C++ Argument forwarding


Example

Template may accept both lvalue and rvalue references using forwarding reference:

template <typename T>
void f(T &&t);

In this case, the real type of t will be deduced depending on the context:

struct X { };

X x;
f(x); // calls f<X&>(x)
f(X()); // calls f<X>(x)

In the first case, the type T is deduced as reference to X (X&), and the type of t is lvalue reference to X, while in the second case the type of T is deduced as X and the type of t as rvalue reference to X (X&&).

Note: It is worth noticing that in the first case, decltype(t) is the same as T, but not in the second.

In order to perfectly forward t to another function ,whether it is an lvalue or rvalue reference, one must use std::forward:

template <typename T>
void f(T &&t) {
    g(std::forward<T>(t));
}

Forwarding references may be used with variadic templates:

template <typename... Args>
void f(Args&&... args) {
    g(std::forward<Args>(args)...);
}

Note: Forwarding references can only be used for template parameters, for instance, in the following code, v is a rvalue reference, not a forwarding reference:

#include <vector>

template <typename T>
void f(std::vector<T> &&v);