Arrays of Pointers

Arrays may contain pointers. A common use of such a data structure is to form an array of pointer-based strings, referred to simply as a string array. Each entry in the array is a string, but in C++ a string is essentially a pointer to its first character, so each entry in an array of strings is simply a pointer to the first character of a string. Consider the declaration of string array suit that might be useful in representing a deck of cards:

const char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" };

The suit[4] portion of the declaration indicates an array of four elements. The const char * portion of the declaration indicates that each element of array suit is of type "pointer to char constant data." The four values to be placed in the array are "Hearts", "Diamonds", "Clubs" and "Spades". Each is stored in memory as a null-terminated character string that is one character longer than the number of characters between quotes. The four strings are seven, nine, six and seven characters long (including their terminating null characters), respectively. Although it appears as though these strings are being placed in the suit array, only pointers are actually stored in the array, as shown in Fig. 8.22. Each pointer points to the first character of its corresponding string. Thus, even though the suit array is fixed in size, it provides access to character strings of any length. This flexibility is one example of C++'s powerful data-structuring capabilities.

Figure 8.22. Graphical representation of the suit array.

The suit strings could be placed into a two-dimensional array, in which each row represents one suit and each column represents one of the letters of a suit name. Such a data structure must have a fixed number of columns per row, and that number must be as large as the largest string. Therefore, considerable memory is wasted when we store a large number of strings, of which most are shorter than the longest string. We use arrays of strings to help represent a deck of cards in the next section.

String arrays are commonly used with command-line arguments that are passed to function main when a program begins execution. Such arguments follow the program name when a program is executed from the command line. A typical use of command-line arguments is to pass options to a program. For example, from the command line on a Windows computer, the user can type

dir /P

to list the contents of the current directory and pause after each screen of information. When the dir command executes, the option /P is passed to dir as a command-line argument. Such arguments are placed in a string array that main receives as an argument. We discuss command-line arguments in Appendix E, C Legacy Code Topics.