std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N (3) - Linux Manuals

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

NAME

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