why seeples no interfaces?


#include<iostream>

using namespace std;

class A {
public:

  A() {
  }

  virtual ~A() {
  }

  void waka() {
    cout << "WakA!";
  }

};


class IWoopaWaka {
public:
  IWoopaWaka() {
  }

  virtual ~IWoopaWaka() {
  }

  virtual void waka() = 0;
  virtual void woopa() = 0;
};

class B : public IWoopaWaka, public A {
public:
  B() : A() {
  }

  virtual ~B() {
  }

  void woopa() {
    cout << "Woooopa!";
  }
};


int main(int argc, char** argv) {
  IWoopaWaka* wapa = new B();
  wapa->waka();
  wapa->woopa();
  delete wapa;
  return 0;
}

$g++ m.cpp
m.cpp: In function ‘int main(int, char**)’:
m.cpp:49: error: cannot allocate an object of abstract type ‘B’
m.cpp:34: note:   because the following virtual functions are pure within ‘B’:
m.cpp:30: note:         virtual void IWoopaWaka::waka()

#include<iostream>

using namespace std;

class A {
public:

  A() {
  }

  virtual ~A() {
  }

  void waka() {
    cout << "WakA!";
  }

};


class IWoopaWaka {
public:
  IWoopaWaka() {
  }

  virtual ~IWoopaWaka() {
  }

  virtual void waka() = 0;
  virtual void woopa() = 0;
};

class B : public IWoopaWaka, public A {
public:
  B() : A() {
  }

  virtual ~B() {
  }

  void waka() {
    A::waka();
  }

  void woopa() {
    cout << "Woooopa!";
  }
};


int main(int argc, char** argv) {
  IWoopaWaka* wapa = new B();
  wapa->waka();
  wapa->woopa();
  delete wapa;
  return 0;
}

$g++ m.cpp
$a.out
WakA!Woooopa!$

>>2
That's horrible. waka() from A is made to implement waka() from IWoopaWaka even though it doesn't extend it. Be glad it doesn't work in C++.

The following would work:

#include<iostream>

using namespace std;

class IWoopaWaka {
public:
  virtual ~IWoopaWaka() {}

  virtual void waka() = 0;
  virtual void woopa() = 0;
};

class A : public IWoopaWaka {
public:
  virtual ~A() {}

  void waka() {
    cout << "WakA!";
  }
};

class B : public A {
public:
  void woopa() {
    cout << "Woooopa!";
  }
};


int main(int argc, char** argv) {
  IWoopaWaka* wapa = new B();
  wapa->waka();
  wapa->woopa();
  delete wapa;
  return 0;
}

>>9
When a derived class is deleted through a pointer to a base class with a non-virtual destructor, the results are undefined. In practice, if the class doesn't contain any data members, whether it's virtual or not typically doesn't matter, but any C++ programmer should know that you shouldn't rely on undefined behaviour.

>>17
C++ has a few kinks, but nothing described here. The problem here is incompetent programmers.