C++ Strings, String Functions and String Operators

The C++ String Class

A class is special data type with built-in functions. The ifstream and ofstream data types you learned earlier are examples of classes. They have built-in information and functions to help you manage files. C++ has a string class with functions and helpful information that help you manage text data. When you declare a variable from a class, that variable can also be called an object. Functions that are built into a class can be called from declared objects using a special dot notation. You have already seen examples of this with the .open(), .get(), and .close() functions. C++ classes can also be enhanced with special operators. In this section we will introduce some string class functions and operators.

Declaring a String

Here are some ways to declare and initialise C++ strings:

		

	 //declare an empty string called str1
	 string str1; 

	 //When you declare a string variable object, 
	 //you may initialize it with a character string literal:
	 string str2 = "Hello there"; // Initialize form 1: Declare and assign
	 string str3 ("Goodbye");  	  // Initialize form 1: Declare and construct

	 //A string variable/object may also be initialized with
	 //a string expression:
	 string str4 = str2;
	 string str5 = str2 + str3;

	 //A string variable/object may also be initialized with
	 //a substring of another string object:
	 string str6 = "ABCDEFGHIJKL";

	 // Initialize str7 as "CDEFG"
	 // Starts at character 2 ('C')
	 // with a length of 5
	 // (or the rest of the string, if shorter)
	 string str7 (str6,2,5); 

	

The .length() and .size() Functions

Both of these functions return the number of characters in the string.

They return a special type: size_type. It is an unsigned integer. We use the qualified name string::size_type because the definition of size_type is otherwise hidden inside the definition of the string type.

		
	string str8 = "Hello";
	string::size_type len;


	//Store returned values in a variable, then use them:
	len = str8.length(); 
	cout << len << endl; // prints 5
  
	len = str8.size(); 
	cout << len << endl; // also prints 5


	// OR just use them directly:
	cout << str8.length() << endl;

The .find() and .substr() Functions

• If the substring is found, .find() returns the position of the first character.
  If not, .find() returns the special value string::npos.
  
  
• The first argument is the search substring.
  It is a string or string literal.
  
  
• The second argument is where to start searching.
  It is an integer greater than or equal to 0.
  If you leave it out, .find() starts at 0.

For example:

	string str16 = "abcdefghi";
	string str17 = "def";

	// Search from the beginning of str16
	string::size_type pos = str16.find(str17);
	cout << pos << endl; // prints 3

	// Search from the beginning of str16
	pos = str16.find(str17,0);
	cout << pos << endl; // prints 3


	// Search from the fifth position of str16
	pos = str16.find(str17,5);
	cout << pos << endl; // prints a REALLY BIG number!!


	pos = str16.find ("AB");
	if (pos == string::npos) 
	{
		cout << "Not found." << endl;
		return 1;
	}

The .substr() function cuts a substring out of a string.

• The .substr() function always returns a substring of the string it is called on
  
  
• the first parameter is the start position of the substring
  
  
• the other parameter is is the length of the substring.
  If you leave it out, .substr() returns everything from the start positon to the end of the string.

For example:

	string str18 = "abcdefghi";
	string str19 = str18.substr(5,2);
	cout << str19 << endl; // prints "fg" 

	string str20 = str18.substr(5);
	cout << str20 << endl; // prints "fghi"
   

String Operators

A number of C++ operators also work with strings.

• The = (assignment) operator may be used in several ways:

  
  	//Assigning one string's value to another string 
  	string string_one = "Hello";
  	string string_two;
  	string_two = string_one; 
  
  
  	//Assigning a character array or string literal to a C++ string
  	//(You'll learn more about arrays and how they relate to strings later.)
  	char str[] = "I'm a character array!";
  	string string_three = str;
  	string_three = "I'm a string literal! My type is const char[] - a character array :O";
  
  
  	//Assigning a single character (char) to a string
  	string string_four;
  	char ch = 'A';
  	string_four = ch;
  	string_four = 'Z'; 
  
  

• The + (plus) operator concatenates:

  
  	// two strings 
  	string str1 = "Hello ";
  	string str2 = "there";
  	string str3 = str1 + str2; // "Hello there" 
  
  	// a string and a character string literal
  	string str1 = "Hello ";
  	string str4 = str1 + "there";  //Also "Hello there"
  
  	// a string and a single character
  	string str5 = "The End";
  	string str6 = str5 + '!'; // "The End!"
  
  

• The plus-equals += operator appends:
  ie: it combines the assignment and concatenation operations in the way that you would expect.
  The right-hand side must be a string object, a string literal, or a single character.

  				
  					string str1 = "Hello ";
  					str1 += "there"; // "Hello there"