blocks|key|2789852|text|在this+paper中描述了其基本原理。引用一句话：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2789853|2789854|在C%2B%2B17中采用span时，它同时使用带符号整数作为索引和大小。这在一定程度上是为了允许使用"-1“作为标记值，以指示其大小在编译时未知的类型。但是让一个STL容器的size()函数返回一个带符号的值是有问题的，所以引入了P1089来“修复”这个问题。它得到了多数人的支持，但没有达到达成共识所需的2比1的优势。|blockquote|2789855|2789856|本文P1227建议添加非成员std::ssize和成员ssize()函数。包含这些将使某些代码更直接，并允许在大小计算中避免不想要的无符号。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的抵抗力将会降低。|2789857|entityMap|0|LINK|mutability|MUTABLE|url|http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1227r1.html^0|1|A|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|-4|5|6|7|W|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|X|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|Z|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|10|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

The rationale is described in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1227r1.html" rel="noreferrer">this paper</a>. A quote:
<blockquote>
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 &quot;-1&quot; 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 &quot;fix&quot; the problem. It received majority support, but not the 2-to-1 margin needed for consensus.
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.
</blockquote>

blocks|key|71939|text|来自Eric+Niebler的stolen：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|71940|当第一次设计STL时，|71941|+'Unsigned+types+signal+that+a+negative+index/size+is+not+sane'是最流行的智慧。但从逻辑上讲，事物的计数不一定是正数。我可能希望在有符号整数中保留一个计数，以表示添加到集合或从集合中删除的元素的数量。然后我想把它和集合的大小结合起来。如果集合的大小是无符号的，那么现在我不得不混合使用有符号和无符号的算法，这就是一个bug农场。编译器对此发出警告，但由于STL的设计很大程度上迫使程序员陷入这种情况，因此该警告非常常见，以至于大多数人将其关闭。这是一种遗憾，因为它隐藏了真正的bug。|blockquote|style|CODE|71942|71943|在接口中使用无符号整数并不像许多人认为的那样有好处。如果用户意外地向API传递了一个轻微的负数，它会突然变成一个巨大的正数。如果API将数字作为有符号的，那么它可以通过断言数字大于或等于零来检测情况。|71944|如果我们将无符号整数的使用限制为位旋转(例如，掩码)，而在其他任何地方使用有符号整数，则错误发生的可能性较小，并且更容易检测到错误发生的时间。|71945|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/ericniebler/stl2/issues/182#issuecomment-257000171^0|F|6|0|0|0|1|1Q|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@$A|Y|B|Z|1|10]]|C|$]]|$1|D|3|E|5|6|7|11|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|12|8|@$A|13|B|14|I|J]]|9|@]|C|$]]|$1|K|3|-4|5|6|7|15|8|@]|9|@]|C|$]]|$1|L|3|M|5|6|7|16|8|@]|9|@]|C|$]]|$1|N|3|O|5|6|7|17|8|@]|9|@]|C|$]]|$1|P|3|-4|5|6|7|18|8|@]|9|@]|C|$]]]|Q|$R|$5|S|T|U|C|$V|W]]]]

Gratuitously <a href="https://github.com/ericniebler/stl2/issues/182#issuecomment-257000171" rel="noreferrer">stolen</a> from Eric Niebler:

<blockquote>
 <code>'Unsigned types signal that a negative index/size is not sane'</code> was
 the prevailing wisdom when the STL was first designed. But logically,
 a count of things need not be positive. I may want to keep a count in
 a signed integer to denote the number of elements either added to or
 removed from a collection. Then I would want to combine that with the
 size of the collection. If the size of the collection is unsigned, now
 I'm forced to mix signed and unsigned arithmetic, which is a bug farm.
 Compilers warn about this, but because the design of the STL pretty
 much forces programmers into this situation, the warning is so common
 that most people turn it off. That's a shame because this hides real
 bugs.
 
 Use of unsigned ints in interfaces isn't the boon many people think it
 is. If by accident a user passes a slightly negative number to the
 API, it suddenly becomes a huge positive number. Had the API taken the
 number as signed, then it can detect the situation by asserting the
 number is greater than or equal to zero.
 
 If we restrict our use of unsigned ints to bit twiddling (e.g., masks)
 and use signed ints everywhere else, bugs are less likely to occur,
 and easier to detect when they do occur.
</blockquote>

<a href="https://en.wikipedia.org/wiki/C%2B%2B20" rel="noreferrer">C++20</a> introduced the <code>std::ssize()</code> free function as below:

<pre class="lang-cpp prettyprint-override"><code>template &lt;class C&gt;
 constexpr auto ssize(const C&amp; c)
 -&gt; std::common_type_t&lt;std::ptrdiff_t,
 std::make_signed_t&lt;decltype(c.size())&gt;&gt;;
</code></pre>

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

Since the <code>size()</code> 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 <code>static_cast</code>. 

Why is <code>std::ssize()</code> introduced in C++20?

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

引入了免费的std::ssize()函数，如下所示：template <class C> constexpr auto ssize(const C& c) -> std::common_type_t<std::ptrdiff_t, std::...

问为什么在C++20中引入std::ssize()？
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