操作符重载成员还是非成员函数选择【c++ primer 】

 
When designing the overloaded operators for a class, we must choose whether to make each 
operator a class member or an ordinary nonmember function. In some cases, the programmerhas no choice; the operator must be a member.
 In other cases, there are some rules of thumb that can help guide the decision. The following guidelines can be of help when deciding whether
to make an operator a member or an ordinary nonmember function: he assignment (= ), subscript ([] ), call (() ), 
and member access arrow (-> ) operators must be defined as members. Defining any of these operators as a nonmember function is
flagged at compile time as an error.
Like assignment, the compound-assignment operators ordinarily ought to be members of the class. Unlike assignment, they are not required to be so and the compiler will not
complain if a nonmember compound-assignment operator is defined.
Other operators that change the state of their object or that are closely tied to their given 
typesuch as increment, decrement, and dereferenceusually should be members of the
class.
Symmetric operators, such as the arithmetic, equality, relational, 
and bitwise operators, are best defined as ordinary nonmember functions.