Re: Question about inheritance

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 22 Feb 2009 13:12:24 -0500

Message-ID:

<daniel_t-E411BE.13122422022009@earthlink.vsrv-sjc.supernews.net>

maverik <maverik.mail@gmail.com> wrote:

Hi all.

There is one question about inheritance:

I have base class, lets say it is IInterface:

class IInterface {
    virtual void foo();
};

and derived class:

class Implementation : public IInterface {
    void foo() { do_something(); }

    // Many more functions here
    void func1();
    void func2();
    ...
    void funcN();
};

I want to create class that allows clients to call func1(), ... , funcN
() function, but doesn't allow to call functions from base class (in
our case foo() ). How can I do this?

The only way is to remove the public inheritance.

One way that will get close:

class IInterface {
public:
   virtual void foo();
};

class Implementation : public IInterface {
private:
   void foo();
public:
   void func1(); // etc.
};

But the caller will still be able to call 'foo()' if he wants by simply
casting the Implementation object or passing it to a context that uses
an IInterface.

I try the following:

class IInterface {
public:
    virtual void foo() = 0;
};

class Implementation: public IInterface {
public:
    void foo() { std::cout << "Implementation::foo();\n"; };
    void bar() { std::cout << "Implementation::bar();\n"; };
};

template<class T>
class MyClass : private IInterface, public T {
public:
    void tee() {}
};

MyClass<Implementation> c;

but VC 9.0 says me:

error C2259: 'MyClass<T>' : cannot instantiate abstract class

Unfortunately can't test this on gcc.

Of course, I can write MyClass as a copy of Implementation class, but
with private or protected inheritance from IInterface, but
Implementation in real is a big class (and not only one, there is
bunch of such classes).

The solution here is to not make "real big" classes.