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< title > The MPL Reference Manual: if_< / title >
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< table class = "header" > < tr class = "header" > < td class = "header-group navigation-bar" > < span class = "navigation-group" > < a href = "./type-selection.html" class = "navigation-link" > Prev< / a > < a href = "./if-c.html" class = "navigation-link" > Next< / a > < / span > < span class = "navigation-group-separator" > | < / span > < span class = "navigation-group" > Back < a href = "./if-c.html" class = "navigation-link" > Along< / a > < / span > < span class = "navigation-group-separator" > | < / span > < span class = "navigation-group" > < a href = "./type-selection.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 = "./type-selection.html" class = "navigation-link" > Type Selection< / a > / < a href = "./if.html" class = "navigation-link" > if_< / a > < / td >
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< div class = "section" id = "if" >
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< h1 > < a class = "toc-backref" href = "./type-selection.html#id1520" > if_< / a > < / h1 >
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< div class = "section" id = "id859" >
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< h3 > < a class = "subsection-title" href = "#synopsis" name = "synopsis" > Synopsis< / a > < / h3 >
< pre class = "literal-block" >
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template<
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typename C
, typename T1
, typename T2
>
struct < a href = "./if.html" class = "identifier" > if_< / a >
{
typedef < em > unspecified< / em > type;
};
< / pre >
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< h3 > < a class = "subsection-title" href = "#description" name = "description" > Description< / a > < / h3 >
< p > Returns one of its two arguments, < tt class = "literal" > < span class = "pre" > T1< / span > < / tt > or < tt class = "literal" > < span class = "pre" > T2< / span > < / tt > , depending on the value < tt class = "literal" > < span class = "pre" > C< / span > < / tt > .< / p >
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< div class = "section" id = "id861" >
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< h3 > < a class = "subsection-title" href = "#header" name = "header" > Header< / a > < / h3 >
< pre class = "literal-block" >
#include < < a href = "../../../../boost/mpl/if.hpp" class = "header" > boost/mpl/if.hpp< / a > >
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< div class = "section" id = "id862" >
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< h3 > < a class = "subsection-title" href = "#parameters" name = "parameters" > Parameters< / a > < / h3 >
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< tr > < th class = "head" > Parameter< / th >
< th class = "head" > Requirement< / th >
< th class = "head" > Description< / th >
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< / tr >
< / thead >
< tbody valign = "top" >
< tr > < td > < tt class = "literal" > < span class = "pre" > C< / span > < / tt > < / td >
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< td > < a class = "reference internal" href = "./integral-constant.html" > Integral Constant< / a > < / td >
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< td > A selection condition.< / td >
< / tr >
< tr > < td > < tt class = "literal" > < span class = "pre" > T1< / span > < / tt > , < tt class = "literal" > < span class = "pre" > T2< / span > < / tt > < / td >
< td > Any type< / td >
< td > Types to select from.< / td >
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< h3 > < a class = "subsection-title" href = "#expression-semantics" name = "expression-semantics" > Expression semantics< / a > < / h3 >
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< p > For any < a class = "reference internal" href = "./integral-constant.html" > Integral Constant< / a > < tt class = "literal" > < span class = "pre" > c< / span > < / tt > and arbitrary types < tt class = "literal" > < span class = "pre" > t1< / span > < / tt > , < tt class = "literal" > < span class = "pre" > t2< / span > < / tt > :< / p >
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< pre class = "literal-block" >
typedef < a href = "./if.html" class = "identifier" > if_< / a > < c,t1,t2> ::type t;
< / 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" > Return type:< / th > < td class = "field-body" > Any type.< / td >
< / tr >
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< tr class = "field" > < th class = "field-name" > Semantics:< / th > < td class = "field-body" > If < tt class = "literal" > < span class = "pre" > c::value< / span > < span class = "pre" > ==< / span > < span class = "pre" > true< / span > < / tt > , < tt class = "literal" > < span class = "pre" > t< / span > < / tt > is identical to < tt class = "literal" > < span class = "pre" > t1< / span > < / tt > ; otherwise < tt class = "literal" > < span class = "pre" > t< / span > < / tt > is
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identical to < tt class = "literal" > < span class = "pre" > t2< / span > < / tt > .< / td >
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< h3 > < a class = "subsection-title" href = "#example" name = "example" > Example< / a > < / h3 >
< pre class = "literal-block" >
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typedef < a href = "./if.html" class = "identifier" > if_< / a > < < a href = "./bool.html" class = "identifier" > true_< / a > ,char,long> ::type t1;
typedef < a href = "./if.html" class = "identifier" > if_< / a > < < a href = "./bool.html" class = "identifier" > false_< / a > ,char,long> ::type t2;
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< a href = "./assert.html" class = "identifier" > BOOST_MPL_ASSERT< / a > (( is_same< t1, char> ));
< a href = "./assert.html" class = "identifier" > BOOST_MPL_ASSERT< / a > (( is_same< t2, long> ));
< / pre >
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< pre class = "literal-block" >
// allocates space for an object of class T on heap or " inplace"
// depending on its < a href = "./size.html" class = "identifier" > size< / a >
template< typename T > struct lightweight
{
// ...
typedef typename < a href = "./if.html" class = "identifier" > if_< / a > <
< a href = "./less-equal.html" class = "identifier" > less_equal< / a > < < a href = "./sizeof.html" class = "identifier" > sizeof_< / a > < T> , < a href = "./sizeof.html" class = "identifier" > sizeof_< / a > < T*> >
, inplace_storage< T>
, heap_storage< T>
> ::type impl_t;
impl_t impl;
};
< / pre >
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< / div >
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< div class = "section" id = "id865" >
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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 = "./integral-constant.html" > Integral Constant< / a > , < a class = "reference internal" href = "./if-c.html" > if_c< / a > , < a class = "reference internal" href = "./eval-if.html" > eval_if< / a > < / p >
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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
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