C++ Inheritance, calling a derived function from the base class

Question

How can I call a derived function from a base class? I mean, being able to replace one function from the base to the derived class.

Ex.

class a
{
public:
   void f1();
   void f2();
};

void a::f1()
{
   this->f2();
}

/* here goes the a::f2() definition, etc */

class b : public a
{
public:
   void f2();
};

/* here goes the b::f2() definition, etc */    

void lala(int s)
{
  a *o; // I want this to be class 'a' in this specific example
  switch(s)
  {
   case 0: // type b
     o = new b();
   break;
   case 1: // type c
     o = new c(); // y'a know, ...
   break;
  }

  o->f1(); // I want this f1 to call f2 on the derived class
}

Maybe I'm taking a wrong approach. Any comment about different designs around would also be appreciated.

David Rodríguez - dribeas · Accepted Answer · 2010-01-22 11:24:30Z

21

Declare f2() virtual in the base class.

class a
{
public:
       void f1();
       virtual void f2();
};

Then whenever a derived class overrides f2() the version from the most derived class will be called depending on the type of the actual object the pointer points to, not the type of the pointer.

edited Jan 22, 2010 at 11:24

David Rodríguez - dribeas

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answered Jan 22, 2010 at 11:12

sharptooth

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4 Comments

huff Over a year ago

Thx, now I know what the 'virtual' thing was... :D

Thorsten79 Over a year ago

Actually, virtual is THE feature that makes real OOP possible in C++.

huff Over a year ago

Yeah, well... at least the inheritance part of it... imio (in my ignorant opinion), OOP would still be practical just using compositions...

John Dibling Over a year ago

@huff: True. virtual isn't exactly the enabling idiom as far as OOP goes, precisely. virtual makes polymorphism possible, which is one part of OOP.

Franci Penov · Accepted Answer · 2010-01-22 11:12:25Z

5

Declare f2() as virtual.

answered Jan 22, 2010 at 11:12

Franci Penov

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