C++ Primer 读书笔记 - 第十四章

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。