Implementing multiple interfaces in c++

by

You can implement each individial interface using a separate template and then chain the templates to construct the derived object as if from building blocks. This method was also used by venerable ATL library to implement COM interfaces (for those of us old enough).

Note that you don’t need virtual inheritance for that.

I slightly modified you example for a more complex derivation C -> B -> A to show how this method scales easily:

#include <stdio.h>

// Interfaces

struct A
{
    virtual void foo() = 0;
};

struct B : A
{
    virtual void testB() = 0;
};

struct C : B
{
    virtual void testC() = 0;
};

// Implementations

template<class I>
struct AImpl : I
{
    void foo() { printf("%s\n", __PRETTY_FUNCTION__); }
};

template<class I>
struct BImpl : I
{
    void testB() { printf("%s\n", __PRETTY_FUNCTION__); }
};

template<class I>
struct CImpl : I
{
    void testC() { printf("%s\n", __PRETTY_FUNCTION__); }
};


// Usage

int main() {
    // Compose derived objects from templates as from building blocks.
    AImpl<A> a;
    BImpl<AImpl<B> > b;
    CImpl<BImpl<AImpl<C> > > c;

    a.foo();

    b.foo();
    b.testB();

    c.foo();
    c.testB();
    c.testC();
}

Output:

void AImpl<I>::foo() [with I = A]
void AImpl<I>::foo() [with I = B]
void BImpl<I>::testB() [with I = AImpl<B>]
void AImpl<I>::foo() [with I = C]
void BImpl<I>::testB() [with I = AImpl<C>]
void CImpl<I>::testC() [with I = BImpl<AImpl<C> >]

More Related Contents:

  1. Which access specifiers will not affect a friend function?
  2. Does the stream operator exist for class member functions?
  3. Weird behavior with OOP and string pointers
  4. Why should I prefer to use member initialization lists?
  5. Function pointer to member function
  6. When should you use a class vs a struct in C++?
  7. Is there a performance difference between i++ and ++i in C++?
  8. Thou shalt not inherit from std::vector
  9. Why should the “PIMPL” idiom be used? [duplicate]
  10. How to call a parent class function from derived class function?
  11. Is the PIMPL idiom really used in practice?
  12. What is the difference between private and protected members of C++ classes?
  13. Separating class code into a header and cpp file
  14. Why are unnamed namespaces used and what are their benefits?
  15. What are the differences between struct and class in C++?
  16. Why should I avoid multiple inheritance in C++?
  17. Vectors and polymorphism in C++
  18. Why can’t a derived class call protected member function in this code?
  19. Why should I use the “using” keyword to access my base class method?
  20. Declaring multiple object pointers on one line causes compiler error
  21. Diamond inheritance (C++)
  22. overloading base class method in derived class
  23. Semicolon after class declaration braces
  24. Best method for storing this pointer for use in WndProc
  25. C++ inheritance and member function pointers
  26. Should I use public or private variables?
  27. How to initialize the reference member variable of a class?
  28. Making a vector of instances of different subclasses
  29. How does the C++ compiler know which implementation of a virtual function to call?
  30. Should objects delete themselves in C++?

Leave a Comment