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

Ian McCulloch writes:

Gennadiy Rozental wrote:

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"Stefan Schild" wrote in message news:eg3r7s$bcb$1@sea.gmane.org...

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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

Did you try:

template<typename T> class B { BOOST_STATIC_ASSERT( boost::is_base_and_derived::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;

Right.

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.

Right. The assertion (and the cast) will compile anyway even if certain bad things are done: class D1 : public B<D1> ... class D2 : public B<D1> ... There's nothing you can do about the 2nd line, because B<D1> is just as valid there, as far as the compiler is concerned, as it is in the first line. I deal with this by saying there's no way to improve on the check that the static_cast gives you (which is pretty good after all), so you may as well just stop worrying about it :) -- Dave Abrahams Boost Consulting www.boost-consulting.com