20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.10 Function object binders [func.bind]

20.14.10.4 Placeholders [func.bind.place]

namespace std::placeholders {
  // M is the implementation-defined number of placeholders
  see below _1;
  see below _2;
              .
              .
              .
  see below _M;
}
1

All placeholder types shall be DefaultConstructible and CopyConstructible, and their default constructors and copy/move constructors shall not throw exceptions. It is implementation-defined whether placeholder types are CopyAssignable. CopyAssignable placeholders' copy assignment operators shall not throw exceptions.

2

Placeholders should be defined as: 

constexpr unspecified _1{};

If they are not, they shall be declared as: 

extern unspecified _1;