Value Ranges for Enumerations

Originally, the only valid values for an enumeration were those named in the declaration. However, C++ has expanded the list of valid values that can be assigned to an enumeration variable through the use of a type cast. Each enumeration has a range, and you can assign any integer value in the range, even if it’s not an enumerator value, by using a type cast to an enumeration variable. For example, suppose that bits and myflag are defined this way:

enum bits{one = 1, two = 2, four = 4, eight = 8};
bits myflag;

In this case, the following is valid:

myflag = bits(6);    // valid, because 6 is in bits range

Here 6 is not one of the enumerations, but it lies in the range the enumerations define.

The range is defined as follows. First, to find the upper limit, you take the largest enumerator value. Then you find the smallest power of two greater than this largest value and subtract one; the result is the upper end of the range. (For example, the largest bigstep value, as previously defined, is 101. The smallest power of two greater than this is 128, so the upper end of the range is 127.) Next, to find the lower limit, you find the smallest enumerator value. If it is 0 or greater, the lower limit for the range is 0. If the smallest enumerator is negative, you use the same approach as for finding the upper limit but toss in a minus sign. (For example, if the smallest enumerator is -6, the next power of two [times a minus sign] is -8, and the lower limit is -7.)

The idea is that the compiler can choose how much space to use to hold an enumeration. It might use 1 byte or less for an enumeration with a small range and 4 bytes for an enumeration with type long values.

C++11 extends enumerations with a form called the scoped enumeration. Chapter 10 discusses this form briefly in the section “Class Scope.”

Pointers and the Free Store

The beginning of Chapter 3 mentions three fundamental properties of which a computer program must keep track when it stores data. To save the book the wear and tear of your thumbing back to that chapter, here are those properties again:

• Where the information is stored

• What value is kept there

• What kind of information is stored

You’ve used one strategy for accomplishing these ends: defining a simple variable. The declaration statement provides the type and a symbolic name for the value. It also causes the program to allocate memory for the value and to keep track of the location internally.

Let’s look at a second strategy now, one that becomes particularly important in developing C++ classes. This strategy is based on pointers, which are variables that store addresses of values rather than the values themselves. But before discussing pointers, let’s talk about how to explicitly find addresses for ordinary variables. You just apply the address operator, represented by &, to a variable to get its location; for example, if home is a variable, &home is its address. Listing 4.14 demonstrates this operator.

Listing 4.14. address.cpp

// address.cpp -- using the & operator to find addresses
#include <iostream>
int main()
{
    using namespace std;
    int donuts = 6;
    double cups = 4.5;

    cout << "donuts value = " << donuts;
    cout << " and donuts address = " << &donuts << endl;
// NOTE: you may need to use unsigned (&donuts)
// and unsigned (&cups)
    cout << "cups value = " << cups;
    cout << " and cups address = " << &cups << endl;
    return 0;
}

Here is the output from the program in Listing 4.14 on one system:

donuts value = 6 and donuts address = 0x0065fd40
cups value = 4.5 and cups address = 0x0065fd44

The particular implementation of cout shown here uses hexadecimal notation when displaying address values because that is the usual notation used to specify a memory address. (Some implementations use base 10 notation instead.) Our implementation stores donuts at a lower memory location than cups. The difference between the two addresses is 0x0065fd44 - 0x0065fd40, or 4. This makes sense because donuts is type int, which uses 4 bytes. Different systems, of course, will give different values for the address. Also some may store cups first, then donuts, giving a difference of 8 bytes because cups is double. And some may not even use adjacent locations.

Using ordinary variables, then, treats the value as a named quantity and the location as a derived quantity. Now let’s look at the pointer strategy, one that is essential to the C++ programming philosophy of memory management. (See the following sidebar, “Pointers and the C++ Philosophy.”)

The new strategy for handling stored data switches things around by treating the location as the named quantity and the value as a derived quantity. A special type of variable—the pointer—holds the address of a value. Thus, the name of the pointer represents the location. Applying the * operator, called the indirect value or the dereferencing operator, yields the value at the location. (Yes, this is the same * symbol used for multiplication; C++ uses the context to determine whether you mean multiplication or dereferencing.) Suppose, for example, that manly is a pointer. In that case, manly represents an address, and *manly represents the value at that address. The combination *manly becomes equivalent to an ordinary type int variable. Listing 4.15 demonstrates these ideas. It also shows how to declare a pointer.

Listing 4.15. pointer.cpp

// pointer.cpp -- our first pointer variable
#include <iostream>
int main()
{
    using namespace std;
    int updates = 6;        // declare a variable
    int * p_updates;        // declare pointer to an int
    p_updates = &updates;   // assign address of int to pointer

// express values two ways
    cout << "Values: updates = " << updates;
    cout << ", *p_updates = " << *p_updates << endl;

// express address two ways
    cout << "Addresses: &updates = " << &updates;
    cout << ", p_updates = " << p_updates << endl;

// use pointer to change value
    *p_updates = *p_updates + 1;
    cout << "Now updates = " << updates << endl;
    return 0;
}

Here is the output from the program in Listing 4.15:

Values: updates = 6, *p_updates = 6
Addresses: &updates = 0x0065fd48, p_updates = 0x0065fd48
Now updates = 7

As you can see, the int variable updates and the pointer variable p_updates are just two sides of the same coin. The updates variable represents the value as primary and uses the & operator to get the address, whereas the p_updates variable represents the address as primary and uses the * operator to get the value (see Figure 4.8). Because p_updates points to updates, *p_updates and updates are completely equivalent. You can use *p_updates exactly as you would use a type int variable. As the program in Listing 4.15 shows, you can even assign values to *p_updates. Doing so changes the value of the pointed-to value, updates.

Figure 4.8. Two sides of a coin.