NAME¶

std::forward - std::forward

Synopsis¶

Defined in header <utility>
template< class T > (since C++11)
T&& forward( typename std::remove_reference<T>::type& (until C++14)
t ) noexcept;
template< class T >
constexpr T&& forward( std::remove_reference_t<T>& t ) (since C++14)
noexcept;
template< class T > (1) (since
T&& forward( typename std::remove_reference<T>::type&& C++11)
t ) noexcept; (until
(2) C++14)
template< class T > (since
constexpr T&& forward( std::remove_reference_t<T>&& t C++14)
) noexcept;

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.

Notes¶

See template argument deduction for the special rules behind forwarding references
(T&& used as a function parameter) and forwarding references for other detail.

Parameters¶

t - the object to be forwarded

Return value¶

static_cast<T&&>(t)

Complexity¶

Constant

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>

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

See also¶

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