[cfe-commits] [PATCH] Fix for reinterpret_cast (bug 11747)

[Eli Friedman]

On Sat, Jan 21, 2012 at 11:38 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
> On Thu, Jan 19, 2012 at 12:00 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
>> On Mon, Jan 16, 2012 at 8:38 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>> On Sun, Jan 15, 2012 at 9:05 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>>> On Sun, Jan 15, 2012 at 1:45 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>>>> This is a patch for fixing 11747, allowing reinterpret_cast to "cast"
>>>>> where the source and destination types are the same as per
>>>>> [expr.reinterpret.cast]p2.  The fix itself is fairly simple -- the
>>>>> code was already there, it just needed to be moved up slightly since
>>>>> the previous check was for source and dest being pointers.
>>>>
>>>> C++11 [expr.reinterpret.cast]p2 is as follows:
>>>>
>>>> The reinterpret_cast operator shall not cast away constness (5.2.11).
>>>> An expression of integral, enumeration, pointer, or pointer-to-member
>>>> type can be explicitly converted to its own type; such a cast yields
>>>> the value of its operand.
>>>
>>> Attached is a new patch to hopefully better address this, as well as
>>> some additional test cases.
>>
>> +    // C++ 5.2.10p2 has a note that mentions that, subject to all other
>> +    // restrictions, a cast to the same type is allowed so long as it does not
>> +    // cast away constness. The intent is not entirely clear here, since all
>> +    // other paragraphs explicitly forbid casts to the same type.
>> +    //
>> +    // The only allowed types are: integral, enumeration, pointer, or
>> +    // pointer-to-member types.
>>
>> This comment is really unclear... you should really explicitly state
>> that the C++98 version is unclear, and C++11 clarifies this case.
>
> I've clarified now, thanks for pointing out the confusion!
>
>> +    } else if (SrcType->isPointerType() ||
>> +      SrcType->isMemberPointerType()) {
>>
>> The original code allowed "DestType->isAnyPointerType() ||
>> DestType->isBlockPointerType()".  I don't see any reason to restrict
>> Objective-C or block pointers.
>
> Nor do I, I've added this in as well.
>
>> +      if (CastsAwayConstness(Self, SrcType, DestType, /*CheckCVR=*/!CStyle,
>> +                           /*CheckObjCLifetime=*/CStyle)) {
>>
>> How could a cast where the source and destination types are identical
>> cast away constness?
>
> From my testing, the equality test does not take cv qualifiers into
> account.  Is there another way I should be testing for equality that
> does?

"==" should definitely be taking qualifiers into account.  Do you have
a testcase?

-Eli