Aliasing with an array

[xsk...@gmail.com]

Hi all,

Please consider the following example:

int a[3] = {0};

auto &b = reinterpret_cast<int(&)[2]>(a);

int c = b[0];

It seems that the value of a has not been accessed until b[0], which is equivalent to *(b + 0), thus access an int object through an int glvalue. 

So Is this well-defined?

Thanks.

xskxzr

[Nicol Bolas]

What about this code would not be well-defined?

[xskxzr]

It is a bit strange. If it is well-defined, what is the semantic of the array-to-pointer conversion while the array object actually does not exist?

[T. C.]

If there is no such array object, then the behavior is undefined by omission.

[xskxzr]

So do you mean the example is undefined?

[Chris Hallock]

On Sunday, December 31, 2017 at 10:31:29 PM UTC-8, xskxzr wrote:

It is a bit strange. If it is well-defined, what is the semantic of the array-to-pointer conversion while the array object actually does not exist?

Well, but the expression does refer to an actual array, just not one that has the expression's type. The good question I think you're asking is, when [conv.array] says "the result is a pointer to the first element of the array", what exactly is meant by "the array"?

1. the (presumed-to-exist) array referred to by the expression, or
2. the (presumed-to-exist) array of the expression's type referred to by the expression?

I think the correct interpretation is #1, because #2 has the absurd implication of not allowing differences in cv-qualification (unless that in itself is a defect...).

[xskxzr]

Then what if the expression refers to an non-array object? For example, change a from int[3] to int. Or do you mean the object is considered an array with a single element in such case?

[Chris Hallock]

On Wednesday, January 3, 2018 at 4:27:10 AM UTC-8, xskxzr wrote:

Then what if the expression refers to an non-array object? For example, change a from int[3] to int. Or do you mean the object is considered an array with a single element in such case?

Undefined-by-omission, as T.C. says. The "1-element array" clause doesn't help because it only applies to pointer arithmetic/comparison with the result of operator &.