20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.15 Function object binders [func.bind]

20.14.15.3 Class template is_­placeholder [func.bind.isplace]

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.
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.