Given an expression E(a,b,c,d ...), each of some templated type, we want f(a,b,c,d...) to be equivalent. So nest E inside f.

This cannot be accomplished by C++03.

• Case 1: you use lvalue refs for f. This fails to handle rvalue cases.
• Case 2: const lvalue refs. Now the arguments are rendered unmodifiable for E.
• Case 3: The infamous const_cast. Can lead to undefined behaviour.
• Case 4: Handle all combinations. Overloads become exponential with the number of parameters.

• If f got an lvalue ref and E needs an lvalue ref, then it obviously works out. Neither of them can take ownership of the object so neither of them will have it.
• If f got an rvalue ref and E needs an rvalue ref, it still checks out. Both can take ownership, both can have it. This mechanism also makes sure that E doesn't receive it as an lvalue, which is what happens with an rvalue ref inside a function block (See rvalue refs)
• If f got an lvalue ref and E needs an rvalue ref, it will be given an lvalue ref. This is because it cannot be given ownership of the object, since f doesn't have it itself.
• If f got an rvalue ref and E needs an lvalue ref, do we even care? Let it have that, it was going to get lvalue ref without intervention anyways.

This combination is called as perfect forwarding.