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# std::forward

From cppreference.com
< cpp‎ | utility

Defined in header `<utility>` |  |

template< class T > T&& forward( typename std::remove_reference<T>::type& t ); | (1) | (since C++11) (until C++14)
template< class T > constexpr T&& forward( typename std::remove_reference<T>::type& t ); | (1) | (since C++14)
template< class T > T&& forward( typename std::remove_reference<T>::type&& t ); | (2) | (since C++11) (until C++14)
template< class T > constexpr T&& forward( typename std::remove_reference<T>::type&& t ); | (2) | (since C++14)

When used according to the following recipe in a function template, forwards the argument to another function exactly as it was passed to the calling function.

For example, if used in wrapper such as the following, the template behaves as described below:

template<class T>
void wrapper(T&& arg)
{
foo(std::forward<T>(arg)); // Forward a single argument.
}

- If a call to `wrapper()` passes an rvalue `std::string`, then `T` is deduced to `std::string` (not `std::string&`, `const std::string&`, or `std::string&&`), and `std::forward` ensures that an rvalue reference is passed to `foo`.
- If a call to `wrapper()` passes a const lvalue `std::string`, then `T` is deduced to `const std::string&`, and `std::forward` ensures that a const lvalue reference is passed to `foo`.
- If a call to `wrapper()` passes a non-const lvalue `std::string`, then `T` is deduced to `std::string&`, and `std::forward` ensures that a non-const lvalue reference is passed to `foo`.

### [edit] Notes

Attempting to forward an rvalue as an lvalue, such as by instantiating the form 2) with lvalue reference type T, is a compile-time error.

See template argument deduction for the special rules behind `T&&` used as a function parameter.

### [edit] Parameters

t | - | the object to be forwarded

### [edit] Return value

static_cast<T&&>(t)

### [edit] Exceptions

`noexcept` specification:
noexcept

### [edit] Example

This example demonstrates perfect forwarding of the parameter(s) to the argument of the constructor of class `T`. Also, perfect forwarding of parameter packs is demonstrated.

Run this code

    #include <iostream>
    #include <memory>
    #include <utility>
    #include <array>

    struct A {
        A(int&& n) { std::cout << "rvalue overload, n=" << n << "\n"; }
        A(int& n)  { std::cout << "lvalue overload, n=" << n << "\n"; }
    };

    class B {
    public:
        template<class T1, class T2, class T3>
        B(T1&& t1, T2&& t2, T3&& t3) :
            a1_{std::forward<T1>(t1)},
            a2_{std::forward<T2>(t2)},
            a3_{std::forward<T3>(t3)}
        {
        }

    private:
        A a1_, a2_, a3_;
    };

    template<class T, class U>
    std::unique_ptr<T> make_unique1(U&& u)
    {
        return std::unique_ptr<T>(new T(std::forward<U>(u)));
    }

    template<class T, class... U>
    std::unique_ptr<T> make_unique(U&&... u)
    {
        return std::unique_ptr<T>(new T(std::forward<U>(u)...));
    }

    int main()
    {
        auto p1 = make_unique1<A>(2); // rvalue
        int i = 1;
        auto p2 = make_unique1<A>(i); // lvalue

        std::cout << "B\n";
        auto t = make_unique<B>(2, i, 3);
    }

Output:

    rvalue overload, n=2
    lvalue overload, n=1
    B
    rvalue overload, n=2
    lvalue overload, n=1
    rvalue overload, n=3

### [edit] Complexity

Constant

### [edit] See also

move (C++11) | obtains an rvalue reference (function template) [edit]
move_if_noexcept (C++11) | obtains an rvalue reference if the move constructor does not throw (function template) [edit]

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