C++ Primer Plus полностью

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.)

Why Must Variables Be Declared?

Some languages, notably BASIC, create a new variable whenever you use a new name, without the aid of explicit declarations. That might seem friendlier to the user, and it is—in the short term. The problem is that if you misspell the name of a variable, you inadvertently can create a new variable without realizing it. That is, in BASIC, you can do something like the following:

CastleDark = 34

...

CastleDank = CastleDark + MoreGhosts

...

PRINT CastleDark

Because CastleDank is misspelled (the r was typed as an n), the changes you make to it leave CastleDark unchanged. This kind of error can be hard to trace because it breaks no rules in BASIC. However, in C++, CastleDark would be declared while the misspelled CastleDank would not be declared. Therefore, the equivalent C++ code breaks the rule about the need to declare a variable for you to use it, so the compiler catches the error and stomps the potential bug.

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++.)

Tip

The C++ style for declaring variables is to declare a variable as close to its first use as possible.

Assignment Statements

An assignment statement assigns a value to a storage location. For example, the following statement assigns the integer 25 to the location represented by the variable carrots:

carrots = 25;

The = symbol is called the assignment operator. One unusual feature of C++ (and C) is that you can use the assignment operator serially. For example, the following is valid code:

int steinway;

int baldwin;

int yamaha;

yamaha = baldwin = steinway = 88;

The assignment works from right to left. First, 88 is assigned to steinway; then the value of steinway, which is now 88, is assigned to baldwin; then baldwin’s value of 88 is assigned to yamaha. (C++ follows C’s penchant for allowing weird-appearing code.)

The second assignment statement in Listing 2.2 demonstrates that you can change the value of a variable:

carrots = carrots - 1;  // modify the variable

The expression to the right of the assignment operator (carrots – 1) is an example of an arithmetic expression. The computer will subtract 1 from 25, the value of carrots, obtaining 24. The assignment operator then stores this new value in the carrots location.

A New Trick for cout

Up until now, the examples in this chapter have given cout strings to print. Listing 2.2 also gives cout a variable whose value is an integer:

cout << carrots;

The program doesn’t print the word carrots; instead, it prints the integer value stored in carrots, which is 25. Actually, this is two tricks in one. First, cout replaces carrots with its current numeric value of 25. Second, it translates the value to the proper output characters.