17 Sep 2021

The std::forward function

17 Sep 2021

The std::forward function as the std::move function aims at implementing move semantics in C++. The function takes a forwarding reference. According to the T template parameter, std::forward identifies whether an lvalue or an rvalue reference has been passed to it and returns a corresponding kind of reference. std::forward helps to implement perfect forwarding. This mechanism implies that objects passed to the function as lvalue expressions should be copied, and objects passed to the function as rvalue expressions should be moved.

If you assign an rvalue reference to some ref variable, then ref is a named entity. Therefore, its category is lvalue, despite the fact that ref is an rvalue reference. Therefore, for ref copy semantics is used instead of move semantics.

The std::forward function solves this problem. Let's consider the following example:

template <typename T>
void foo(T &&arg)
{
  std::vector<int> var = arg;
  ....
}

std::vector<int> vect(1'000'000, 1);
foo(std::move(vect));

That's what happens in the code fragment:

The std::move function is called. It returns an rvalue reference to vect.
This reference is passed via a forwarding link to the instance of the foo function as the arg argument. arg is an lvalue object of the rvalue reference type.
The var variable is initialized. It calls the copy constructor. vector vect is copied element-by-element to the var variable.

Now let's use std::forward:

template <typename T>
void foo(T &&arg)
{
  std::vector<int> var = std::forward<T>(arg);
  ....
}

std::vector<int> vect(1'000'000, 1);
foo(std::move(vect));

Here's what happens:

The std::move function is called. It returns an rvalue reference to vect.
This reference is passed via a forwarding link to the instance of the foo function as the arg argument. arg is an lvalue object of the rvalue reference type.
The std::forward function is called. It returns an xvalue object with the rvalue reference type.
The var variable is initialized. It calls the move constructor. vector vect is moved to the var variable.