Lab 8: Object Layout and Vtables

A PDF version of this document is located here.

Learning objectives:

• Understand object layout with inheritance
• Understand vtable layout in object inheritance hierarchies

This lab does not have any distribution code.

1. Vtables. Consider the following C++ code:

   struct A {
     void foo() {
       cout << "A::foo()" << endl;
     }
     virtual void bar() {
       cout << "A::bar()" << endl;
     }
   };
   
   struct B : A {
     virtual void foo() {
       cout << "B::foo()" << endl;
     }
     void bar() {
       cout << "B::bar()" << endl;
     }
   };
   
   int main() {
     A *aptr = new B;
     aptr->foo();
     aptr->bar();
   }
   

   This code prints the following when run:

   A::foo()
   B::bar()
   

   1. Draw the vtables for A and B. Clearly indicate the name of each member as well as which type contains its definition (e.g. T::member).
   2. Briefly explain how the compiler translates the method calls in main().

2. Vtable and object layout. Consider the following C++ code:

   #include <iostream>
   
   using std::cout;
   using std::endl;
   
   struct A {
     int x;
     virtual void spam() { cout << "A::spam()" << endl; }
     virtual void eggs() { cout << "A::eggs()" << endl; }
   };
   
   struct B : A {
     int x;
     virtual void eggs() { cout << "B::eggs()" << endl; }
   };
   
   struct C : B {
     int z;
     virtual void spam() { cout << "C::spam()" << endl; }
   };
   

   1. Draw a picture illustrating the contents of an object of type C. Clearly indicate what each entry in the object is, and if it is a member, which type contains its definition (e.g. T::member).
   2. Draw the layout of the vtable for type B. Clearly indicate the name of each member as well as which type contains its definition.
   3. Draw the layout of the vtable for type C. Clearly indicate the name of each member as well as which type contains its definition.