std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N(3)

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 (since C++17)
   extern /*unspecified*/ _1; is still allowed by the standard.
   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, where
   std::is_placeholder<decltype(_N)> is derived from std::integral_constant<int, N>.

Example
   The following code shows the creation of function objects with placeholder
   arguments. // 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()
 {
     using namespace std::placeholders;
     using ExampleFunction = std::function<void(const std::string&)>;
     Object instance;
     std::string str("World");
     ExampleFunction f = std::bind(&Object::hello, &instance, _1);
     f(str); // equivalent to instance.hello(str)
     f = std::bind(&goodbye, std::placeholders::_1);
     f(str); // equivalent to goodbye(str)
     auto lambda = [](std::string pre, char o, int rep, std::string post) {
         std::cout << pre;
         while (rep-- > 0) std::cout << o;
         std::cout << post << '\n';
     };
     // binding the lambda:
     std::function<void(std::string, char, int, std::string)> g =
         std::bind(&decltype(lambda)::operator(), &lambda, _1, _2, _3, _4);
     g("G", 'o', 'o'-'g', "gol");
 }

Output:
 Hello World
 Goodbye World
 Goooooooogol

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