Classes

C++ classes are similar to C structures in many ways. In fact, a C++ struct is really a class that has only public data members. In the following explanation of how classes work, we will use a stack class as an example.

Member functions. Here is a (partial) example of a class with a member function and some data members:
```
class Stack {
  public:
    void Push(int value); // Push an integer, checking for overflow.
    int top;          // Index of the top of the stack.
    int stack[10];    // The elements of the stack.
};

void
Stack::Push(int value) {
    ASSERT(top < 10);		// stack should never overflow
    stack[top++] = value;
}
```
This class has two data members, top and stack, and one member function, Push. The notation class:: function denotes the function member of the class class. (In the style we use, most function names are capitalized.) The function is defined beneath it.
As an aside, note that we use a call to ASSERT to check that the stack hasn't overflowed; ASSERT drops into the debugger if the condition is false. It is an extremely good idea for you to use ASSERT statements liberally throughout your code to document assumptions made by your implementation. Better to catch errors automatically via ASSERTs than to let them go by and have your program overwrite random locations.
In actual usage, the definition of class Stack would typically go in the file stack.h and the definitions of the member functions, like Stack::Push, would go in the file stack.cc.
If we have a pointer to a Stack object called s, we can access the top element as s->top, just as in C. However, in C++ we can also call the member function using the following syntax:
```
    s->Push(17);
```
Of course, as in C, s must point to a valid Stack object.
Inside a member function, one may refer to the members of the class by their names alone. In other words, the class definition creates a scope that includes the member (function and data) definitions.
Note that if you are inside a member function, you can get a pointer to the object you were called on by using the variable this. If you want to call another member function on the same object, you do not need to use the this pointer, however. Let's extend the Stack example to illustrate this by adding a Full() function.
```
class Stack {
  public:
    void Push(int value); // Push an integer, checking for overflow.
    bool Full();       // Returns TRUE if the stack is full, FALSE otherwise.
    int top;          // Index of the lowest unused position.
    int stack[10];    // A pointer to an array that holds the contents.
};
```
```
bool
Stack::Full() {
    return (top == 10);
}
```
Now we can rewrite Push this way:
```
void
Stack::Push(int value) {
    ASSERT(!Full());
    stack[top++] = value;
}
```
We could have also written the ASSERT:
```
    ASSERT(!(this->Full());
```
but in a member function, the this-> is implicit.
The purpose of member functions is to encapsulate the functionality of a type of object along with the data that the object contains. A member function does not take up space in an object of the class.
Private members. One can declare some members of a class to be private, which are hidden to all but the member functions of that class, and some to be public, which are visible and accessible to everybody. Both data and function members can be either public or private.
In our stack example, note that once we have the Full() function, we really don't need to look at the top or stack members outside of the class -- in fact, we'd rather that users of the Stack abstraction not know about its internal implementation, in case we change it. Thus we can rewrite the class as follows:
```
class Stack {
  public:
    void Push(int value); // Push an integer, checking for overflow.
    bool Full();       // Returns TRUE if the stack is full, FALSE otherwise.
  private:
    int top;          // Index of the top of the stack.
    int stack[10];    // The elements of the stack.
};
```
Before, given a pointer to a Stack object, say s, any part of the program could access s->top, in potentially bad ways. Now, since the top member is private, only a member function, such as Full(), can access it. If any other part of the program attempts to use s->top the compiler will report an error.
You can have alternating public: and private: sections in a class. Before you specify either of these, class members are private, thus the above example could have been written:
```
class Stack {
    int top;          // Index of the top of the stack.
    int stack[10];    // The elements of the stack.
  public:
    void Push(int value); // Push an integer, checking for overflow.
    bool Full();       // Returns TRUE if the stack is full, FALSE otherwise.
};
```
Which form you prefer is a matter of style, but it's usually best to be explicit, so that it is obvious what is intended. In Nachos, we make everything explicit.
What is not a matter of style: all data members of a class should be private. All operations on data should be via that class' member functions. Keeping data private adds to the modularity of the system, since you can redefine how the data members are stored without changing how you access them.
Constructors and the operator new. In C, in order to create a new object of type Stack, one might write:
```
    struct Stack *s = (struct Stack *) malloc(sizeof (struct Stack));
    InitStack(s, 17);
```
The InitStack() function might take the second argument as the size of the stack to create, and use malloc() again to get an array of 17 integers.
The way this is done in C++ is as follows:
```
    Stack *s = new Stack(17);
```
The new function takes the place of malloc(). To specify how the object should be initialized, one declares a constructor function as a member of the class, with the name of the function being the same as the class name:
```
class Stack {
  public:
    Stack(int sz);    // Constructor:  initialize variables, allocate space.
    void Push(int value); // Push an integer, checking for overflow.
    bool Full();       // Returns TRUE if the stack is full, FALSE otherwise.
  private:
    int size;         // The maximum capacity of the stack.
    int top;          // Index of the lowest unused position.
    int* stack;       // A pointer to an array that holds the contents.
};

Stack::Stack(int sz) {
    size = sz;
    top = 0;
    stack = new int[size];   // Let's get an array of integers.
}
```
There are a few things going on here, so we will describe them one at a time.
The new operator automatically creates (i.e. allocates) the object and then calls the constructor function for the new object. This same sequence happens even if, for instance, you declare an object as an automatic variable inside a function or block -- the compiler allocates space for the object on the stack, and calls the constructor function on it.
In this example, we create two stacks of different sizes, one by declaring it as an automatic variable, and one by using new.
```
void
test() {
    Stack s1(17);
    Stack* s2 = new Stack(23);
}
```
Note there are two ways of providing arguments to constructors: with new, you put the argument list after the class name, and with automatic or global variables, you put them after the variable name.
It is crucial that you always define a constructor for every class you define, and that the constructor initialize every data member of the class. If you don't define your own constructor, the compiler will automatically define one for you, and believe me, it won't do what you want (``the unhelpful compiler''). The data members will be initialized to random, unrepeatable values, and while your program may work anyway, it might not the next time you recompile (or vice versa!).
As with normal C variables, variables declared inside a function are deallocated automatically when the function returns; for example, the s1 object is deallocated when test returns. Data allocated with new (such as s2) is stored on the heap, however, and remains after the function returns; heap data must be explicitly disposed of using delete, described below.
The new operator can also be used to allocate arrays, illustrated above in allocating an array of ints, of dimension size:
```
    stack = new int[size];
```
Note that you can use new and delete (described below) with built-in types like int and char as well as with class objects like Stack.
Destructors and the operator delete. Just as new is the replacement for malloc(), the replacement for free() is delete. To get rid of the Stack object we allocated above with new, one can do:
```
    delete s2;
```
This will deallocate the object, but first it will call the destructor for the Stack class, if there is one. This destructor is a member function of Stack called ~Stack():
```
class Stack {
  public:
    Stack(int sz);    // Constructor:  initialize variables, allocate space.
    ~Stack();         // Destructor:   deallocate space allocated above.
    void Push(int value); // Push an integer, checking for overflow.
    bool Full();      // Returns TRUE if the stack is full, FALSE otherwise.
  private:
    int size;         // The maximum capacity of the stack.
    int top;          // Index of the lowest unused position.
    int* stack;       // A pointer to an array that holds the contents.
};

Stack::~Stack() {
    delete [] stack;  // delete an array of integers
}
```
The destructor has the job of deallocating the data the constructor allocated. Many classes won't need destructors, and some will use them to close files and otherwise clean up after themselves.
The destructor for an object is called when the object is deallocated. If the object was created with new, then you must call delete on the object, or else the object will continue to occupy space until the program is over -- this is called ``a memory leak.'' Memory leaks are bad things -- although virtual memory is supposed to be unlimited, you can in fact run out of it -- and so you should be careful to always delete what you allocate. Of course, it is even worse to call delete too early -- delete calls the destructor and puts the space back on the heap for later re-use. If you are still using the object, you will get random and non-repeatable results that will be very difficult to debug. In my experience, using data that has already been deleted is major source of hard-to-locate bugs in student (and professional) programs, so hey, be careful out there!
If the object is an automatic, allocated on the execution stack of a function, the destructor will be called and the space deallocated when the function returns; in the test() example above, s1 will be deallocated when test() returns, without you having to do anything.
In Nachos, we always explicitly allocate and deallocate objects with new and delete, to make it clear when the constructor and destructor is being called. For example, if an object contains another object as a member variable, we use new to explicitly allocated and initialize the member variable, instead of implicitly allocating it as part of the containing object. C++ has strange, non-intuitive rules for the order in which the constructors and destructors are called when you implicitly allocate and deallocate objects. In practice, although simpler, explicit allocation is slightly slower and it makes it more likely that you will forget to deallocate an object (a bad thing!), and so some would disagree with this approach.
When you deallocate an array, you have to tell the compiler that you are deallocating an array, as opposed to a single element in the array. Hence to delete the array of integers in Stack::~Stack:
```
    delete [] stack;
```