Description

A Placeholder Expression is a type that is either a placeholder or a class template specialization with at least one argument that itself is a Placeholder Expression.

Expression requirements

If X is a class template, and a1,... an are arbitrary types, then X<a1,...,an> is a Placeholder Expression if and only if all of the following conditions hold:

• At least one of the template arguments a1,... an is a placeholder or a Placeholder Expression.
• All of X's template parameters, including the default ones, are types.
• The number of X's template parameters, including the default ones, is less or equal to the value of BOOST_MPL_LIMIT_METAFUNCTION_ARITY configuration macro.

Models

• _1
• plus<_, int_<2> >
• if_< less<_1, int_<7> >, plus<_1,_2>, _1 >

See also

Lambda Expression, Placeholders, Metafunctions, apply, lambda