Defined in header <functional> /*see below*/ _1;
/*see below*/ _2;
.
.
/*see below*/ _N;
The std::placeholders namespace contains the placeholder objects [_1, . . . _N] where N is an implementation defined maximum number.
When used as an argument in a std::bind expression, the placeholder objects are stored in the generated function object, and when that function object is invoked with unbound arguments, each placeholder _N is replaced by the corresponding Nth unbound argum