[cfe-dev] GCC-specific, Clang-specific and ObjC-specific casts in C++

steve naroff snaroff at apple.com
Tue Jul 28 11:54:21 PDT 2009

Hi Sebastian et.al.,

I just had a brief chat with Doug on this topic...I think it's best to  
defer this issue for now.

At some point, Doug, Fariborz, and I will need to discuss how ObjC  
will fit with C++ casts.

Until C++ is more complete, it's not worth worrying about.


On Jul 28, 2009, at 2:08 PM, Sebastian Redl wrote:

> Ken Ferry wrote:
>> On Sat, Jul 25, 2009 at 3:17 PM, Daniel Dunbar <daniel at zuster.org
>> <mailto:daniel at zuster.org>> wrote:
>>    On Sat, Jul 25, 2009 at 9:04 AM, Sebastian
>>    Redl<sebastian.redl at getdesigned.at
>>    <mailto:sebastian.redl at getdesigned.at>> wrote:
>>> 4) Are there formalized Objective-C semantics? Which casts are
>>    allowed,
>>> which are disallowed? Which are implicit, and which, in
>>    Objective-C++,
>>> should be allowed by static_cast and reinterpret_cast respectively?
>>    Formalized, no. One general rule of thumb is that interface behave
>>    like pointers to structs, since this is historically their
>>    implementation. Of course, that doesn't always make a lot of  
>> sense so
>>    its mostly a good rule of thumb for what gcc will do, not what is
>>    necessarily best.
>> There are a few people on this list who have lots of experience with
>> ObjC and less with compilers.
>> If you can phrase your questions in terms of objective-C code, we can
>> answer them.
> I'm afraid I know nothing about Objective-C, so I need someone who  
> knows
> both languages, at least to translate the questions. Here's the  
> issue in
> C++:
> Given three classes, Base, Derived and Other, where Derived is derived
> from Base and Other is unrelated to either, C++ allows the following
> conversions:
> Pointer to Derived -> Pointer to Base: implicit conversion, always  
> safe
> (except in some obscure multiple inheritance cases).
> Pointer to Base -> Pointer to Derived: explicit and unchecked via
> static_cast, requires the programmer to know what he's doing, or
> explicit and runtime-checked via dynamic_cast.
> Pointer to Base <-> Pointer to Other: explicit and unsafe via
> reinterpret_cast, overrules any type safety there is
> I expect that you can convert pointers to related interfaces
> (protocols?) and classes in Objective-C as well, and I wonder how
> Objective-C++ should treat attempts to cast Obj-C types via C++ cast
> operators.

> Sebastian
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