std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N (3) - Linux Man Pages
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 argument.
Each placeholder is declared as if by extern /*unspecified*/ _1; (until C++17)
Implementations are encouraged to declare the placeholders as if by inline constexpr /*unspecified*/ _1;, although declaring them by extern /*unspecified*/ _1; is still allowed by the standard. (since C++17)
The types of the placeholder objects are DefaultConstructible and CopyConstructible, their default copy/move constructors do not throw exceptions, and for any placeholder _N, the type std::is_placeholder<decltype(_N)> is defined and is derived from std::integral_constant<int, N>.
The following code shows the creation of function objects with a placeholder argument.
// Run this code
bind binds one or more arguments to a function object
is_placeholder indicates that an object is a standard placeholder or can be used as one