[cfe-commits] r148348 - in /cfe/trunk: include/clang/Sema/Sema.h lib/Sema/Sema.cpp lib/Sema/SemaDeclCXX.cpp

[Richard Smith]

On Wed, January 18, 2012 01:31, Sebastian Redl wrote:
> On 18.01.2012, at 00:34, Richard Smith wrote:
>> On Tue, January 17, 2012 22:49, Sebastian Redl wrote:
>>
>>> Author: cornedbee
>>> Date: Tue Jan 17 16:49:33 2012
>>> New Revision: 148348
>>>
>>>
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=148348&view=rev
>>> Log:
>>> Add Sema::isStdInitializerList, which will be necessary for the upcoming
>>> operations.
>>>
>>> Modified:
>>> cfe/trunk/include/clang/Sema/Sema.h cfe/trunk/lib/Sema/Sema.cpp
>>> cfe/trunk/lib/Sema/SemaDeclCXX.cpp
>>>
>>> Modified: cfe/trunk/lib/Sema/SemaDeclCXX.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclCXX.cpp?rev
>>> =14
>>> 8348&r1=148347&r2=148348&view=diff
>>> ==========================================================================
>>> ====
>>> --- cfe/trunk/lib/Sema/SemaDeclCXX.cpp (original)
>>> +++ cfe/trunk/lib/Sema/SemaDeclCXX.cpp Tue Jan 17 16:49:33 2012
>>> @@ -5767,6 +5767,59 @@
>>> return getStdNamespace(); }
>>>
>>>
>>> +bool Sema::isStdInitializerList(QualType Ty, QualType *Element) {
>>> +  assert(getLangOptions().CPlusPlus &&
>>> +         "Looking for std::initializer_list outside of C++.");
>>> +
>>> +  // We're looking for implicit instantiations of
>>> +  // template <typename E> class std::initializer_list.
>>> +
>>> +  if (!StdNamespace) // If we haven't seen namespace std yet, this can't
>>> be it. +    return false; +
>>> +  const RecordType *RT = Ty->getAs<RecordType>();
>>> +  if (!RT)
>>> +    return false;
>>> +
>>> +  ClassTemplateSpecializationDecl *Specialization =
>>> +      dyn_cast<ClassTemplateSpecializationDecl>(RT->getDecl());
>>> +  if (!Specialization)
>>> +    return false;
>>> +
>>> +  if (Specialization->getSpecializationKind() !=
>>> TSK_ImplicitInstantiation)
>>> +    return false;
>>> +
>>> +  ClassTemplateDecl *Template = Specialization->getSpecializedTemplate();
>>>
>>
>> It's not clear how we should behave if this isn't a specialization of the
>> primary template -- [namespace.std]p1 seems to allow user-specialization of
>> std::initializer_list, though that seems unimplementable in general.
>>
>
> I don't think we can do anything reasonable in such a case, so I haven't made
> any decision, just wrote the code that was easiest to write. Option: disallow
> specialization. That would probably be the most reasonable approach, all
> things considered. Option: allow specialization, but don't give special
> treatment to specializations of std::initializer_list (i.e. treat them as
> normal types). That's unintuitive, though. Option: allow specialization and
> still give special treatment. As you said, unimplementable.

I've brought this up on the core reflector. It looks likely that the
conclusion will be that users are not permitted to specialize
std::initializer_list.

> I'm still somewhat worried about the lack of specification on
> initializer_list in general. As far as I can tell, the common C++ ABI doesn't
> specify the layout or at least the constructor signature of initializer_list.
> This is a problem for codegen. Do we use a constructor of initializer_list?
> If so, which one? (E*, size_t), as in libstdc++ and libc++? (E*, E*) as in
> MSVC's stdlib? template <size_t N> (E &[N]), as it would be perfectly valid
> to have in some stdlib? Do we just look for these constructors and use the
> first we find? Or do we just initialize the initializer_list object in-place?
> Again, what's the layout? Pointer+size or pointer+pointer?

I don't believe you're intended to call a constructor. I think the most robust
approach would be to look at the fields and assume that (ptr,ptr) is
(begin,end) and (ptr,int) is (begin,size).

> "The lifetime of the array is the same as that of the initializer_list
> object." So if a temporary is constructed, wouldn't the lifetime of the array
> be that of the temporary?

This is DR1290. If I understand correctly, the intent is that the lifetime of
the array acts like initializer_list is a reference type binding to the array
(under the rules in 12.2/5).

- Richard