// https://cirosantilli.com/linux-kernel-module-cheat#cpp

#include <cassert>

int main() {

// Hello world.

{

struct Base {

virtual int f() { return 0; }

};

struct Derived1 : public Base {

virtual int f() override { return 1; }

};

struct Derived2 : public Base {

// virtual not required on the last derived method.

int f() override { return 2; }

};

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

// This is what is called 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!

// We get name hiding instead.

{

struct Base {

int f() { return 0; }

};

struct Derived : public Base {

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.

//

// This is why you should always use override to prevent such bugs.

{

struct Base {

virtual int f() { return 0; }

};

class Derived : public Base {

virtual int f(int i) /* override */ { 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.

#if 0

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

#endif

}

// Polymorphic calls cannot be made from constructors.

// https://stackoverflow.com/questions/1453131/how-can-i-get-polymorphic-behavior-in-a-c-constructor

{

struct Base {

int i;

Base() {

i = f();

}

virtual int f() { return 0; }

};

class Derived : public Base {

int f() override { return i; }

};

Base b;

assert(b.i == 0);

Derived d;

assert(d.i == 0);

}

}