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std::forward

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Revision as of 12:31, 19 June 2013 by P12 (Talk | contribs)

Defined in header <utility>
template< class T >
T&& forward( typename std::remove_reference<T>::type& t );
(1) (since C++11)
template< class T >
T&& forward( typename std::remove_reference<T>::type&& t );
(2) (since C++11)

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.

Contents

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.

Parameters

t - the object to be forwarded

Return value

static_cast<T&&>(t)

Exceptions

noexcept specification:  
noexcept
  

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.

#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

Complexity

Constant

See also

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