Re: My attempt at a small hierarchy. Why doesn't my code compile?

"Alf P. Steinbach" <>
Wed, 18 Nov 2009 12:59:08 +0100
* PeteUK:

See example code below. The instantiation of a Concrete object (see
main function) makes the compiler (VS 2003) complain that Concrete is
an abstract class because MethodsA::callA() and MethodsB::callB() are
not defined.

Hopefully it's obvious what I'm trying to acheive. Can someone tell me
where I'm going wrong?

C++ isn't Java.

To emulate Java's interfaces, inherit every interface class virtually.

Since there's then only one base class sub-object of each interface, any
implementation of its member routines serves as implementation. :-)

#ifdef _MSC_VER
#pragma warning( disable: 4250 ) // inherits via dominance

// Low level interface
struct MethodsA
     virtual ~MethodsA() {}
     virtual void callA() = 0;

// Another low level interface
struct MethodsB
     virtual ~MethodsB() {}
     virtual void callB() = 0;

// Higher level interface - the main base class of concrete objects
struct MainBase
     : public virtual MethodsA
     , public virtual MethodsB

// A suggested implementation of a first low level interface
// Can be used as a mixin to easily create concrete objects
class ImplOfAMixin : public virtual MethodsA
     void callA() {}

// A suggested implementation of a second low level interface
// Can be used as a mixin to easily create concrete objects
class ImplOfBMixin : public virtual MethodsB
     void callB() {}

// Concrete class
class Concrete
     : public MainBase, // "isa" main base
          public ImplOfAMixin, // pull in this mixin
          public ImplOfBMixin // and this mixin

int main()
     Concrete obj;

Cheers & hth.,

- Alf

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