20 General utilities library [utilities]

20.9 Function objects [function.objects]

20.9.9 Function object binders [func.bind]

20.9.9.2 Class template is_placeholder [func.bind.isplace]

namespace std {
  template<class T> struct is_placeholder; // see below
}
1

is_placeholder can be used to detect the standard placeholders _1, _2, and so on. bind uses is_placeholder to detect placeholders.

2

Instantiations of the is_placeholder template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has the BaseCharacteristic of integral_constant<int, J> if T is the type of std::placeholders::_J, otherwise it shall have a BaseCharacteristic of integral_constant<int, 0>. A program may specialize this template for a user-defined type T to have a BaseCharacteristic of integral_constant<int, N> with N > 0 to indicate that T should be treated as a placeholder type.