明确定义了哪些指针值以进行计算? [英] What pointer values are well-defined to compute?

问题描述

我的印象是，虽然取消引用未指向有效对象的指针是UB，但只需计算此类指针就可以了.

I was under the impression that while dereferencing pointers that don't point to a valid object is UB, simply computing such pointers is fine.

但是，如果我了解 expr.add [4] 正确，事实并非如此.

However, if I'm understanding expr.add[4] correctly, that's not the case.

那么这些指针计算中哪些是定义明确的?

So which of these pointer computations are well-defined?

int a = 42;
int *p = &a;
p;           // valid, and obviously ok
p++;         // invalid, but ok, because one past the end of 'array' containing 1 element?
p++;         // UB ?

这种情况怎么样?

int *p = nullptr;
p;                // invalid, and obviously ok (considered one past the end?)
p++;              // one past the end? or UB?

推荐答案

在您的第一个示例中，第一个 p ++ 是定义明确的，因为非数组被视为单长度数组.

In your first example, the first p++ is well-defined, because a non-array is considered a one-length array.

这是相关的报价( basic.compound/3.4 ):

出于指针算术([expr.add])和比较([expr.rel]，[expr.eq])的目的，将指针从n个元素的数组x的最后一个元素的末尾算起等效于指向x的假设数组元素n的指针，并且不是数组元素的类型T的对象被认为属于一个元素类型T的数组.

在 p ++ ， p 之后，它将指向(假设的)数组的最后一个(也是唯一的)元素，该元素定义明确.它不是无效的，但可以"，因为指向对象末尾的指针不是无效的指针， basic.compound/3.2 :

After p++, p it will point past the last (and only) element of the (hypothetical) array, which is well-defined. It is not "invalid, but ok", as pointers pointing to past the end of an object are not invalid pointers, basic.compound/3.2:

指针类型的每个值都是以下之一:

Every value of pointer type is one of the following:

• [...]

• [...]

指向对象末尾的指针

[...]

无效的指针值.

第一个示例的第二个 p ++ 是UB，因为结果将指向未定义的假设(&a)[1]元素之后.

The second p++ of the first example is UB, because the result will point after the hypothetical (&a)[1] element, which is not defined.

在第二个示例中， p ++ 是UB，因为只能将0添加到 nullptr (

In your second example, p++ is UB, because only 0 can be added to a nullptr (expr.add/4.1):

• 如果P评估为空指针值，而J评估为0 ，则结果为空指针值.

[...]

否则，行为未定义.

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