C++核心准则C.87:小心基类的相等运算符

C.87: Beware of == on base classes

C.87:小心基类的相等运算符

Reason（原因）

It is really hard to write a foolproof and useful == for a hierarchy.

为继承体系写出简单又好用的相等运算符真的很难。

Example, bad（反面示例）

class B {
    string name;
    int number;
    virtual bool operator==(const B& a) const
    {
         return name == a.name && number == a.number;
    }
    // ...
};

B's comparison accepts conversions for its second operand, but not its first.

B的相等比较运算符的第二个操作数接受类型转换，但是第一个不行。

class D :B {
    char character;
    virtual bool operator==(const D& a) const
    {
        return name == a.name && number == a.number && character == a.character;
    }
    // ...
};

B b = ...
D d = ...
b == d;    // compares name and number, ignores d's character
d == b;    // error: no == defined
D d2;
d == d2;   // compares name, number, and character
B& b2 = d2;
b2 == d;   // compares name and number, ignores d2's and d's character

Of course there are ways of making == work in a hierarchy, but the naive approaches do not scale

当然有办法让相等比较运算符在继承体系中动作，但是简单的方法不行。

Note（注意）

This rule applies to all the usual comparison operators: !=, <, <=, >, and >=.

本规则适用于所有的常见比较运算符：!=, <, <=, >, 和 >=。

Enforcement（实施建议）

• Flag a virtual operator==(); same for other comparison operators: !=, <, <=, >, and >=.
• 提示被定义为虚函数的相等比较运算符；其他比较运算符也一样：!=, <, <=, >, 和 >=。

原文链接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c87-beware-of--on-base-classes

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