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

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< 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, 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 <iostream>
    #include <string>

    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 (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)
ignore (C++11) | placeholder to skip an element when unpacking a `tuple` using `tie` (constant)

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