Abstract base class

[ttuethao]

Hi! I am writing a program and recent encountered a problem with the concept of "abstract class". In a simplified form, my problem is: I have a function f(x), where x can be either of class A1 or class A2. As f requires specific member methods of in A1 and A2, I really don't want to use template (which looks like everything can be inserted there). I choose to use a common base class instead. (After all, in reality, A1, A2 are really two types of a thing). More precisely:

class A 
{
public: 
 virtual void say(){};
};

void f (A  X) {X.say();};

class A1: A 
{
public:
 void say(){std::cout<<"Meow"<<std::endl;};
};

class A2: A 
{
public:
 void say(){std::cout<<"Geow"<<std::endl;};
};

int main()
{
 A1 X1; A2 X2;
 f(X1);
 f(X2);
 return 0;
};

This works just fine and load the proper function. However as class A does not do anything in A::say(), it is some sort of "virtual class" to serve as argument for f. To prevent any usage of this class A, I try to make it abstract by setting virtual say() = 0;. But this does not compile because the compiler complains that A::say() was not implemented.

My question is: how to prevent any real usage of A then? Or template is the preferred solution after all? Other question: is there also computational cost in overloading virtual functions in comparison to using templates?

Thank you for your interest and comments!

[shadowmouse]

Try virtual void say() = 0; Also, I really wouldn't use templates for this, if you absolutely have to, just overload the function.

[Gamer2015]

you simply have to make the parameter of f a pointer to an A
when writing f(A) you say that the function will get an Instance of A which is not possible because it has a pure virtual method

// Example program
#include <iostream>
#include <string>

class A 
{
public: 
    virtual void say() = 0; // = 0 marks pure virtual methods
};

void f (A* X) {X->say();};

class A1 : public A 
{
public:
    void say(){std::cout<<"Meow"<<std::endl;};
};

class A2 : public A 
{
public:
    void say(){std::cout<<"Geow"<<std::endl;};
};

int main()
{
    A1 X1; A2 X2;
    f(&X1);
    f(&X2);
    return 0;
}

some sort of "virtual class" to serve as argument for f

something like this is called an interface, I just want to clear the terminology
an interface is a class with only pure virtual methods
an abstract class is a class with at least one pure virtual and at least one implemented method
(if someone knows better please correct me)

[ttuethao]

Thank you both. It indeed works for that minimal example. Somehow it still does not work for my bigger program.

./src/vsystem.cpp:26:24: warning: inline function 'virtual double vsystem::getTEntropy() const' used but never defined
virtual inline double getTEntropy() const = 0;

But maybe it is just another problem. I am trying to fix.

[ttuethao]

Hmm, OK. I found that virtual function should not be declared as "inline", thus the warning. Although the program runs just fine. Thanks!