1. On the other hand, a class or nonmember function need not have been declared to be made a friend. 另一方面，不必预先声明类和非成员函数来将它们设为友元。
2. When resolving the use of an operator, it is possible for both nonmember and member versions of the operator to be candidates. 而确定操作符的使用时，操作符的非成员和成员版本可能都是候选者。
3. Symmetric operators, such as the arithmetic, equality, relational, and bitwise operators, are best defined as ordinary nonmember functions. 对称的操作符，如算术操作符、相等操作符、关系操作符和位操作符，最好定义为普通非成员函数。
