administrivia

midterm review Thursday

midterm exam Friday, here

today’s plan

how does g++ implement: inheritance & virtual

learn how to explore this question

single inheritance

class A {
 public:
  int a;
};

class B : public A {
 public:
  int b;
};

#include <iostream>
int main() {
  B x;
  std::cout << &x.a << " " << &x.b << std::endl;
  A *pa = &x;
  std::cout << pa << " " << &x << std::endl;
}

inherited data

multiple inheritance

struct A { int a; };

struct B { int b; };

struct C : public A, public B { int c; };

#include <iostream>
int main() {
  C x;
  std::cout << &x.a << " " << &x.b << " " << &x.c << std::endl;
  A *pa = &x;
  B *pb = &x;
  C *pc = &x;
  std::cout << pa << " " << pb << " " << pc << std::endl;
}

inherited data

virtual revisited

lec 10: example

#include <iostream>

struct A {
  virtual void foo() { std::cout << "A" << std::endl; }
};

struct B : public A {
  virtual void foo() override { std::cout << "B" << std::endl; }
};

int main() {
  B b;
  A *pa = &b;
  pa->foo();
}

how does g++ know which to invoke?

polymorphism in C

Linux kernel: struct file

vtable example

#include <iostream>

struct A {
  virtual void foo() { std::cout << "A" << std::endl; }
  virtual void bar() { }
  void not_virtual();
};

struct B : public A {
  virtual void foo() override { std::cout << "B" << std::endl; }
};

g++ -fdump-class-hierarchy -c

gdb: info vtbl

vtable

q: what’s the cost of calling a virtual function?

virtual destrutor

#include <iostream>

struct A {
  virtual void foo() { std::cout << "A" << std::endl; }
  ~A() { std::cout << "~A" << std::endl; } // BAD
};

struct B : public A {
  virtual void foo() override { std::cout << "B" << std::endl; }
  ~B() { std::cout << "~B" << std::endl; }
};

int main() {
  B *pb = new B;
  A *pa = pb;
  delete pa;
}

what’s the output? mark destructors as virtual!

how about just

mark every method and destructor as virtual?

performance

see pure virtual, = default, = delete

sum up

understand the cost of C++ abstractions

learn how to explore memory layout details

see also “Inside the C++ Object Model” by Stanley B. Lippman

good luck with midterm!