What kind of problems for not forwarding universal reference?

Question

As far as I know, in C++11, universal reference should always be used with std::forward, but I am not sure of what kind of problem can occur if std::forward

Answer 1

Let's say f is called like this:

f(someType{});

If the body of f performs some kind of operation on x

foo(x);

and there are two overloads for foo

void foo(someType const&);

void foo(someType&&);

without using std::forward, as x is an lvalue the following overload is called

void foo(someType const&);

which might cost you a potential optimization.

Using

foo(std::forward(x));

makes sure the correct overload of foo is selected.