A string

• Is a series of characters stored in consecutive bytes of memory

C++ has two ways of dealing with strings

• The first, taken from C and often called a C-style string, is the first one this page examines
• Later, we will discuss an alternative method based on a string class library

The idea of a series of characters stored in consecutive bytes

• Implies that you can store a string in an array of char
  • With each character kept in its own array element

Strings provide a convenient way to store text information

• Such as messages to the user (“Please tell me your secret Swiss bank account number”)
  •  Or responses from the user (“You must be joking”)

C-style strings have a special feature:

• The last character of every string is the null character
  • This character, written \0, is the character with ASCII code 0, and it serves to mark the string’s end
    • For example, consider the following two declarations (A-1):

Example of (A-1)

char dog [5] = { 'h', 'u', 's', 'k', 'y'}; // not a string!
char cat[5] = {'f', 'a', 't', 's', '\0'}; // a string!

(A-1) both of these arrays are arrays of char

• But only the second is a string

The null character plays a fundamental role in C-style strings

• For example, C++ has many functions that handle strings, including those used by cout
• They all work by processing a string character-by-character until they reach the null character
• If you ask cout to display a nice string like cat in the preceding example
• It displays the first four characters, detects the null character, and stops
• But if you are ungracious enough to tell cout to display the dog array from the preceding example, which is not a string
• cout prints the five letters in the array and then keeps marching through memory byte-by-byte, interpreting each byte as a character to print, until it reached a null character
• Because null characters, which really are bytes set to zero
• Tend to be common in memory, the damage is usually contained quickly; nonetheless, you should not treat nonstring character arrays as strings

The cat array example makes initializing an array to a string look tedious

• All those single quotes and then having to remember the null character
  • Don’t worry. There is a better way to initialize a character array to a string
    • Just use a quoted string, called a string constant or string literal, as in the following (A-2):

Example of (A-2)

char bird[10] = "Mr. Cheeps"; // the \0 is understood
char fish[] = "Bubbles"; // let the compiler count

Quoted strings always include the terminating null character implicitly

• So you don’t have to spell it out
  • See (Image-1)

(Image-1) Initializing an array to a string
 

Also, the various C++ input facilities for reading a string from keyboard input into a char array

• Automatically add the terminating null character for you

Of course, you should make sure the array

• Is large enough to hold all the characters of the string
  • Including the null character

Initializing a character array with a string constant

• Is one case where it may be safer
  • To let the compiler count the number of elements for you

 


There is no harm, other than wasted space, in making an array larger than the string

• That’s because functions that work with strings are guided by the location of the null character
  • Not by the size of the array

C++ imposes

• No limits on the length of a string

Remember

• When determining the minimum array size necessary to hold a string
  • Remember to include the terminating null character in your count

Note that a string constant (with double quotes)

• Is not interchangeable with a character constant (with single quotes)

A character constant

• Such as ‘S’, is a shorthand notation for the code for a character
  • On an ASCII system, ‘S’ is just another way of writing 83
    • Thus, the following statement assigns the value 83 to shirt_size (A-3):

Example of (A-3)

char shirt_size = 'S'; // this is fine

But “S” represents the string consisting of two characters

• The S and the \0 characters
  • Even worse, “S” actually represents the memory address at which the string is stored
    • So the following statement attempts to assign a memory address to shirt_size (A-4):

Example of (A-4)

char shirt_size = "S"; // illegal type mismatch

Because an address is a separate type in C++

• A C++ compiler won’t allow this sort of nonsense
  • (We’ll return to this point later, after we’ve discussed pointers.)