On Wednesday, 13 July 2016 at 11:48:15 UTC, John Colvin wrote:
On Wednesday, 13 July 2016 at 11:28:11 UTC, Andrei Alexandrescu
wrote:
On 7/13/16 5:19 AM, John Colvin wrote:
"Casting away immutable is undefined behaviour": the
following code has
undefined results (note, not implementation defined, not
if-you-know-what-you're-doing defined, undefined), despite
not doing
anything much:
void foo()
{
immutable a = new int;
auto b = cast(int*)a;
}
"modifying immutable data is undefined": The above code is
fine, but the
following is still undefined:
void foo()
{
immutable a = new int;
auto b = cast(int*)a;
b = 3;
}
Interesting distinction. We must render the latter undefined
but not the former. Consider:
struct S { immutable int a; int b; }
S s;
immutable int* p = &s.a;
It may be the case that you need to get to s.b (and change it)
when all you have is p, which is a pointer to s.a. This is
essentially what makes AffixAllocator work.
Andrei
Hmm. You have to create a mutable reference to immutable data
to do that (although you are casting away immutable).
Woops, I meant "You don't have to create".
