为什么numeric_limits对引用类型不起作用? [英] Is there a reason why numeric_limits do not work on reference types?

问题描述

如果您错误地执行了以下操作:

If you mistakenly do something like:

#include<limits>
int arr[3];
auto x  = std::numeric_limits<decltype(arr[0])>::max();

您将从STL实现中的文件中获得无用的错误消息.

You will get unhelpful error message from the file in the STL implementation.

问题在于模板参数是一个引用，因此解决方法是将其删除:

Problem is that template argument is a reference, so the fix is to remove it:

auto x  = std::numeric_limits<std::remove_reference_t<decltype(arr[0])>>::max();

现在我的问题是为什么 numeric_limits 不知道自己这样做?我会理解您不希望删除指针性(因为 char 指针的 max 和 char 的 max 完全不同)，但是我认为只要您将引用作为 numeric_limits 的参数，您都会对删除它所获得的结果感到满意.

Now my question is why numeric_limits do not know to do this by themselves? I would understand that you do not want to remove pointerness(since max of char pointer and max of char are very very different things), but I would assume that whenever you have a reference as an argument to numeric_limits you would be happy with result that is obtained by removing it.

推荐答案

从技术角度来看，没有理由说明 std :: numeric_limits< T> 不能与引用一起使用.添加像这样的部分专业化所需的一切:

From a technical point of view, there is no reason why std::numeric_limits<T> couldn't work with references. All what would be needed it to add a partial specialisations like this:

namespace std {
    template <typename T> struct numeric_limits<T&>: numeric_limits<T> {};
    template <typename T> struct numeric_limits<T&&>: numeric_limits<T> {};
    template <typename T> struct numeric_limits<T const>: numeric_limits<T> {};
    template <typename T> struct numeric_limits<T volatile>: numeric_limits<T> {};
    template <typename T> struct numeric_limits<T const volatile>: numeric_limits<T> {};
}

用户当然不能添加这些专业.但是，这不是一个很大的限制，因为可以在适当的命名空间中创建 numeric_limits 的自定义变体.

A user can't add these specialisations, of course. However, that's not a huge constraint as a custom variant of numeric_limits can be created in a suitable namespace.

由于从技术上讲可行，现在的问题是为什么该标准不提供这些声明.我认为不会有一个结论性的答案(除非对该想法进行了讨论并以合适且仍可访问的记录进行了丢弃).以下是一些可能的答案:

As it is technically doable the question now becomes why the standard doesn't provide these declarations. I don't think there will be a conclusive answer (unless this idea was discussed and discarded with a suitable and still accessible record). Here are some of the potential answers:

1. 未提出该功能.当引入 std :: numeric_limits 时，它专门针对于使用更多的C ++方法替换< limits.h> 中的宏.诸如 decltype(expr)之类的东西和转发引用都不存在，即模板参数不会被偶然"推导为引用类型.因此，当时不考虑删除预选赛.
2. 我不确定是否在历史上添加 numeric_limits 时已经存在部分模板专门化功能.即使它存在，也不存在类似于模板元编程的任何东西.结果，可能不可能或假定不可能以必要的方式来插入模板参数类型.
3. 即使考虑到这一点，我也怀疑委员会是否会添加部分专业知识: numeric_limits< T> 会检查 T 类型的特征，但引用类型不会没有 max()或 digits .另外，如果支持引用类型是因为显然"所需的属性必须是要停止的基础类型之一:应该 std :: numeric_limits< int *> :: max()提供以下内容:与 std :: numeric_limits< int> :: max()的值也一样吗?毕竟，它对指针也没有任何意义.
4. 考虑到原始提案几乎肯定没有涵盖限定类型的情况(请参见上文)，因此该功能不可用的另一个原因是根本没有提出该提案:没有提案，标准就不会换衣服.如果提出该功能，是否会更改标准是一个单独的问题.在此常规空间中有一个建议( P0437r0 )，但浏览它，我认为该建议不包括合格的建议类型.

1. The feature wasn't proposed. When std::numeric_limits was introduced it specifically targeted replacing the macros in <limits.h> with a more a C++ approach. Something like decltype(expr) and forwarding references didn't exist, i.e., template arguments wouldn't be "accidentally" deduced as reference types. Thus, removing qualifiers wasn't a concern at the time.
2. I'm not sure if at the point in history when numeric_limits were added partial template specialisation already existed. Even if it existed, anything resembling template meta programming didn't exist. As a result, it may not have been possible or assumed to be possible to meddle with the template argument type in the necessary way.
3. Even if it were considered, I doubt the committee would have gone with adding the partial specialisations: numeric_limits<T> inspects the traits of type T but reference types don't have a max() or digits. Also, if reference types are supported because "clearly" the desired property must be the one of the underlying type where to stop: should std::numeric_limits<int*>::max() provide the same value as std::numeric_limits<int>::max(), too? After all, it also doesn't make any sense on pointers.
4. Considering that the original proposal almost certainly didn't cover the case of qualified types (see above), another reason why the feature isn't available is that it simply wasn't proposed: without a proposal the standard won't get changed. Whether the standard would get changed if the feature were proposed is a separate question. There is a proposal in this general space (P0437r0) but browsing over it I don't think this proposal covers qualified types, either.

这篇关于为什么numeric_limits对引用类型不起作用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！