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 == 的定义应当对峙一致。

5. 定义下标操纵的类，应当有两个版本：返回引用的非const成员，返回const引用的const成员。

6. 不消为ScreenPtr类定义默认机关函数，是以，一个ScreenPtr对象将老是指向一个Screen对象，不会有未绑定的ScreenPtr，这一点和内置指针不合。

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;

}



因为类重载了（）操纵符，所以pred（first）仍然可用。