COMS W4995 C++ Deep Dive for C Programmers

Inheritance

Lecture Outline

inherit1

1. Inheritance basics

   • default member initializer
   • B::sum() - invoking the base class’s member function
   • w() macros for convenience
   • protected access modifier

2. Memory layout of derived classes

   • reinterpret_cast

3. Static vs. dynamic binding

   • Pointer or reference to B can bind to a D object

   • Static binding by default unlike Java

   • Dynamic binding requries “virtual” function specifier:

       virtual uint64_t sum() const { return b; }
     

   • “override” specifier ensures that the function is virtual and is overriding a virtual function from a base class:

       uint64_t sum() const override { return B::sum() + d; }
     

4. Virtual function table (vtable)

   • Object size & layout changes when it contains virtual functions
   • Diagram of vtable and vtable pointer
   • Calling a virtual function manually

inherit2

1. Multiple inheritance object layout

   • Multiple inheritance diagram

2. Base pointers have different addresses

   • The compiler must change the memory addresses of base-type pointers to the same multiply derived object in order for the base-type pointers to point to the right part of the object.

3. static_cast

4. Multiple inheritance with virtual functions

   • pb->sum() and pc->sum() polymorphically invoke D::sum()

   • Two vptrs are added to the object

5. dynamic_cast

6. (Optional) vtable layout

inherit3

1. The diamond problem: D object contains two A portions

2. Base classes B & C still behave the same

3. But referring to the base class A is ambiguous

inherit4

1. Virtual inheritance fixes the diamond problem

2. sum() behaves polymorphically via any base pointer

3. (Optional) D’s vtable layout

inherit5

1. Order of initialization: base classes, data members, constructor body

   • Note the base class initialization syntaxt: B{100}

   • Destructors are called in the opposite order

2. Pure virtual function & abstract class

3. Virtual destructor