function template
<utility>
std::forward lvalue (1)template <class T> T&& forward (typename remove_reference<T>::type& arg) noexcept;rvalue (2)
template <class T> T&& forward (typename remove_reference<T>::type&& arg) noexcept;
Forward argument
Returns an rvalue reference to arg if arg is not an lvalue reference.If arg is an lvalue reference, the function returns arg without modifying its type.
This is a helper function to allow perfect forwarding of arguments taken as rvalue references to deduced types, preserving any potential move semantics involved.
The need for this function stems from the fact that all named values (such as function parameters) always evaluate as lvalues (even those declared as rvalue references), and this poses difficulties in preserving potential move semantics on template functions that forward arguments to other functions.
Both signatures return the same as:
1
static_cast<decltype(arg)&&>(arg)T&&) that refers to arg that can be used to pass an rvalue.
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// forward example
#include <utility> // std::forward
#include <iostream> // std::cout
// function with lvalue and rvalue reference overloads:
void overloaded (const int& x) {std::cout << "[lvalue]";}
void overloaded (int&& x) {std::cout << "[rvalue]";}
// function template taking rvalue reference to deduced type:
template <class T> void fn (T&& x) {
overloaded (x); // always an lvalue
overloaded (std::forward<T>(x)); // rvalue if argument is rvalue
}
int main () {
int a;
std::cout << "calling fn with lvalue: ";
fn (a);
std::cout << '\n';
std::cout << "calling fn with rvalue: ";
fn (0);
std::cout << '\n';
return 0;
}calling fn with lvalue: [lvalue][lvalue] calling fn with rvalue: [lvalue][rvalue]
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