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<cstdint>对比 std::size_t 类型

[英]<cstdint> vs std::size_t types

原文 2014-05-14 16:54:25 6 3 c++/ c++11/ stl

From the peeks I've made into boost and libstdc++ , the libraries usually make use of std::size_t and std::ssize_t whenever the upper/lower limit of an unsigned/signed index is not known in advance.从我对boost和libstdc++的std::ssize_t只要事先不知道无符号/有符号索引的上限/下限，这些库通常会使用std::size_t和std::ssize_t 。 My question is: Why not rather use uintmax_t from <cstdint> instead of std::size_t and intmax_t instead of std::ssize_t ?我的问题是：为什么不情愿使用uintmax_t从<cstdint>代替std::size_t和intmax_t代替std::ssize_t ？

3 个解决方案

The former are part of the C++ standard, the latter are not.前者是 C++ 标准的一部分，后者不是。 More precisely, the cstdint header was only recently introduced (in C++11).更准确地说， cstdint头文件最近才被引入（在 C++11 中）。 The reason for this is that stdint.h itself is part of C99, which is newer than C++98.这样做的原因是stdint.h本身是 C99 的一部分，它比 C++98 更新。

Because the size_t types are intended to describe the sizes of things.因为 size_t 类型旨在描述事物的大小。 Using them for sizes is more descriptive than uint_t.将它们用于大小比 uint_t 更具描述性。

Also, it might be possible for an architecture to be limited to smaller sizes of things so size_t might not always be the biggest integer type.此外，架构可能仅限于较小尺寸的事物，因此 size_t 可能并不总是最大的整数类型。 Although I think that would be a bit weird.虽然我觉得那会有点奇怪。

The C++11 standard (section 18.2) says: C++11 标准（第 18.2 节）说：

(5). (5). The type ptrdiff_t is an implementation-deﬁned signed integer type that can hold the diﬀerence of two subscripts in an array object....类型ptrdiff_t是一个实现定义的有符号整数类型，它可以在一个数组对象中保存两个下标的差异......

(6). (6). The type size_t is an implementation-deﬁned unsigned integer type that is large enough to contain the size in bytes of any object.类型size_t是实现定义的无符号整数类型，它足够大以包含任何对象的字节大小。

(7). (7). [ Note: It is recommended that implementations choose types for ptrdiff_t and size_t whose integer conversion ranks (4.13) are no greater than that of signed long int unless a larger size is necessary to contain all the possible values. [注意：建议实现为ptrdiff_t和size_t选择整数转换等级 (4.13) 不大于有signed long int除非需要更大的大小来包含所有可能的值。 —end note ] ——尾注]

From this we see that:由此我们看到：

size_t is specifically for byte-sizes of objects, and its companion ptrdiff_t is specifically for math with array indices. size_t专门用于对象的字节大小，它的配套ptrdiff_t专门用于具有数组索引的数学。 uintmax_t , on the other hand, is the largest unsigned integral type.另一方面， uintmax_t是最大的无符号整数类型。

Depending on the platform uintmax_t could be larger than size_t .根据平台uintmax_t可能大于size_t 。

We also know that:我们也知道：

sizeof returns a size_t , and the STL container size_type s are typically identical to size_t , so it makes sense to use size_t in code that deals with sizeof or STL containers. sizeof返回一个size_t ，并且 STL 容器size_type s 通常与size_t相同，因此在处理sizeof或 STL 容器的代码中使用size_t是有意义的。

Now mix in the fact the <cstdint> is new-ish to C++, and I think it's pretty clear why established libraries like Boost have been using size_t .现在混合<cstdint>是 C++ 的新事实，我认为很清楚为什么像 Boost 这样的成熟库一直在使用size_t 。