namespace std {
template<class T> struct is_placeholder; // see below
}
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.
Instantiations of the is_placeholder template shall meet the UnaryTypeTrait requirements. The implementation shall provide a definition that has the base characteristic of integral_constant<int, J> if T is the type of std::placeholders::_J, otherwise it shall have a base characteristic of integral_constant<int, 0>. A program may specialize this template for a user-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.