Prevent user from deriving from incorrect CRTP base

Question

I cannot think about a proper question title to describe the problem. Hopefully the details below explains my problem clear.

Consider the following code

Answer 1

If you can't count with C++11, you could try this trick:

1. Add a static function in Base that returns a pointer to its specialied type:

   static Derived *derived( ) { return NULL; }

2. Add a static check function template to base that takes a pointer:

   template< typename T > static bool check( T *derived_this ) { return ( derived_this == Base< Derived >::derived( ) ); }

3. In your Dn constructors, call check( this ):

   check( this )

Now if you try to compile:

$ g++ -Wall check_inherit.cpp -o check_inherit
check_inherit.cpp: In instantiation of ‘static bool Base::check(T*) [with T = D2; Derived = D1]’:
check_inherit.cpp:46:16:   required from here
check_inherit.cpp:19:62: error: comparison between distinct pointer types ‘D2*’ and ‘D1*’ lacks a cast                                                                                                                             
check_inherit.cpp: In static member function ‘static bool Base::check(T*) [with T = D2; Derived = D1]’:                                                                                                                   
check_inherit.cpp:20:5: warning: control reaches end of non-void function [-Wreturn-type]