[cfe-dev] Why qualifiers for array elements on the array itself?

[Ray Zhang via cfe-dev]

Hi Eli and Aaron,

Thank you for your help! This makes a lot of sense, and thank you for
citing the standard :). For pointer types I used dyn_cast (and so far it's
been working well) and so in my opinion the getAsArrayType() function seems
a bit of an unintuitive edge case (but I'm sure there were justifiable
reasons for it). For anyone who might run into this issue in the future, if
you perform dyn_cast instead of getAsArrayType(), the qualifiers on the
elements will be dropped.

Thanks!
Ray

On Fri, Oct 9, 2020 at 1:02 PM Eli Friedman <efriedma at quicinc.com> wrote:

> The reason it’s represented this way is to preserve the source as
> written.  Suppose you have something like the following:
>
>
>
> typedef int a[10];
>
> typedef const a b;
>
>
>
> We want to preserve the TypedefType, so the const has to stay outside it.
>
>
>
> Because of this, you usually want to use ASTContext:: getAsArrayType(),
> not dyn_cast<ArrayType>(MyQualType).  The ASTContext method will rewrite
> the element type so it has the right qualifiers.
>
>
>
> -Eli
>
>
>
> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> *On Behalf Of *Ray Zhang
> via cfe-dev
> *Sent:* Friday, October 9, 2020 1:23 AM
> *To:* Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* [EXT] [cfe-dev] Why qualifiers for array elements on the array
> itself?
>
>
>
> Hi,
>
>
>
> I'm currently working on a clang cast tool and ran into a small oddity.
> This might be a stupid mistake but I'd love some pointers(no pun intended)
> or insight into it:
>
>
>
> const int arr[2] {1,2};
>
>
>
> should be "an array of fixed size 2 of constant integers". The QualType
> associated with arr is an array type, and we can observe that with
> Expr->isArrayType(). I checked the qualifiers and found that the qualifier
> for const is actually on this level, and not for the individual elements of
> ints. I found the elements inside of the array with the following:
>
>
>
> const ArrayType* ArrType = dyn_cast<ArrayType>(MyQualType);
>
> ArrType->getElementType();
>
>
>
> which I was hoping could give me the const qualifier for the integers. If
> it helps at all, I'm using the getLocalFastQualifiers() function to return
> an unsigned int, and I'm seeing if 0x1 is masked for constness.
>
>
>
> Since arrays can't really have qualifiers(the only example I see is on the
> standards <https://en.cppreference.com/w/c/language/array> for function
> parameters for C99 and not C++), was there a reason for why clang decided
> to put the qualifiers on the array qualtype rather than the elements'
> qualtype?
>
>
>
> If I made any unreasonable assumptions to reach this conclusion, please
> let me know :)
>
>
>
> Best,
>
> Ray
>
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