Declaration Statements and Variables

Computers are precise, orderly machines. To store an item of information in a computer, you must identify both the storage location and how much memory storage space the information requires. One relatively painless way to do this in C++ is to use a declaration statement to indicate the type of storage and to provide a label for the location. For example, the program in Listing 2.2 has this declaration statement (note the semicolon):

int carrots;

This statement provides two kinds of information: the type of memory storage needed and a label to attach to that storage. In particular, the statement declares that the program requires enough storage to hold an integer, for which C++ uses the label int. The compiler takes care of the details of allocating and labeling memory for that task. C++ can handle several kinds, or types, of data, and the int is the most basic data type. It corresponds to an integer, a number with no fractional part. The C++ int type can be positive or negative, but the size range depends on the implementation. Chapter 3 provides the details on int and the other basic types.

Naming the storage is the second task achieved. In this case, the declaration statement declares that henceforth the program will use the name carrots to identify the value stored at that location. carrots is called a variable because you can change its value. In C++ you must declare all variables. If you were to omit the declaration in carrots.cpp, the compiler would report an error when the program attempts to use carrots further on. (In fact, you might want to try omitting the declaration just to see how your compiler responds. Then if you see that response in the future, you’ll know to check for omitted declarations.)

In general, then, a declaration indicates the type of data to be stored and the name the program will use for the data that’s stored there. In this particular case, the program creates a variable called carrots in which it can store an integer (see Figure 2.4).

Figure 2.4. A variable declaration.

The declaration statement in the program is called a defining declaration statement, or definition, for short. This means that its presence causes the compiler to allocate memory space for the variable. In more complex situations, you can also have reference declarations. These tell the computer to use a variable that has already been defined elsewhere. In general, a declaration need not be a definition, but in this example it is.

If you’re familiar with C or Pascal, you’re already familiar with variable declarations. You also might have a modest surprise in store for you. In C and Pascal, all variable declarations normally come at the very beginning of a function or procedure. But C++ has no such restriction. Indeed, the usual C++ style is to declare a variable just before it is first used. That way, you don’t have to rummage back through a program to see what the type is. You’ll see an example of this later in this chapter. This style does have the disadvantage of not gathering all your variable names in one place; thus, you can’t tell at a glance what variables a function uses. (Incidentally, C99 now makes the rules for C declarations much the same as for C++.)