1. An overloaded operator must have at least one operand of class or enumeration type.
2. Default arguments for overloaded operators are illegal, except for operator(), the function-call operator.
3. The 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 he state of their object or that are closely tied to their given type - such as increment, decrement, and dereference - usually should be members of the class.
Symmetric operators, such as the arithmetic, equality, relational, and bitwise operators, are best defined as ordinary nonmember functions.
4. < 和 == 的定义应该保持一致。
5. 定义下标操作的类，应该有两个版本：返回引用的非const成员，返回const引用的const成员。
6. 不用为ScreenPtr类定义默认构造函数，因此，一个ScreenPtr对象将总是指向一个Screen对象，不会有未绑定的ScreenPtr，这一点和内置指针不同。
6. Function Object
class GT_cls { public: GT_cls(size_t val = 0) : bound(val) { } bool operator() (const string &s) { return s.size() >= bound; } private: std::string::size_type bound; }; count_if(words.begin(), words.end(), GT_cls(6)); count_if的原型是 template <class InputIterator, class UnaryPredicate> typename iterator_traits<InputIterator>::difference_type count (InputIterator first, InputIterator last, UnaryPredicate pred) { typename iterator_traits<InputIterator>::difference_type ret = 0; while (first!=last) { if (pred(*first)) ++ret; ++first; } return ret; } 由于类重载了()操作符，所以pred(*first)仍然可用。