Re: [boost] type_traits for checking derived type

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-06-17 07:41:21

John Maddock wrote:
>> some code:
>> 
>> template <typename DerivedT>
>> class A
>> {
>> BOOST_STATIC_ASSERT((boost::is_convertible<A<DerivedT>*,
>> B*>::value)); [...]
>> };
>> 
>> class B : public A<B>
>> {
>> };
>> 
>> I want to check if the template parameter really specifies a derived
>> type. But the assert is always false.
>> How can i solve this?
> 
> I think your code should be:
> 
> template <typename DerivedT>
> class A
> {
>    BOOST_STATIC_ASSERT((::boost::is_convertible<DerivedT*,
> A<DerivedT>*>::value));
>    // this should work as well:
>    BOOST_STATIC_ASSERT((::boost::is_base_and_derived<A<DerivedT>,
> DerivedT>::value));
> };
> 
> class B : public A<B>
> {
> };
> 
> However, the static asserts still always get triggered, and I'm not
> sure why: any language lawyers understand this?

At the instantiation point B and A<B> are still incomplete.