std::placeholders::_1, std::placeholders::_2, ..., std::placeholders::_N - cppreference.com

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

From cppreference.com
< cpp‎ | utility‎ | functional

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>.

### [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) [edit]
is_placeholder (C++11) | indicates that an object is a standard placeholder or can be used as one (class template) [edit]

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