12.7. (Optional) Polymorphism, Virtual Functions and Dynamic Binding “Under the Hood”

C++ makes polymorphism easy to program. It’s certainly possible to program for polymorphism in non-object-oriented languages such as C, but doing so requires complex and potentially dangerous pointer manipulations. This section discusses how C++ can implement polymorphism, virtual functions and dynamic binding internally. This will give you a solid understanding of how these capabilities really work. More importantly, it will help you appreciate the overhead of polymorphism—in terms of additional memory consumption and processor time. This can help you determine when to use polymorphism and when to avoid it. C++ Standard Library classes like array and vector are implemented without polymorphism and virtual functions to avoid the associated execution-time overhead and achieve optimal performance.

First, we’ll explain the data structures that the compiler builds at compile time to support polymorphism at execution time. You’ll see that polymorphism is accomplished through three levels of pointers, i.e., triple indirection. Then we’ll show how an executing program uses these data structures to execute virtual functions and achieve the dynamic binding associated with polymorphism. Our discussion explains one possible implementation; this is not a language requirement.

When C++ compiles a class that has one or more virtual functions, it builds a virtual function table (vtable) for that class. The vtable contains pointers to the class’s virtual functions. Just as the name of a built-in array contains the address in memory of the array’s first element, a pointer to a function contains the starting address in memory of the code that performs the function’s task. An executing program uses the vtable to select the proper function implementation each time a virtual function of that class is called. The leftmost column of Fig. 12.18 illustrates the vtables for the classes Employee, SalariedEmployee, CommissionEmployee and BasePlusCommissionEmployee.

Fig. 12.18. How virtual function calls work.