undefined | Better HN

0 pointsBuge9y ago0 comments

It's a complicated situation. There's a pointer to an array, and that pointer is dereferenced, resulting in an array (that then decays to a pointer). But that second array/pointer is not dereferenced. I'm not sure if it's legal.

0 comments

cbsmith9y ago

Where is it dereferencing the array?

    *(&arr + 1) - arr

That translates to taking the address one point past the array and subtracting the address of the array from it. It doesn't actually dereference the location past the end of the array.

While:

    (&arr)[1] - arr

might appear to be doing something different, it actually isn't.

BugeOP9y ago

&arr is a pointer to an array (it points to the existing array).

&arr + 1 is a pointer to an array that begins just after the existing array.

* is the dereference operator, so it seems to me that *(&arr + 1) dereferences the pointer to the array, resulting in an array (or a reference to an array), which then decays to a pointer.

feelix9y ago

>so it seems to me that (&arr + 1) dereferences the pointer to the array

It doesn't. Because an array is already a pointer, in (&arr + 1) &arr is a pointer to a pointer (ie, a handle) so *(&arr) is dereferencing the handle to the pointer. So it's still one pointer level deep - it doesn't dereference it completely.

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cbsmith9y ago

Ah, but since it is dereferencing an array... you haven't actually dereferenced to the memory location yet. If you had, you wouldn't be possible to subtract a pointer from it.

j / k navigate · click thread line to collapse

0 comments

cbsmith9y ago

Where is it dereferencing the array?

    *(&arr + 1) - arr

That translates to taking the address one point past the array and subtracting the address of the array from it. It doesn't actually dereference the location past the end of the array.

While:

    (&arr)[1] - arr

might appear to be doing something different, it actually isn't.

BugeOP9y ago

&arr is a pointer to an array (it points to the existing array).

&arr + 1 is a pointer to an array that begins just after the existing array.

* is the dereference operator, so it seems to me that *(&arr + 1) dereferences the pointer to the array, resulting in an array (or a reference to an array), which then decays to a pointer.

feelix9y ago

>so it seems to me that (&arr + 1) dereferences the pointer to the array

1 more reply

cbsmith9y ago

Ah, but since it is dereferencing an array... you haven't actually dereferenced to the memory location yet. If you had, you wouldn't be possible to subtract a pointer from it.

j / k navigate · click thread line to collapse