">// examines std namespace because the left argument is in</span>
<span class="co1">// std and finds std::operator<<(std::ostream&, const char*)</span>
operator<span class="sy1"><<</span><span class="br0">(</span><a href="../io/cout.html"><span class="kw1451">std::<span class="me2">cout</span></span></a>, <span class="st0">"Test<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same, using function call notation</span>
 
<span class="co1">// however,</span>
<a href="../io/cout.html"><span class="kw1451">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> endl<span class="sy4">;</span> <span class="co1">// Error: 'endl' is not declared in this namespace.</span>
<span class="co1">// This is not a function call to endl(), so ADL does not apply</span>
 
endl<span class="br0">(</span><a href="../io/cout.html"><span class="kw1451">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: this is a function call: ADL examines std namespace</span>
<span class="co1">// because the argument of endl is in std, and finds std::endl</span>
 
<span class="br0">(</span>endl<span class="br0">)</span><span class="br0">(</span><a href="../io/cout.html"><span class="kw1451">std::<span class="me2">cout</span></span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: 'endl' is not declared in this namespace.</span>
<span class="co1">// The sub-expression (endl) is not a function call expression</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="adl.html#Details"><span class="tocnumber">1</span> <span class="toctext">Details</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="adl.html#Notes"><span class="tocnumber">2</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="adl.html#Examples"><span class="tocnumber">3</span> <span class="toctext">Examples</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="adl.html#See_also"><span class="tocnumber">4</span> <span class="toctext">See also</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="adl.html#References"><span class="tocnumber">5</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/adl&action=edit&section=1" title="Edit section: Details">edit</a>]</span> <span class="mw-headline" id="Details">Details</span></h3>
<p>First, if the lookup set produced by usual <a href="lookup.html" title="cpp/language/lookup">unqualified lookup</a> contains any of the following:
</p>
<div class="t-li1"><span class="t-li">1)</span> a declaration of a class member</div>
<div class="t-li1"><span class="t-li">2)</span> a declaration of a function at block scope (that's not a <a href="namespace.html#Using-declarations" title="cpp/language/namespace">using-declaration</a>)</div>
<div class="t-li1"><span class="t-li">3)</span> any declaration that is not a function or a function template (e.g. a function object or another variable whose name conflicts with the name of the function that's being looked up)</div>
<p>Then the argument-dependent lookup is not considered.
</p><p>Otherwise, for every argument in a function call expression and for every template template argument of a template function, its type is examined to determine the <i>associated set of namespaces and classes</i> that it will add to the lookup
</p>
<div class="t-li1"><span class="t-li">1)</span> For arguments of fundamental type, the associated set of namespaces and classes is empty</div>
<div class="t-li1"><span class="t-li">2)</span> For arguments of class type (including union), the set consists of</div>
<div class="t-li2"><span class="t-li">a)</span> The class itself</div>
<div class="t-li2"><span class="t-li">b)</span> All of its direct and indirect base classes</div>
<div class="t-li2"><span class="t-li">c)</span> If the class is a <a href="nested_classes.html" title="cpp/language/nested types">member of another class</a>, the class of which it is a member</div>
<div class="t-li2"><span class="t-li">d)</span> The innermost enclosing namespaces in the classes added to the set</div>
<div class="t-li1"><span class="t-li">3)</span> For arguments whose type is a class template specialization, in addition to the class rules, the following types are examined and their associated classes and namespaces are added to the set</div>
<div class="t-li2"><span class="t-li">a)</span> The types of all template arguments provided for type template parameters (skipping non-type template parameters and skipping template template parameters)</div>
<div class="t-li2"><span class="t-li">b)</span> The namespaces in which any template template arguments are members</div>
<div class="t-li2"><span class="t-li">c)</span> The classes in which any template template arguments are members (if they happen to be class member templates)</div>
<div class="t-li1"><span class="t-li">4)</span> For arguments of enumeration type, the namespace in which the enumeration is defined is added to the set. If the enumeration type is a member of a class, that class is added to the set.</div>
<div class="t-li1"><span class="t-li">5)</span> For arguments of type pointer to T or pointer to an array of T, the type T is examined and its associated set of classes and namespaces is added to the set.</div>
<div class="t-li1"><span class="t-li">6)</span> For arguments of function type, the function parameter types and the function return type are examined and their associated set of classes and namespaces are added to the set.</div>
<div class="t-li1"><span class="t-li">7)</span> For arguments of type pointer to member function F of class X, the function parameter types, the function return type, and the class X are examined and their associated set of classes and namespaces are added to the set.</div>
<div class="t-li1"><span class="t-li">8)</span> For arguments of type pointer to data member T of class X, the member type and the type X are both examined and their associated set of classes and namespaces are added to the set.</div>
<div class="t-li1"><span class="t-li">9)</span> If the argument is the name or the <a href="overloaded_address.html" title="cpp/language/overloaded address">address-of expression for a set of overloaded functions</a> (or function templates), every function in the overload set is examined and its associated set of classes and namespaces is added to the set.</div>
<div class="t-li2"><span class="t-li">a)</span> Additionally, if the set of overloads is named by a template-id (template name with template arguments), all of its type template arguments and template template arguments (but not non-type template arguments) are examined and their associated set of classes and namespaces are added to the set.</div>
<p>If any namespace in the associated set of classes and namespaces is an <a href="namespace.html" title="cpp/language/namespace">inline namespace</a>, its enclosing namespace is also added to the set.
</p><p>If any namespace in the associated set of classes and namespaces directly contains an inline namespace, that inline namespace is added to the set.
</p><p>After the associated set of classes and namespaces is determined, all declarations found in classes of this set are discarded for the purpose of further ADL processing, except namespace-scoped friend functions and function templates, as stated in point 2 below .
</p><p>The set of declarations found by ordinary <a href="lookup.html" title="cpp/language/lookup">unqualified lookup</a> and the set of declarations found in all elements of the associated set produced by ADL, are merged, with the following special rules
</p>
<div class="t-li1"><span class="t-li">1)</span> <a href="namespace.html#Using-directives" title="cpp/language/namespace">using-directives</a> in the associated namespaces are ignored</div>
<div class="t-li1"><span class="t-li">2)</span> namespace-scoped friend functions (and function templates) that are declared in an associated class are visible through ADL even if they are not visible through ordinary lookup</div>
<div class="t-li1"><span class="t-li">3)</span> all names except for the functions and function templates are ignored (no collision with variables)</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/adl&action=edit&section=2" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Because of argument-dependent lookup, non-member functions and non-member operators defined in the same namespace as a class are considered part of the public interface of that class (if they are found through ADL) <sup id="cite_ref-1" class="reference"><a href="adl.html#cite_note-1">[1]</a></sup>.
</p>
ADL is the reason behind the established idiom for swapping two objects in generic code:<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">using</span> <a href="../algorithm/swap.html"><span class="kw1303">std::<span class="me2">swap</span></span></a><span class="sy4">;</span>
swap<span class="br0">(</span>obj1, obj2<span class="br0">)</span><span class="sy4">;</span></pre></div></div>because calling <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../algorithm/swap.html"><span class="kw1303">std::<span class="me2">swap</span></span></a><span class="br0">(</span>obj1, obj2<span class="br0">)</span></span></span> directly would not consider the user-defined swap() functions that could be defined in the same namespace as the types of obj1 or obj2, and just calling the unqualified <span class="t-c"><span class="mw-geshi cpp source-cpp">swap<span class="br0">(</span>obj1, obj2<span class="br0">)</span></span></span> would call nothing if no user-defined overload was provided. In particular, <span class="t-lc"><a href="../algorithm/iter_swap.html" title="cpp/algorithm/iter swap">std::iter_swap</a></span> and all other standard library algorithms use this approach when dealing with <a href="../concept/Swappable.html" title="cpp/concept/Swappable"><code>Swappable</code></a> types.
<p>Name lookup rules make it impractical to declare operators in global or user-defined namespace that operate on types from the std namespace, e.g. a custom <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1">>></span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">+</span></span></span> for <span class="t-lc"><a href="../container/vector.html" title="cpp/container/vector">std::vector</a></span> or for <span class="t-lc"><a href="../utility/pair.html" title="cpp/utility/pair">std::pair</a></span> (unless the element types of the vector/pair are user-defined types, which would add their namespace to ADL). Such operators would not be looked up from template instantiations, such as the standard library algorithms. See <a href="dependent_name.html" title="cpp/language/dependent name">dependent names</a> for further details.
</p><p>ADL can find a <a href="friend.html" title="cpp/language/friend">friend function</a> (typically, an overloaded operator) that is defined entirely within a class or class template, even if it was never declared at namespace level.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">struct</span> number
<span class="br0">{</span>
number<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">friend</span> number gcd<span class="br0">(</span>number x, number y<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// definition within</span>
<span class="co1">// a class template</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// unless a matching declaration is provided gcd is an invisible (except through ADL)</span>
<span class="co1">// member of this namespace</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
number<span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span> a<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span>, b<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
a <span class="sy1">=</span> gcd<span class="br0">(</span>a,b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// finds gcd because number<double> is an associated class,</span>
<span class="co1">// making gcd visible in its namespace (global scope)</span>
<span class="co1">// b = gcd(3,4); // Error; gcd is not visible</span>
<span class="br0">}</span></pre></div></div>
<p>Although a function call can be resolved through ADL even if ordinary lookup finds nothing, a function call to a <a href="function_template.html" title="cpp/language/function template">function template</a> with explicitly-specified template arguments requires that there is a declaration of the template found by ordinary lookup (otherwise, it is a syntax error to encounter an unknown name followed by a less-than character)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N1 <span class="br0">{</span>
<span class="kw1">struct</span> S <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw4">int</span> X<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>S<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">namespace</span> N2 <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>T t<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span>N1<span class="sy4">::</span><span class="me2">S</span> s<span class="br0">)</span> <span class="br0">{</span>
f<span class="sy1"><</span><span class="nu0">3</span><span class="sy1">></span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Syntax error (unqualified lookup finds no f)</span>
N1<span class="sy4">::</span><span class="me2">f</span><span class="sy1"><</span><span class="nu0">3</span><span class="sy1">></span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, qualified lookup finds the template 'f'</span>
N2<span class="sy4">::</span><span class="me2">f</span><span class="sy1"><</span><span class="nu0">3</span><span class="sy1">></span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: N2::f does not take a non-type parameter</span>
<span class="co1">// N1::f is not looked up because ADL only works</span>
<span class="co1">// with unqualified names</span>
<span class="kw1">using</span> N2<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span>
f<span class="sy1"><</span><span class="nu0">3</span><span class="sy1">></span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: Unqualified lookup now finds N2::f</span>
<span class="co1">// then ADL kicks in because this name is unqualified</span>
<span class="co1">// and finds N1::f</span>
<span class="br0">}</span></pre></div></div>
<p>There are two contexts where ADL-only lookup (that is, lookup in associated namespaces only) takes place:
</p>
<ul><li> the lookup of non-member functions <code>begin</code> and <code>end</code> performed by the <a href="range-for.html" title="cpp/language/range-for">range-for</a> loop if member lookup fails
</li><li> the <a href="dependent_name.html#Lookup_rules" title="cpp/language/dependent name">dependent name lookup</a> from the point of template instantiation.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/adl&action=edit&section=3" title="Edit section: Examples">edit</a>]</span> <span class="mw-headline" id="Examples">Examples</span></h3>
<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: more examples </td></tr></table>
<div class="t-example"><p>Example from <a rel="nofollow" class="external free" href="http://www.gotw.ca/gotw/030.htm">http://www.gotw.ca/gotw/030.htm</a>
</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> A <span class="br0">{</span>
<span class="kw1">struct</span> X<span class="sy4">;</span>
<span class="kw1">struct</span> Y<span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">void</span> g<span class="br0">(</span>X<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">namespace</span> B <span class="br0">{</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span>
f<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::f (endless recursion)</span>
<span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span>A<span class="sy4">::</span><span class="me2">X</span> x<span class="br0">)</span> <span class="br0">{</span>
g<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: ambiguous between B::g (ordinary lookup)</span>
<span class="co1">// and A::g (argument-dependent lookup)</span>
<span class="br0">}</span>
<span class="kw4">void</span> h<span class="br0">(</span>A<span class="sy4">::</span><span class="me2">Y</span> y<span class="br0">)</span> <span class="br0">{</span>
h<span class="br0">(</span>y<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls B::h (endless recursion): ADL examines the A namespace</span>
<span class="co1">// but finds no A::h, so only B::h from ordinary lookup is used</span>
<span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/adl&action=edit&section=4" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="lookup.html" title="cpp/language/lookup">Name lookup</a>
</li><li> <a href="function_template.html" title="cpp/language/function template">Template argument deduction</a>
</li><li> <a href="overload_resolution.html" title="cpp/language/overload resolution">Overload resolution</a>
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/adl&action=edit&section=5" 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="adl.html#cite_ref-1">↑</a></span> <span class="reference-text"> H. Sutter (1998) <a rel="nofollow" class="external text" href="http://www.gotw.ca/publications/mill02.htm">"What's In a Class? - The Interface Principle"</a> in C++ Report, 10(3)</span>
</li>
</ol>
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