In this homework, you will explore the dynamic dispatch mechanism of virtual functions in object-oriented languages. We will hand-build a vtable for a group of classes in the C language, and show how the compiler uses a vtable to select a derived function to call at run time based on the type of the value it is called on.

Function Pointers

C and C++ allow programmers to declare variables that point to functions rather than values. When such a pointer is dereferenced, the actual function that it points to is called, using arguments supplied by the programmer. C and C++ do type checking of function pointers to make sure the right number and types of arguments are provided at compile time. Example:

int max(int a, int b) { return a >= b ? a : b; } // return the larger integer

// in main():

int (*pFunc)(int, int); // pFunc is a pointer to a function that takes 2 int arguments

// and returns int

pFunc = max; // pFunc now points to the real function max printf( %d , pFunc(5, 10)); // dereference pFunc, use its return value

In F#, we say that a function taking two integers and returning integer has type int->int->int. In C, we say it has type int (*)(int, int). (As if we replaced the name of the function with (*), removed the parameter names, and left everything else the same.)

Like all pointers, we do not necessarily know at compile time what a function pointer actually points to, so the actual function to be executed will not be known until run-time. We can use this with some trickery to implement function calls that execute di erent function bodies depending on run-time values.

Faking Objects and Polymorphism

We will implement this lab in C, which means we do not have access to objects but we can fake it with structs. After all, a struct is a class without member functions, inheritance, or polymorphism but it can store a pointer to a vtable, and through this we will implement polymorphism and dynamic dispatch.

Consider the following C++ code showing the polymorphism feature we want to emulate:

class Employee { int age;

public:

int GetAge() { return age; } virtual void Speak() = 0; virtual double GetPay() = 0;

};

class HourlyEmployee : public Employee { double hourly_rate; double hours;

public:

virtual void Speak() { cout << I work for << hourly_rate << dollars per hour 🙁 ;

};

class CommissionEmployee : public Employee { double sales_amount;

public:

virtual void Speak() { cout << I make commision on << sales_amount << dollars in sales! ;

};

// in main()

Employee *e = // suppose e points to either a CommissionEmployee or HourlyEmployee. cout << e->GetPay(); // which function gets called? Who knows!

We can emulate this in C using:

1. A struct Employee, with two member elds: a pointer to vtable (as a void**); and an integer eld age.
2. A struct HourlyEmployee, with four member elds: the same elds as Employee, and doubles hourly_rate and hours.
3. A struct CommissionEmployee, likewise, but with a double sales_amount.
4. Global functions to emulate each of the member functions of the objects:
   • Speak_Hourly which takes an Employee pointer, casts it to a HourlyEmployee pointer, and prints the employees message;
   • GetPay_Hourly which also takes an Employee pointer and returns the employees total pay (see below);
   • Construct_Hourly which takes a HourlyEmployee pointer and initializes its elds to their default values, most importantly initializing the objectss vtable pointer (more on this below).

and then following some tedious steps to create variables of our types and call the appropriate functions:

1. To make a HourlyEmployee, declare a HourlyEmployee variable (either on the stack or with malloc) and then pass it by pointer to Construct_Hourly.
2. To use subtype polymorphism to point an Employee at a HourlyEmployee, declare an Employee pointer and initialize it by casting your HourlyEmployees address to an Employee pointer.
3. To use dynamic dispatch , dereference the Employee pointers vtable pointer, index the table to the appropriate method, cast that pointer to the correct function pointer type, and invoke the method by passing the Employee pointer and any other necessary parameters. Easy!

Vtables

A vtable (short for virtual table) is a table of function pointers, with one entry in the table for each virtual function in a class (or its ancestors). Since the Employee class has two virtual functions, the vtable for any Employee-derived object will have two pointers in it for Speak and GetCost, plus additional entries for any more virtual functions introduced by the derived class. C++ handles the creation and use of vtables automatically; in this lab, we will simulate the work that a C++ compiler performs to transform virtual method calls into vtable lookups.

Suppose we have two functions: void Speak_Hourly(struct HourlyEmployee*) and double GetPay_Hourly(struct HourlyEmployee*). We can build a table (really an array) of two pointers to functions as such:

void* Vtable_Hourly[2] = {Speak_Hourly, GetPay_Hourly};

The void* type in C lets us create a pointer to anything, but the compiler wont help us use such a pointer; we will need to cast it to something speci c in order to use it. Suppose we have a struct HourlyEmployee h variable that we want to call Speak_Hourly on, but we dont want to use Speak_Hourly directly instead, we want to use Vtable_Hourly. We note that the rst entry in Vtable_Hourly is a pointer to Speak_Hourly, and thus try to use that pointer to call the function it points to:

Vtable_Hourly[0]((struct HourlyEmployee *)&h);

This makes sense in our head: the element 0 of Vtable_Hourlyis a pointer to Speak_Hourly, which wants a single parameter of type struct Employee *. Unfortunately C sees Vtable_Hourly[0] as a pointer to void, not to a function; we have to tell the compiler that it actually points to a function that returns void and takes a single parameter of type struct Employee *. A cast will accomplish this task:

((void (*)(struct Employee*))Vtable_Hourly[0])((struct Employee *)&h);^[1]

which tells the compiler to invoke the function pointed to by Vtable_Hourly[0], passing it the address of h as its parameter. Success!

All we need to do now is add a new member variable to our structs: a void** pointer to a vtable appropriate to the class. Create one vtable variable globally for all derived Employee types, point the vtable pointers to the appropriate global tables in constructor methods, and voila! We can now call a virtual function through a base class pointer by accessing the appropriate index from the vtable associated with the variable and invoking that function by hand.

Program

Implement the following code:

1. Create Employee, HourlyEmployee, and CommissionEmployee structs as described above. The rst member of each struct should be a void** vtable variable.
2. Implement the Speak_Hourly and Speak_Commission functions for the HourlyEmployee and CommissionEmployee structs as global functions.
3. Add HourlyEmployee and CommissionEmployee implementations of a GetPay function, which takes an Employee pointer and returns a double as such:
   • The pay for an hourly employee is the number of hours multiplied by their hourly rate.
   • The pay for a commission employee is 10% of their total sales, plus 40,000.
4. Create a Vtable_XX global array for each employee derived type, initialized with pointers to the appropriate Speak and GetPay functions for that type, in that order.
5. Implement Construct_XX functions for HouryEmployee and CommissionEmployee, which initialize the member variables to 0 values, and most importantly, sets the employees vtable pointer to the appropriate global Vtable_XX variable. (Dont overthink this. Its as easy as writing parameter->vtable = Vtable_Hourly;, for example.)
6. Add a new class SeniorSalesman, which derives from CommissionEmployee by duplicating all of CommissionEmployees member variables. SeniorSalesman will override the GetPay method but will use CommissionEmployees version of Speak. Create a vtable and constructor for the SeniorSalesman class, and a GetPay method that returns 20% of the salesmans sales, plus 50,000, plus another 5% of sales if the employee is at least 40 years old.

Then write a main program that does the following:

1. Declare an Employee pointer.
2. Ask the user to choose either an hourly employee, a commission employee, or a senior salesman.
3. Use malloc to create space for the appropriate employee, for example, HourlyEmployee *h = (HourlyEmployee*)mallo (a) Ask the user how old the employee is.
   • If the user selects an hourly, ask them for the employees pay rate and hours.
   • If the user selects a commission or a senior salesman, ask for the employees amount of sales.
   • Use your Construct_ function to initialize the memory given back from malloc, passing the appropriate parameters for the type selected.
   • Point your Employee to the variable.
   • Past this point of the program, you can have no code referring to CommissionEmployee, HourlyEmployee, or SeniorSalesman explicitly everything must be through Employee pointers.
4. Tell the Employee to speak, then inform the user how much money they make.

(a) To do this, you will access the vtable pointer from the Employee pointer in your main, index it to the appropriate position for each function, cast that pointer as described above, and invoke the function it is pointing to.

[1] We know that were working with function pointers in C when our code starts to look like line noise.