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/*

# virtual

Keyword that enables polymorphism.

http://stackoverflow.com/questions/2391679/can-someone-explain-c-virtual-methods

*/

#include "common.hpp"

int main() {

/*

# Polymorphism

The pointer t the base class can have different effects

depending on what actual object it points to.

# Override method

Method of the derived class with compatible signature to base class

overrides the base method.

For incompatible signatures, or base methods that are not virtual,

_hiding_ happnes, not overhiding.

*/

{

class Base {

public:

virtual int f() { return 0; }

};

class Derived1 : public Base {

public:

virtual int f() { return 1; }

};

class Derived2 : public Base {

public:

virtual int f() { return 2; }

};

// Depending on what bp points to, we call different functions.

// This is polymorphism.

Base *bp;

Base b;

bp = &b;

assert(bp->f() == 0);

Derived1 d1;

bp = &d1;

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

Derived2 d2;

bp = &d2;

assert(bp->f() == 2);

}

// Without `virtual`, no polymorphism happens!

{

class Base {

public:

int f() { return 0; }

};

class Derived : public Base {

public:

int f() { return 1; }

};

Base *bp;

Base b;

bp = &b;

assert(bp->f() == 0);

Derived d;

bp = &d;

// Base method called!

assert(bp->f() == 0);

}

/*

If the derived signature is not compatible with the virtual,

no polymorphism happens!

Also see "covariant return" for compatibility of return types.

*/

{

class Base {

public:

virtual int f() { return 0; }

};

class Derived : public Base {

public:

virtual int f(int i) { return i; }

};

Base *bp;

Base b;

bp = &b;

assert(bp->f() == 0);

Derived d;

bp = &d;

// Base method called!

assert(bp->f() == 0);

// ERROR: no matching function.

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

}

/*

`virtual` on the overriding method is optional, although highly recommended.

Even if not given, the method is still `virtual`.

So just always use it.

*/

{

class Base {

public:

virtual int f() { return 0; }

};

class Derived1 : public Base {

public:

/* Note no virtual here. */

int f() { return 1; }

};

class Derived2 : public Derived1 {

public:

int f() { return 2; }

};

// Polymorphism works. So Derived::f is actually `virtual`.

Derived1 *dp;

Derived2 d2;

dp = &d2;

assert(dp->f() == 2);

}

}