22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.15 Function object binders [func.bind]

22.10.15.3 Class template is_placeholder [func.bind.isplace]

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namespace std { template<class T> struct is_placeholder; // see below }
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The class template is_placeholder can be used to detect the standard placeholders _1, _2, and so on ([func.bind.place]).
The function template bind uses is_placeholder to detect placeholders.
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Specializations of the is_placeholder template shall meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]).
The implementation provides a definition that has the base characteristic of integral_constant<int, J> if T is the type of std​::​placeholders​::​_J, otherwise it has a base characteristic of integral_constant<int, 0>.
A program may specialize this template for a program-defined type T to have a base characteristic of integral_constant<int, N> with N > 0 to indicate that T should be treated as a placeholder type.