overloading ++ --

[jacek]

could you show me any examples of operations (assigment or other) for which at first the returned value of operator+= has to be 'const' and at second, examples for which returned value of operator+= mustn't be 'const' ?

There are no "must" or "musn't" in this case, but you don't gain anything if you return const reference instead of plain reference.