[cfe-dev] MSVC RTTI implementation status

[Nikola Smiljanic]

And what library implements __dynamic_cast for clang on Linux/OS X?


On Tue, Feb 25, 2014 at 7:44 PM, David Majnemer <david.majnemer at gmail.com>wrote:

> The code that actually performs the dynamic_cast is not actually emitted
> by the compiler.  The behavior you are seeing is not coming from the
> compiler but from the __RTDynamicCast function.
>
> Clang's job is to call __RTDynamicCast with the correct arguments, nothing
> more.
>
> The Itanium ABI, utilized by OS X and Linux, specifies that this cast is
> performed by __dynamic_cast.  Again, all we do is provide the necessary
> argument to this function and let the underlying runtime do the rest.
>
> This behavior is not really up to us to emulate.
>
> Also note that the ability to handle these kinds of memory exceptions
> happens under the aegis of SEH, making an already heavyweight operation
> even heavier.
>
> On Mon Feb 24 2014 at 1:33:35 AM, Domagoj Saric <
> domagoj.saric at littleendian.com> wrote:
>
>> On 24.2.2014. 7:53, David Majnemer wrote:
>> > I don't think it would be unreasonable for us to maintain a list of
>> things we do
>> > and do not claim to support.
>>
>> FWIW, MSVC's RTTI implementation has one nice property (which I'd like if
>> Clang
>> could also 'emulate'): it (dynamic_cast) does not crash on objects w/o an
>> RTTI
>> record (while Clang under OSX always crashed for me in those cases).
>> Such a case, for example, often arises if you use a (lame) GUI library
>> which
>> requires RTTI (because it relies on dynamic_cast and cross-casting) so you
>> enable RTTI for its sources but disable it for others/your own sources
>> (in order
>> to minimize bloat and the leaking of internal implementation details as
>> plain
>> text in the final binary). Then when the GUI lib calls dynamic_cast on
>> one of
>> your own objects (because you, for example, added it to some widget
>> container)
>> it will simply get a nullptr result with MSVC (while, as I mentioned,
>> you'll get
>> a crash with Clang)...
>>
>>
>>
>> --
>> Domagoj Saric
>> Software Architect
>> www.LittleEndian.com
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>
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