On my platform (and on most of them I think) std::size_t and std::ptrdiff_t have the same size and the same alignment. Is there any platform where that is not true? In short: is it required by the standard?
In short: is it required by the standard?
No. The only requirement is from [support.types.layout]/2 and it is:
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].
There is 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 ]
but notes are non-normative and it is only a recommendation, not a requirement.
std::size_t is defined as
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]).
in paragraph 3 and it also has no requirement that they be the same.