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std::forward

From cppreference.com
< cpp‎ | utility
 
 
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Defined in header <utility>
(1)
template< class T >
T&& forward( typename std::remove_reference<T>::type& t ) noexcept;
(since C++11)
(until C++14)
template< class T >
constexpr T&& forward( std::remove_reference_t<T>& t ) noexcept;
(since C++14)
(2)
template< class T >
T&& forward( typename std::remove_reference<T>::type&& t ) noexcept;
(since C++11)
(until C++14)
template< class T >
constexpr T&& forward( std::remove_reference_t<T>&& t ) noexcept;
(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 declared as 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 a 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) and forwarding references for other detail.

[edit] Parameters

t - the object to be forwarded

[edit] Return value

static_cast<T&&>(t)

[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>
 
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_unique2(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_unique2<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]