20 General utilities library [utilities]

20.9 Function objects [function.objects]

20.9.9 Function object binders [func.bind]

20.9.9.4 Placeholders [func.bind.place]

namespace std {
  namespace placeholders {
    // M is the implementation-defined number of placeholders
    extern unspecified _1;
    extern unspecified _2;
                .
                .
                .
    extern unspecified _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.