C++ Primer, Fifth Edition

Choosing Member or Nonmember Implementation

When we define an overloaded operator, we must decide whether to make the operator a class member or an ordinary nonmember function. In some cases, there is no choice—some operators are required to be members; in other cases, we may not be able to define the operator appropriately if it is a member.

The following guidelines can be of help in deciding whether to make an operator a member or an ordinary nonmember function:

• The assignment (=), subscript ([]), call (()), and member access arrow (->) operators must be defined as members.

• The compound-assignment operators ordinarily ought to be members. However, unlike assignment, they are not required to be members.

• Operators that change the state ...