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/*

# Pure virtual

Virtual function without implementation. Syntax:

virtual f() = 0

Makes it impossible instantiate the class.

It is only possible to instantiate derived classes that override it.

If a class has a pure virtual method is called as an *abstract class* or *interface*.

In Java there is a language difference between those two terms,

and it might be a good idea to differentiate them when speaking about C++:

- interface: no data

- abstract: data

*/

#include "common.hpp"

int main() {

// Basic example.

{

// This is an abstract type because it has a pure virtual method.

class Base {

public:

virtual int f() = 0;

};

class Derived : public Base {

public:

virtual int f() { return 1; }

};

// ERROR: abstract type.

//Base b;

Derived d;

assert(d.f() == 1);

// We can still have abstract pointers and references to abstract classes.

Base *bp = &d;

Base &br = d;

assert(bp->f() == 1);

assert(br.f() == 1);

}

/*

It is not possible to implement pure virtual methods on another base class

with multiple-inheritance: they must be implemented on the Derived class.

In some languages where this is possible, this pattern is called a mixin:

http://stackoverflow.com/a/20022860/895245

*/

{

class Base {

public:

virtual int f() = 0;

};

class Implementor {

public:

virtual int f() {

return 1;

}

};

class Derived : public Base, public Implementor {};

// ERROR: abstract.

//Derived d;

}

}