C++ Perfect forwarding.

Perfect forwarding comes from std::forward() is one of the special C++ technique to reduce the code length.

More information can be found on cppreference.com.

How it works

Basically it forwards a rvalue reference to the nested function inside a function template.

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 T is a lvalue reference, std::forward<T>() does nothing, arg will be passed as lvalue reference as normal.
• If T is a rvalue reference, since arg is treated as an lvalue in the body of the function, std::forward<T>() converts it back to rvalue reference.

For both cases, foo(), which takes an rvalue parameter OR lvalue reference, will work correctly.

Difference from std::move()

The difference from std::move() is that, std::forward<T>() is conventionally used in function template only.

• For case 1 above, std::forward<T>() is no-op, it doesn't do anything.
• For case 2 above, std::forward<T>() is same as std::move() here.

You can think std::forward<T>() is a smarter version of std::move(), however it requires a typename to work.