std::forward

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.

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.

[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

[edit] Return value

static_cast<T&&>(t)

[edit] Complexity

Constant.

[edit] Example

#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)...));
}
 
auto make_B(auto&&... args) // since C++20
{
 return B(std::forward<decltype(args)>(args)...);
}
 
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);
 
 std::cout << "make_B\n";
 [[maybe_unused]] B b = make_B(4, i, 5);
}

rvalue overload, n=2
lvalue overload, n=1
B
rvalue overload, n=2
lvalue overload, n=1
rvalue overload, n=3
make_B
rvalue overload, n=4
lvalue overload, n=1
rvalue overload, n=5

[edit] See also

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