Re: [Boost-users] Enforcing CRTP with the MPL?

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2006-10-06 12:55:56

Gennadiy Rozental wrote:
>
> "Stefan Schild" <schilds_at_[hidden]> wrote in message
> news:eg3r7s$bcb$1_at_sea.gmane.org...
>> Hi there
>>
>> Having a templatized base class B
>>
>> template<class T>
>> class B .....
>>
>> is it possible to enforce that a class D can derive from B IF AND ONLY
>> IF the curiously recurring template pattern is applied, that is
>>
>> class D1 : public B<D1> ... // ok, compile
>> class D2 : public B<X> ... // not ok, do not compile
>>
>> ???
>>
>> Thx
>> Stefan
> 
> Did you try:
> 
> template<typename T>
> class B {
>     BOOST_STATIC_ASSERT( boost::is_base_and_derived<B<T>, T>::value );
>    ....
> };

This doesn't quite work?  At this point, T is incomplete.  But, it ought to
be possible to put this check inside some member function of B; probably
the one that casts *this to T& would be the ideal place.  But then it
should be pretty much unnecessary as the static_cast will fail if the
conversion is not possible.  Stefan, can you give more details, i.e. a
*real* example where you want it to fail to compile, but it does?

Cheers,
Ian McCulloch