C++ STL string class

The C++ standard template library (STL) provides us with many common containers that we can use in our programs.

One common container provided by the C++ library is a string. The string container is used to store a sequence of characters, and generally replaces the troublesome null-terminated character arrays used in C. Note that there are programming situations when a character array is required. C++ strings may be converted to null-terminated C-style char arrays when necessary. For the moment, think of C++ strings as a new data type with a slightly strange syntax.

To use strings, you must include its associated header file --- #include <string>

What are some common operations you would expect a string class to support?

  1. Support to initialize a string object with the empty string
    The code string firstName;
    constructs a variable called "firstName" as a string which is initialized to the empty string (no characters).

  2. Support to initialize a string with a sequence of characters
    The code string middleName ("bob");
    or equivalently string middleName = "bob";
    constructs a variable called "middleName" as a string initialized to the sequence of characters "bob".

  3. Support to initialize a string as a copy of an existing string
    The code string lastName (middleName);
    constructs a variable called "lastName" as a string initialized as a copy of the existing string above called "middleName"

  4. Support to copy one string to another
    With primitive types such as int, float and char, we copy the contents of one existing variable into another using assignment with code like x = y;
    Very often (almost always) it is possible to copy one existing variable into another existing variable of the same type also by using the standard assignment statement. Using the string constructed above,
    the statement firstName = lastName;
    copies the contents of "lastName" into the string "firstName".

  5. Support to access the individual characters of the string
    We can access the individual characters of a string in a manner similar to the C-style character array using square brackets ( [ ] ) char char2; char2 = firstName[1]; assigns (copies) the second character of the string object "firstName" to the character variable char2.

    Similarly,

    firstName[0] = 'x'; changes the first character of firstName to 'x'

  6. Support to compare two strings for equality
    We can compare two integers (or other primitive type) using the equality operator "==" as the following code illustrates if (x == y) { // do something } The same thing is true of strings and many other classes. The equality operator for string compares two strings and returns true if the are equal. if (firstName == lastName) { // do something; } Other comparators such as <, >, and != can also be used with strings and produce the expected, intuitive results.

  7. Support of string concatenation
    We can concatenate (append one string to the end of another) two strings together using "string addition". string hello = "Hello"; string world( "World" ); string result = hello + world + "!"; cout << result; prints HelloWorld!

  8. Support to know how many characters are contained in the string
    We can ask a string how many characters it contains by using the string's size( ) operation. Note the syntax that is similar to that used for structures in C. int nrChars; nrChars = firstName.size( ); This syntax (the dot and parentheses) indicate that we are requesting that the string "firstName" perform the size( ) operation. We know from the string documentation that the size( ) operation requires no parameters and returns an integer value.

  9. Support to know if the string is empty.
    A similar operation provided by strings is the boolean empty( ) operation. From the string documentation we know that empty( ) takes no parameters and returns a boolean -- true if the string is empty, false if not empty. We can now write code such as if ( ! lastName.empty( ) ) // do something if not empty. Of course, we could have chosen to write if ( lastName.size( ) != 0 )

  10. Treat the C++ string as a C-style string. If we desire to treat the C++ string as a C-style string (perhaps to pass to some function), we ask the string to perform the c_str() operation. // given the declarations string s1 = "bob"; char s2[] = "This is a C-style string"; // this code copies s1 onto s2 using strcpy() strcpy (s2, s1.c_str());

  11. Access a substring
    Sometimes it's useful to be able to access just part (consecutive characters) of the string (a "substring"). The string operation substr(int index, int length ) gives us that access. Consider the following code. string sentence = "The quick brown fox jumped over the lazy dog."; // these statements print // The fox cout << sentence.substr( 0, 4); cout << sentence.substr( 16, 3) << endl;
Other operations are possible with strings and there are several common functions that work with string. See the C++ STL strings reference page and section 9.3 of the text for more details.

I/O with strings

You can output strings using operator << and cout, just like you do with other basic data types. string firstName = "Mary"; string lastName = "Jones"; cout << lastName << "," << firstName << endl; outputs Jones,Mary

You can use the input operator >> and cin to input strings from the user. However, remember that the >> operator ignores leading whitespace and stops reading when it sees the next whitespace. This technique is used for reading "words". Consider this code

string firstName, lastName; cin >> firstName; cin >> lastName If the user types Mary Jones is cool.
Then firstName contains "Mary" and lastName contains "Jones". The words is cool are waiting to be read.

To read a "sentence" (multiple words separated by blanks), use the function getline( ) as in the code below.

string sentence; getline (cin, sentence); This time if the user types Mary Jones is cool.
Then the entire line of text is read and sentence contains "Mary Jones is cool.".

The difference between using >> and getline( ) is illustrated in the example code.

Analysis