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

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 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>.

[edit] 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(int argc, char* argv[])
{
    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

[edit] See also

bind (C++11)	binds one or more arguments to a function object (function template)
is_placeholder (C++11)	indicates that an object is a standard placeholder or can be used as one (class template)

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