为什么C ++ 20中引入了std :: ssize（）？ [英] Why is std::ssize() introduced in C++20?

问题描述

C ++ 20 介绍了 std： ：ssize（）如下所示的免费功能：

C++20 introduced the std::ssize() free function as below:

template <class C>
    constexpr auto ssize(const C& c)
        -> std::common_type_t<std::ptrdiff_t,
                              std::make_signed_t<decltype(c.size())>>;

可能的实现方式似乎是使用 static_cast 将cl ass C 成员函数 size（）的返回值转换为其签名的对应值。

A possible implementation seems using static_cast, to convert the return value of the size() member function of class C into its signed counterpart.

由于C的 size（）成员函数总是返回非负值，所以为什么有人要将它们存储在带符号的变量中？如果确实要这样做，则可以通过简单的 static_cast 来解决。

Since the size() member function of C always returns non-negative values, why would anyone want to store them in signed variables? In case one really wants to, it is a matter of simple static_cast.

为什么在C ++ 20中引入了 std :: ssize（）？

Why is std::ssize() introduced in C++20?

推荐答案

本文。引号：

在C ++ 17中采用span时，它使用带符号整数作为索引和大小。部分原因是为了允许使用 -1。作为前哨值，指示在编译时大小未知的类型。但是，让STL容器的size（）函数返回一个带符号的值是有问题的，因此将P1089引入到修复目录中。问题。它获得了多数支持，但没有获得共识所需的2比1保证金。

When span was adopted into C++17, it used a signed integer both as an index and a size. Partly this was to allow for the use of "-1" as a sentinel value to indicate a type whose size was not known at compile time. But having an STL container whose size() function returned a signed value was problematic, so P1089 was introduced to "fix" the problem. It received majority support, but not the 2-to-1 margin needed for consensus.

本文P1227是一项建议，建议添加非成员std :: ssize和成员ssize（ ） 功能。包含这些内容将使某些代码更加简单明了，并可以避免在大小计算中出现不必要的无符号性。这个想法是，如果通过std :: ssize（）和作为成员函数使ssize（）可用于所有容器，则对P1089的抵抗力将降低。

This paper, P1227, was a proposal to add non-member std::ssize and member ssize() functions. The inclusion of these would make certain code much more straightforward and allow for the avoidance of unwanted unsigned-ness in size computations. The idea was that the resistance to P1089 would decrease if ssize() were made available for all containers, both through std::ssize() and as member functions.

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