C + + operator overloading-How to determine as a member function or a non-member function

The decision between Member and Non-member

The binary Operators = (Assignment), [] (array subscription), -> (member access), as well as the N-ary () (fu Nction call) operator, must always be implemented as member functions, because the syntax of the language require s them to.

Other operators can is implemented either as members or as non-members. Some of them, however, usually has to being implemented as non-member functions, because their left operand cannot be modifi Ed by you. The most prominent of these is the input and output operators << >> and, whose left operands is stream classes from The standard library which your cannot change.

For any operators where you have to choose to either implement them as a member function or a non-member function, use The following rules of thumb to decide:

1. If It is a unary operator, implement it as a member function.
2. If A binary operator treats both operands equally (it leaves them unchanged), implement this operator as a no N-member function.
3. If A binary operator does not treat both of its operands equally (usually it would change its left op Erand), it might is useful to make it a member function of the It left operand ' s type, if it have to access the Ope Rand ' s private parts.

Of course, as with the all rules of thumb, there is exceptions. If you have a type

enum Month {Jan, Feb, ..., Nov, Dec}

And you want to overload the increment and decrement operators for it, you cannot does this as a member functions, since in C + +, enum types cannot has member functions. So you had to overload it as a free function. And for operator<() a class template nested within a class template was much easier to write and read when do as a member Funct Ion inline in the class definition. But these is indeed rare exceptions.

(However, if you make a exception, do not forget the issue of const -ness for the operand, for member Functi ONS, becomes the implicit this argument. If the operator as a non-member function would take its left-most argument as a const reference, the same operator as a me Mber function needs to has a at the const end to make *this a const reference.)

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Original: http://stackoverflow.com/questions/4421706/operator-overloading/4421729#4421729

C + + operator overloading-How to determine as a member function or a non-member function