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< title > The MPL Reference Manual: Metafunction< / title >
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< table class = "header" > < tr class = "header" > < td class = "header-group navigation-bar" > < span class = "navigation-group" > < a href = "./metafunctions-concepts.html" class = "navigation-link" > Prev< / a > < a href = "./metafunction-class.html" class = "navigation-link" > Next< / a > < / span > < span class = "navigation-group-separator" > | < / span > < span class = "navigation-group" > Back < a href = "./metafunction-class.html" class = "navigation-link" > Along< / a > < / span > < span class = "navigation-group-separator" > | < / span > < span class = "navigation-group" > < a href = "./metafunctions-concepts.html" class = "navigation-link" > Up< / a > < a href = "../refmanual.html" class = "navigation-link" > Home< / a > < / span > < span class = "navigation-group-separator" > | < / span > < span class = "navigation-group" > < a href = "./refmanual_toc.html" class = "navigation-link" > Full TOC< / a > < / span > < / td >
< td class = "header-group page-location" > < a href = "../refmanual.html" class = "navigation-link" > Front Page< / a > / < a href = "./metafunctions.html" class = "navigation-link" > Metafunctions< / a > / < a href = "./metafunctions-concepts.html" class = "navigation-link" > Concepts< / a > / < a href = "./metafunction.html" class = "navigation-link" > Metafunction< / a > < / td >
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< div class = "section" id = "metafunction" >
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< h1 > < a class = "toc-backref" href = "./metafunctions-concepts.html#id1512" > Metafunction< / a > < / h1 >
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< div class = "section" id = "id822" >
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< h3 > < a class = "subsection-title" href = "#description" name = "description" > Description< / a > < / h3 >
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< p id = "nullary-metafunction" > A < em > metafunction< / em > is a class or a class template that represents a
function invocable at compile-time. An non-nullary metafunction is
invoked by instantiating the class template with particular
template parameters (metafunction arguments); the result of the
metafunction application is accessible through the instantiation's
nested < tt class = "literal" > < span class = "pre" > type< / span > < / tt > typedef. All metafunction's arguments must be types
(i.e. only < em > type template parameters< / em > are allowed). A metafunction
can have a variable number of parameters. A < em > nullary metafunction< / em > is
represented as a (template) class with a nested < tt class = "literal" > < span class = "pre" > type< / span > < / tt > typename
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member.< / p >
< / div >
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< div class = "section" id = "id823" >
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< h3 > < a class = "subsection-title" href = "#expression-requirements" name = "expression-requirements" > Expression requirements< / a > < / h3 >
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< p > In the following table and subsequent specifications, < tt class = "literal" > < span class = "pre" > f< / span > < / tt > is a < a class = "reference internal" href = "./metafunction.html" > Metafunction< / a > .< / p >
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< colgroup >
< col width = "38%" / >
< col width = "28%" / >
< col width = "33%" / >
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< tr > < th class = "head" > Expression< / th >
< th class = "head" > Type< / th >
< th class = "head" > Complexity< / th >
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< / tr >
< / thead >
< tbody valign = "top" >
< tr > < td > < tt class = "literal" > < span class = "pre" > f::type< / span > < / tt > < / td >
< td > Any type< / td >
< td > Unspecified.< / td >
< / tr >
< tr > < td > < tt class = "literal" > < span class = "pre" > f< > ::type< / span > < / tt > < / td >
< td > Any type< / td >
< td > Unspecified.< / td >
< / tr >
< tr > < td > < tt class = "literal" > < span class = "pre" > f< a1,..,an> ::type< / span > < / tt > < / td >
< td > Any type< / td >
< td > Unspecified.< / td >
< / tr >
< / tbody >
< / table >
< / div >
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< div class = "section" id = "id824" >
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< h3 > < a class = "subsection-title" href = "#expression-semantics" name = "expression-semantics" > Expression semantics< / a > < / h3 >
< pre class = "literal-block" >
typedef f::type x;
< / pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" / >
< col class = "field-body" / >
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< tr class = "field" > < th class = "field-name" > Precondition:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > f< / span > < / tt > is a nullary < a class = "reference internal" href = "./metafunction.html" > Metafunction< / a > ; < tt class = "literal" > < span class = "pre" > f::type< / span > < / tt > is a < em > type-name< / em > .< / td >
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< / tr >
< tr class = "field" > < th class = "field-name" > Semantics:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > x< / span > < / tt > is the result of the metafunction invocation.< / td >
< / tr >
< / tbody >
< / table >
<!-- ................................................................................... -->
< pre class = "literal-block" >
typedef f< > ::type x;
< / pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" / >
< col class = "field-body" / >
< tbody valign = "top" >
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< tr class = "field" > < th class = "field-name" > Precondition:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > f< / span > < / tt > is a nullary < a class = "reference internal" href = "./metafunction.html" > Metafunction< / a > ; < tt class = "literal" > < span class = "pre" > f< > ::type< / span > < / tt > is a < em > type-name< / em > .< / td >
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< / tr >
< tr class = "field" > < th class = "field-name" > Semantics:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > x< / span > < / tt > is the result of the metafunction invocation.< / td >
< / tr >
< / tbody >
< / table >
<!-- ................................................................................... -->
< pre class = "literal-block" >
typedef f< a1,< em > ...< / em > a< em > n< / em > > ::type x;
< / pre >
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< table class = "docutils field-list" frame = "void" rules = "none" >
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< col class = "field-name" / >
< col class = "field-body" / >
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< tr class = "field" > < th class = "field-name" > Precondition:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > f< / span > < / tt > is an < em > n< / em > -ary < a class = "reference internal" href = "./metafunction.html" > Metafunction< / a > ; < tt class = "literal" > < span class = "pre" > a1< / span > < / tt > ,... < tt class = "literal" > < span class = "pre" > an< / span > < / tt > are types;
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< tt class = "literal" > < span class = "pre" > f< a1,...an> ::type< / span > < / tt > is a < em > type-name< / em > .< / td >
< / tr >
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< tr class = "field" > < th class = "field-name" > Semantics:< / th > < td class = "field-body" > < tt class = "literal" > < span class = "pre" > x< / span > < / tt > is the result of the metafunction invocation
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with the actual arguments < tt class = "literal" > < span class = "pre" > a1< / span > < / tt > ,... < tt class = "literal" > < span class = "pre" > an< / span > < / tt > .< / td >
< / tr >
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< / table >
< / div >
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< div class = "section" id = "id825" >
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< h3 > < a class = "subsection-title" href = "#models" name = "models" > Models< / a > < / h3 >
< ul class = "simple" >
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< li > < a class = "reference internal" href = "./identity.html" > identity< / a > < / li >
< li > < a class = "reference internal" href = "./plus.html" > plus< / a > < / li >
< li > < a class = "reference internal" href = "./begin.html" > begin< / a > < / li >
< li > < a class = "reference internal" href = "./insert.html" > insert< / a > < / li >
< li > < a class = "reference internal" href = "./fold.html" > fold< / a > < / li >
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< / ul >
< / div >
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< div class = "section" id = "id826" >
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< h3 > < a class = "subsection-title" href = "#see-also" name = "see-also" > See also< / a > < / h3 >
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< p > < a class = "reference internal" href = "./metafunctions.html" > Metafunctions< / a > , < a class = "reference internal" href = "./metafunction-class.html" > Metafunction Class< / a > , < a class = "reference internal" href = "./lambda-expression.html" > Lambda Expression< / a > , < a class = "reference internal" href = "./invocation.html" > invocation< / a > , < a class = "reference internal" href = "./apply.html" > apply< / a > , < a class = "reference internal" href = "./lambda.html" > lambda< / a > , < a class = "reference internal" href = "./bind.html" > bind< / a > < / p >
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<!-- Metafunctions/Concepts//Metafunction Class |20 -->
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< td > < div class = "copyright-footer" > < div class = "copyright" > Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams< / div >
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at < a class = "reference external" href = "http://www.boost.org/LICENSE_1_0.txt" target = "_top" > http://www.boost.org/LICENSE_1_0.txt< / a > )< / div > < / td > < / tr > < / table > < / body >
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