[boost] Re: [any] checked convertion to reference?

From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-02-18 03:33:17

Alexander Nasonov wrote:

> 
> 
> Vladimir Prus wrote:
>> Given that overall reaction is positive, I'll make the change,
> starting with
>> the 'cast to reference' logic I wanted in the first place.
>> 
>> The only difference between my version and the one you mention above
> is this
>> snippet in 'any_cast':
>> 
>> #ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
>>         // Please use BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION
> macro
>>         // to generate specialization of remove_reference for your
> class
>>         // See type traits library documentation for details
>>         BOOST_STATIC_ASSERT(!::boost::is_reference<nonref>::value);
>> #endif
>> 
>> 
>> It looks to me that this assert will always be triggered when your
> pass
>> reference type and BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION is
> defined,
>> right? The assert will fire even if used has specialized
> remove_reference,
>> no? If that's true, the comment is not correct and casting to
> reference is
>> just not supported for broken compilers.
> 
> On broken compilers the assert for any_cast<int cv&> fires only if this
> thing is not in TU before first use of any_cast<int v&>:
> 
> BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION(int)
> 
> IMHO, if it's there, everything should work. BTW, there is a testcase in
> my
> earlier post in that thread. You can try to comment one such workaround
> to see
> the difference on a broken compiler or with
> BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION defined.

I think you're right, and trying gcc +
BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION confirms that. 
I've committed this part of the patch.

>> I see that your patch also:
>> 1. Strips cv-qualification in constructor (which is reasonable).
> 
> const is already stripped because ctor's argument is 'const ValueType &
> value'.
> So, it's only for volatile. remove_cv is used only to avoid inclusion of
> remove_volatile.hpp.

Does this use case matters, btw?
 
>> 2. Strips cv-qualification when casting to pointer.
>> 
>> Why the second aspect is needed?
> 
> Quoting myself:
> 
>> I forgot to mention that the original version has small problem with
>> any_cast
>> to const type. If any stores T and you cast it to T const you're
> trying
>> this sequence of static_casts:
>> 
>> holder<T>* -> (in ctor) -> placeholder* -> (in any_cast) -> holder<T>
> const>*
>> 
>> Obviously, it's not getting back to original holder<T> type.

What about declaring that 'T' in all variants should have *no*
cv-qualification? It's a bit strange that the user can cast to volatile
type, and the any_cast will just throw that away. And cast *from* volalite
any does not work, i.e.

    volatile boost::any a = 1;
    volatile int& i = boost::any_cast<volatile int&>(a);

does not compile. So what's the point of allowing volatile type at all?

- Volodya