2004-11-28 03:35:12 +00:00
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2004-11-28 03:35:12 +00:00
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< table class = "header" >< tr class = "header" >< td class = "header-group navigation-bar" >< span class = "navigation-group" >< a href = "./lambda-expression.html" class = "navigation-link" > Prev</ a > < a href = "./tag-dispatched.html" class = "navigation-link" > Next</ a ></ span >< span class = "navigation-group-separator" > | </ span >< span class = "navigation-group" >< a href = "./lambda-expression.html" class = "navigation-link" > Back</ a > < a href = "./tag-dispatched.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 = "./placeholder-expression.html" class = "navigation-link" > Placeholder Expression</ a ></ td >
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< div class = "section" id = "placeholder-expression" >
< h1 >< a class = "toc-backref" href = "./metafunctions-concepts.html#id441" name = "placeholder-expression" > Placeholder Expression</ a ></ h1 >
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< h3 >< a class = "subsection-title" href = "#description" name = "description" > Description</ a ></ h3 >
< p > A < a class = "reference" href = "./placeholder-expression.html" > Placeholder Expression</ a > is a type that is either a < a class = "reference" href = "./placeholders.html" > placeholder</ a > or a class
template specialization with at least one argument that itself is a
< a class = "reference" href = "./placeholder-expression.html" > Placeholder Expression</ a > .</ p >
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< div class = "section" id = "placeholder-expression-requirements" >
< h3 >< a class = "subsection-title" href = "#expression-requirements" name = "expression-requirements" > Expression requirements</ a ></ h3 >
< p > If < tt class = "literal" >< span class = "pre" > X</ span ></ tt > is a class template, and < tt class = "literal" >< span class = "pre" > a1</ span ></ tt > ,... < tt class = "literal" >< span class = "pre" > an</ span ></ tt > are arbitrary types, then
< tt class = "literal" >< span class = "pre" > X< a1,...,an> </ span ></ tt > is a < a class = "reference" href = "./placeholder-expression.html" > Placeholder Expression</ a > if and only if all of the following
conditions hold:</ p >
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< li > At least one of the template arguments < tt class = "literal" >< span class = "pre" > a1</ span ></ tt > ,... < tt class = "literal" >< span class = "pre" > an</ span ></ tt > is a < a class = "reference" href = "./placeholders.html" > placeholder</ a >
or a < a class = "reference" href = "./placeholder-expression.html" > Placeholder Expression</ a > .</ li >
< li > All of < tt class = "literal" >< span class = "pre" > X</ span ></ tt > 's template parameters, including the default ones, are types.</ li >
< li > The number of < tt class = "literal" >< span class = "pre" > X</ span ></ tt > 's template parameters, including the default ones, is
less or equal to the value of < tt class = "literal" >< span class = "pre" >< a href = "./limit-metafunction-arity.html" class = "identifier" > BOOST_MPL_LIMIT_METAFUNCTION_ARITY</ a ></ span ></ tt >
< a class = "reference" href = "./configuration.html" > configuration macro</ a > .</ li >
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< h3 >< a class = "subsection-title" href = "#models" name = "models" > Models</ a ></ h3 >
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< li >< a class = "refentry reference" href = "./placeholders.html" >< tt class = "refentry literal" >< span class = "pre" > _1</ span ></ tt ></ a ></ li >
< li >< tt class = "literal" >< span class = "pre" >< a href = "./plus.html" class = "identifier" > plus</ a > < _,</ span > < span class = "pre" >< a href = "./int.html" class = "identifier" > int_</ a > < 2> </ span > < span class = "pre" > > </ span ></ tt ></ li >
< li >< tt class = "literal" >< span class = "pre" >< a href = "./if.html" class = "identifier" > if_</ a > < </ span > < span class = "pre" >< a href = "./less.html" class = "identifier" > less</ a > < _1,</ span > < span class = "pre" >< a href = "./int.html" class = "identifier" > int_</ a > < 7> </ span > < span class = "pre" > > ,</ span > < span class = "pre" >< a href = "./plus.html" class = "identifier" > plus</ a > < _1,_2> ,</ span > < span class = "pre" > _1</ span > < span class = "pre" > > </ span ></ tt ></ li >
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< h3 >< a class = "subsection-title" href = "#see-also" name = "see-also" > See also</ a ></ h3 >
< p >< a class = "reference" href = "./lambda-expression.html" > Lambda Expression</ a > , < a class = "reference" href = "./placeholders.html" > Placeholders</ a > , < a class = "reference" href = "./metafunctions.html" > Metafunctions</ a > , < a class = "refentry reference" href = "./apply.html" >< tt class = "refentry literal" >< span class = "pre" > apply</ span ></ tt ></ a > , < a class = "refentry reference" href = "./lambda.html" >< tt class = "refentry literal" >< span class = "pre" > lambda</ span ></ tt ></ a ></ p >
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