为什么C ++编译器不定义operator ==和operator！=？ [英] Why don't C++ compilers define operator== and operator!=?

问题描述

我是一个很大的粉丝，让编译器做尽可能多的工作，为你尽可能。当编写一个简单类时，编译器可以为'free'提供以下内容：

I am a big fan of letting the compiler do as much work for you as possible. When writing a simple class the compiler can give you the following for 'free':

• 默认（空）构造函数


• 一个复制构造函数


• 一个析构函数


• 赋值运算符（ operator = ）

• A default (empty) constructor
• A copy constructor
• A destructor
• An assignment operator (operator=)

但它似乎不能给你任何比较运算符，例如 operator == 或 operator！= 。例如：

But it cannot seem to give you any comparison operators - such as operator== or operator!=. For example:

class foo
{
public:
    std::string str_;
    int n_;
};

foo f1;        // Works
foo f2(f1);    // Works
foo f3;
f3 = f2;       // Works

if (f3 == f2)  // Fails
{ }

if (f3 != f2)  // Fails
{ }

这是否有很好的理由？为什么要执行逐个成员比较是一个问题？显然，如果类分配内存，那么你需要小心，但对于一个简单的类肯定是​​编译器可以为你做这个？

Is there a good reason for this? Why would performing a member-by-member comparison be a problem? Obviously if the class allocates memory then you'd want to be careful, but for a simple class surely the compiler could do this for you?

推荐答案

编译器不知道你是想要一个指针比较还是一个深层（内部）比较。

The compiler wouldn't know whether you wanted a pointer comparison or a deep (internal) comparison.

只是没有实现它，让程序员做自己。然后他们可以做出他们喜欢的所有假设。

It's safer to just not implement it and let the programmer do that themselves. Then they can make all the assumptions they like.

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