c++ - size_t 是否与 ptrdiff_t 具有相同的大小和对齐方式？

Question

在我的平台上(以及我认为的大多数平台上)std::size_t和 std::ptrdiff_t具有相同的大小和相同的对齐方式。有没有哪个平台不是真的？简而言之:标准是否要求？

Answer 1

In short: is it required by the standard?

不。唯一的要求来自 [support.types.layout]/2它是:

The type ptrdiff_­t is an implementation-defined signed integer type that can hold the difference of two subscripts in an array object, as described in [expr.add].

有 paragraph 4

[ Note: It is recommended that implementations choose types for ptrdiff_­t and size_­t whose integer conversion ranks are no greater than that of signed long int unless a larger size is necessary to contain all the possible values. — end note ]

但注释是非规范性的，它只是一个建议，而不是一个要求。
std::size_t被定义为

The type size_­t is an implementation-defined unsigned integer type that is large enough to contain the size in bytes of any object ([expr.sizeof]).

在 paragraph 3并且它也没有要求它们是相同的。