没有为C ++ 20中的自定义太空飞船运算符实现定义等式运算符 [英] Equality operator does not get defined for a custom spaceship operator implementation in C++20

问题描述

在C ++ 20中，新的太空飞船操作员<=>遇到了奇怪的行为.我正在将Visual Studio 2019编译器与/std:c++latest一起使用.

I'm running into a strange behavior with the new spaceship operator <=> in C++20. I'm using Visual Studio 2019 compiler with /std:c++latest.

此代码可以正常编译:

#include <compare>

struct X
{
    int Dummy = 0;
    auto operator<=>(const X&) const = default; // Default implementation
};

int main()
{
    X a, b;

    a == b; // OK!

    return 0;
}

但是，如果我将 X 更改为此:

However, if I change X to this:

struct X
{
    int Dummy = 0;
    auto operator<=>(const X& other) const
    {
        return Dummy <=> other.Dummy;
    }
};

我收到以下编译器错误:

I get the following compiler error:

error C2676: binary '==': 'X' does not define this operator or a conversion to a type acceptable to the predefined operator

我也在clang上尝试了此操作，并且得到了类似的行为.

I tried this on clang as well, and I get similar behavior.

我希望您能对默认实现为何能正确生成operator==却不能自定义生成正确解释的一些解释.

I would appreciate some explanation on why the default implementation generates operator== correctly, but the custom one doesn't.

推荐答案

这是设计使然.

[class.compare.default] (强调我的意思)

[class.compare.default] (emphasis mine)

3 如果类定义未明确声明== 运算符函数，但声明默认的三向比较 运算符函数，==运算符函数被隐式声明 具有与三向比较运算符功能相同的访问权限. X类的隐式声明的==运算符是内联的 成员，并且在X的定义中定义为默认值.

3 If the class definition does not explicitly declare an == operator function, but declares a defaulted three-way comparison operator function, an == operator function is declared implicitly with the same access as the three-way comparison operator function. The implicitly-declared == operator for a class X is an inline member and is defined as defaulted in the definition of X.

只有默认的<=>允许存在合成的==.理由是像std::vector这样的类不能使用默认的<=>.另外，对于==使用<=>并不是比较向量的最有效方法. <=>必须给出确切的顺序，而==可能会通过先比较大小来提早保释.

Only a defaulted <=> allows a synthesized == to exist. The rationale is that classes like std::vector cannot use a defaulted <=>. In addition, using <=> for == is not the most efficient way to compare vectors. <=> must give the exact ordering, whereas == may bail early by comparing sizes first.

如果一个类在其三向比较中做一些特殊的事情，则可能需要在其==中做一些特殊的事情.因此，该语言不会产生不合理的默认值，而是将其留给程序员.

If a class does something special in its three-way comparison, it will likely need to do something special in its ==. Thus, instead of generating a non-sensible default, the language leaves it up to the programmer.

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