C++ Primer, Fifth Edition

14.1. Basic Concepts

Overloaded operators are functions with special names: the keyword operator followed by the symbol for the operator being defined. Like any other function, an overloaded operator has a return type, a parameter list, and a body.

An overloaded operator function has the same number of parameters as the operator has operands. A unary operator has one parameter; a binary operator has two. In a binary operator, the left-hand operand is passed to the first parameter and the right-hand operand to the second. Except for the overloaded function-call operator, operator(), an overloaded operator may not have default arguments (§ 6.5.1 ...