std::placeholders::_1

std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N

/*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 funct