When you define a variable in C++, you must tell the compiler what kind of variable it is: an integer, a character, and so forth. This information tells the compiler how much room to set aside and what kind of value you want to store in your variable.

Each cubbyhole is one byte large. If the type of variable you create is two bytes in size, it needs two bytes of memory, or two cubbyholes. The type of the variable (for example, integer) tells the compiler how much memory (how many cubbyholes) to set aside for the variable.

Because computers use bits and bytes to represent values, and because memory is measured in bytes, it is important that you understand and are comfortable with these concepts. For a full review of this topic, please read Appendix B, "C++ Keywords."

On any one computer, each variable type takes up a single, unchanging amount of room. That is, an integer might be two bytes on one machine, and four on another, but on either computer it is always the same, day in and day out.

A char variable (used to hold characters) is most often one byte long. A short integer is two bytes on most computers, a long integer is usually four bytes, and an integer (without the keyword short or long) can be two or four bytes. Listing 3.1 should help you determine the exact size of these types on your computer.

New Term: A character is a single letter, number, or symbol that takes up one byte of memory.

Listing 3.1. Determining the size of variable types on your computer.

1: #include <iostream.h>

2:

3: int main()

4: {

5: cout << "The size of an int is:\t\t" << sizeof(int) << " bytes.\n";

6: cout << "The size of a short int is:\t" << sizeof(short) << " bytes.\n";

7: cout << "The size of a long int is:\t" << sizeof(long) << " bytes.\n";

8: cout << "The size of a char is:\t\t" << sizeof(char) << " bytes.\n";

9: cout << "The size of a float is:\t\t" << sizeof(float) << " bytes.\n";

10: cout << "The size of a double is:\t" << sizeof(double) << " bytes.\n";

11:

12: return 0;

13: }

OUTPUT: 

The size of an int is: 2 bytes.

The size of a short int is: 2 bytes.

The size of a long int is: 4 bytes.

The size of a char is: 1 bytes.

The size of a float is: 4 bytes.

The size of a double is: 8 bytes.

NOTE: On your computer, the number of bytes presented might be different.

Analysis: Most of Listing 3.1 should be pretty familiar. The one new feature is the use of the sizeof() function in lines 5 through 10. sizeof() is provided by your compiler, and it tells you the size of the object you pass in as a parameter. For example, on line 5 the keyword int is passed into sizeof(). Using sizeof(), I was able to determine that on my computer an int is equal to a short int, which is 2 bytes.

In addition, all integer types come in two varieties: signed and unsigned. The idea here is that sometimes you need negative numbers, and sometimes you don’t. Integers (short and long) without the word "unsigned" are assumed to be signed. Signed integers are either negative or positive. Unsigned integers are always positive.

Because you have the same number of bytes for both signed and unsigned integers, the largest number you can store in an unsigned integer is twice as big as the largest positive number you can store in a signed integer. An unsigned short integer can handle numbers from 0 to 65,535. Half the numbers represented by a signed short are negative, thus a signed short can only represent numbers from -32,768 to 32,767. If this is confusing, be sure to read Appendix A, "Operator Precedence."

Several other variable types are built into C++. They can be conveniently divided into integer variables (the type discussed so far), floating-point variables, and character variables.

Floating-point variables have values that can be expressed as fractions—that is, they are real numbers. Character variables hold a single byte and are used for holding the 256 characters and symbols of the ASCII and extended ASCII character sets.

New Term: The ASCII character set is the set of characters standardized for use on computers. ASCII is an acronym for American Standard Code for Information Interchange. Nearly every computer operating system supports ASCII, though many support other international character sets as well.

The types of variables used in C++ programs are described in Table 3.1. This table shows the variable type, how much room this book assumes it takes in memory, and what kinds of values can be stored in these variables. The values that can be stored are determined by the size of the variable types, so check your output from Listing 3.1.

Table 3.1. Variable Types.

NOTE: The sizes of variables might be different from those shown in Table 3.1, depending on the compiler and the computer you are using. If your computer had the same output as was presented in Listing 3.1, Table 3.1 should apply to your compiler. If your output from Listing 3.1 was different, you should consult your compiler’s manual for the values that your variable types can hold.