为什么 boost::variant 不提供运算符 !=
给定两个相同的 boost::variant 实例 a 和 b，表达式 <允许strong>( a == b ) 。
然而 ( a != b ) 似乎未定义。 为什么是这样？
Given two identical boost::variant instances a and b, the expression ( a == b ) is permitted.
However ( a != b ) seems to be undefined. Why is this?
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（3）
我认为它只是没有添加到库中。 Boost.Operators 不会真正有帮助，因为任何一个变体都会派生自 boost::operator::equality_comparable。 David Pierre 说你可以使用它是正确的，但你的回答也是正确的，ADL 不会找到新的运算符！=，所以你需要一个 using 运算符。
我会在 boost-users 邮件列表上询问这个问题。
编辑 @AFoglia 的评论：
七个月后，我正在研究 Boost.Variant，我偶然发现了这个对省略列表的更好解释。
http://boost.org/Archives/boost/2006/06/105895.php
operator==为当前变体中的实际类调用operator==。 同样，调用operator!=也应该调用该类的operator!=。 （因为，理论上，可以定义一个类，因此a!=b与!(a==b)不同。）因此，这会增加另一个要求：变体中的类有一个操作符!=。 （关于是否可以在邮件列表线程中做出此假设存在争议。）I think it's just not added to the library. The Boost.Operators won't really help, because either variant would have been derived from boost::operator::equality_comparable. David Pierre is right to say you can use that, but your response is correct too, that the new operator!= won't be found by ADL, so you'll need a using operator.
I'd ask this on the boost-users mailing list.
Edit from @AFoglia's comment:
Seven months later, and I'm studying Boost.Variant, and I stumble over this better explanation of the omission lists.
http://boost.org/Archives/boost/2006/06/105895.php
operator==callsoperator==for the actual class currently in the variant. Likewise callingoperator!=should also calloperator!=of the class. (Because, theoretically, a class can be defined soa!=bis not the same as!(a==b).) So that would add another requirement that the classes in the variant have anoperator!=. (There is a debate over whether you can make this assumption in the mailing list thread.)这是作者本人的答案的链接问题是在 boost 邮件列表上提出的。
总结一下，在作者看来，实现比较运算符（！= 和 <）将对用于创建变体类型的类型增加更多要求。
但我不同意他的观点，因为 != 可以以与 == 相同的方式实现，而不必隐藏这些运算符对于构成变体的每种类型的可能实现
This is a link to the answer from the author himself when this question was formulated on boost mailing list
Summarizing it, in the author opinion, implementing comparison operators (!= and <) would add more requirements on the types used to create the variant type.
I don't agree with his point of view though, since != can be implemented in the same way as ==, without necessarily hiding the possible implementations of these operators for each of the types making up the variant
因为它不需要。
Boost 有一个操作符库，它定义了操作符！=就运算符而言==
Because it doesn't need to.
Boost has an operators library which defines operator!= in term of operator==