Hello, This might be an off-topic question, but there must be here some clever guy who can answer my question. class Impl { public: Impl() { std::cout << "j"; } // want to eliminate, but cannot :( Impl(int j) { i = j; std::cout << "i"; } int i; }; template<typename TImpl> class A : virtual protected TImpl { public: void geti() const { std::cout << "gi" << i << "\n"; } }; template<typename TImpl> class B : public A<TImpl> { public: void seti(int a) { this->i = a; } }; class CA : public A<Impl> { public: CA() : Impl(3) { std::cout << "ca " << i;} }; class CB : public B<Impl> { public: CB() : Impl(2) { std::cout << "cb " << i;} }; //abuse of interfaces template<typename TImpl> void foo( A<TImpl>& p_a ) { p_a.geti(); } template<typename TImpl> void foo( B<TImpl>& p_a ) { p_a.seti( 2 ); } template<typename TImpl> void foo2( A<TImpl>& p_a ) { p_a.geti(); } int main( int, char*[] ) { CA a; std::cout << "\n"; CB b; std::cout << "\n"; foo(a); foo(b); foo2(a); foo2(b); return 0; } The basic concept was that: there is a simple implementation class and using some interfaces (bridge classes) the appropriate function call (foo) can be ensured using compile time type checking. The implementation might provide implementation to multiple interfaces (bridge classes) and these interfaces guarantee the required operation for each (template)functions. The role of A and B is to provide the interface and bridge it to the implementation. The only missing part was, how to construct a CA and CB object. They might have completely different construction parameters and the virtual protected inheritance partially solved the problem. BUT the default constructor is always required (VC2008) even tough it gets never called (I don't want to create any object from A,B (who are missing these default ctors) but I couldn't tell it to the compiler) "The declare and don't define" doesn't solve the problem either as the linker requires the default Impl ctor even though it never gets called. For example: impl1: a range iterator for an 1D array: ctor requires the array.begin, array.end impl2 a random generator range: ctor requires the random seed CA: isEmpty, front, popFront CB: put, mutable front copy from random range to array is valid (template foo(A,B)), but not the other way (no template foo(B,A)) Thanks, Gzp -- View this message in context: http://boost.2283326.n4.nabble.com/virtual-protected-inheritance-and-default... Sent from the Boost - Dev mailing list archive at Nabble.com.