std::placeholders::_1,std::placeholders::_2,…,std::placeholders::_N (3) Linux Manual Page

std::placeholders::_1,std::placeholders::_2,…,std::placeholders::_N – std::placeholders::_1,std::placeholders::_2,…,std::placeholders::_N

Synopsis

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>.

Example

The following code shows the creation of function objects with a placeholder argument.
// Run this code

#include <functional>

#include <string>

#include <iostream>
void goodbye(const std::string &s)

{
    std::cout << "Goodbye " << s << '\n';
}
class Object
{

  public:
    void hello(const std::string &s)
    {
        std::cout << "Hello " << s << '\n';
    }
};
int main()

{
    typedef std::function<void(const std::string &)> ExampleFunction;
    Object instance;
    std::string str("World");
    ExampleFunction f = std::bind(&Object::hello, &instance,
                                  std::placeholders::_1);
    // equivalent to instance.hello(str)
    f(str);
    f = std::bind(&goodbye, std::placeholders::_1);
    // equivalent to goodbye(str)
    f(str);
    return 0;
}

Output:

  Hello World
  Goodbye World

See also

bind binds one or more arguments to a function object
               (function template)
(C++11)
is_placeholder indicates that an object is a standard placeholder or can be used as one
               (class template)
(C++11)
               placeholder to skip an element when unpacking a tuple using tie
ignore (constant)