为什么C ++标准算法会“计数”？返回一个difference_type而不是size_t？ [英] Why does the C++ standard algorithm "count" return a difference_type instead of size_t?

问题描述

为什么返回类型为 std :: count 迭代器的 difference_type （通常是 ptrdiff_t ）。

Why is the return type of std::count the difference_type of the iterators (often a ptrdiff_t).

由于计数永远不能为负， size_t 从技术上来说不是正确的选择吗？而且，如果计数超出 ptrdiff_t 的范围，怎么办，因为数组的理论可能大小为 size_t ？

Since count can never be negative, isn't size_t technically the right choice? And what if the count exceeds the range of ptrdiff_t since the theoretical possible size of an array can be size_t?

编辑：到目前为止，对于该函数为什么返回<$尚无合适的答案。 c $ c> ptrdiff_t 。从以下答案中收集到的一些解释是，返回类型为 iterator_traits< InputIterator> :: difference_type ，它是通用的，可以是任何值。直到那一点才有意义。在某些情况下，计数可能会超过 size_t 。但是，对于标准迭代器，为什么返回类型为 typedef ptrdiff_t iterator_traits< InputIterator> :: difference_type 而不是 typedef size_t iterator_traits< lt ; InputIterator> :: difference_type 。

So far there is no suitable answer as to why the function returns ptrdiff_t. Some explanation gathered from the answers below is that the return type is iterator_traits<InputIterator>::difference_type which is generic and can be anything. Up until that point it makes sense. There are cases where the count may exceed size_t. However, it still does not make sense why the return type is typedef ptrdiff_t iterator_traits<InputIterator>::difference_type for the standard iterators instead of typedef size_t iterator_traits<InputIterator>::difference_type.

推荐答案

std :: count （）算法依靠迭代器类型来定义一个足够大的整数类型，以表示范围的任何大小。容器的可能实现包括文件和网络流等。不能保证整个范围都一次适合进程的地址空间，因此 std :: size_t 可能是

The std::count() algorithm relies on the iterator type to define an integral type large enough to represent any size of a range. Possible implementation of containers include files and network streams, etc. There is no guarantee that the entire range fits into the process' address space at once, so std::size_t might be too small.

标准 std :: iterator_traits<> 提供的唯一整数类型是 std :: iterator_traits<> :: difference_type ，适合表示两个迭代器之间的距离。对于实现为（包装）指针的迭代器，此类型为 std :: ptrdiff_t 。迭代器特征没有 size_type 之类，因此没有其他选择。

The only integral type offered by the standard std::iterator_traits<> is std::iterator_traits<>::difference_type, which is suitable for representing "distances" between two iterators. For iterators implemented as (wrappers of) pointers, this type is std::ptrdiff_t. There is no size_type or the like from iterator traits, so there is no other choice.

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