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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 ) noexcept;
(1) (since C++11)
(until C++14)
template< class T >
constexpr T&& forward( typename std::remove_reference<T>::type& t ) noexcept;
(1) (since C++14)
template< class T >
T&& forward( typename std::remove_reference<T>::type&& t ) noexcept;
(2) (since C++11)
(until C++14)
template< class T >
constexpr T&& forward( typename std::remove_reference<T>::type&& t ) noexcept;
(2) (since C++14)
1) Forwards lvalues as either lvalues or as rvalues, depending on T

When t is a forwarding reference (a function argument that is an rvalue reference to a cv-unqualified function template parameter), this overload forwards the argument to another function with the value category it had when 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) 
{
    // arg is always lvalue
    foo(std::forward<T>(arg)); // Forward as lvalue or as rvalue, depending on T
}
  • 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.
2) Forwards rvalues as rvalues and prohibits forwarding of rvalues as lvalues

This overload makes it possible to forward a result of an expression (such as function call), which may be rvalue or lvalue, as the original value category of a forwarding reference argument.

For example, if a wrapper does not just forward its argument, but calls a member function on the argument, and forwards its result:

// transforming wrapper 
template<class T>
void wrapper(T&& arg)
{
    foo(forward<decltype(forward<T>(arg).get())>(forward<T>(arg).get()));
}

where the type of arg may be

struct Arg
{
    int i = 1;
    int  get() && { return i; } // call to this overload is rvalue
    int& get() &  { return i; } // call to this overload is lvalue
};

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.

Contents

[edit] Notes

See template argument deduction for the special rules behind forwarding references (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.

#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

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