ANSI String Class

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The ANSI string class implements a first-class character string data type that avoids many problems associated with simple character arrays ("C-style strings"). You can define a string object very simply, as shown in the following example:

#include <string>
using namespace std;
...
string first_name = "Bjarne";
string last_name;

last_name = "Stroustrup";

string names = first_name + " " + last_name;
cout << names << endl;

names = last_name + ", " + first_name;
cout << names << endl;

Member functions

The string class defines many member functions. A few of the basic ones are described below:

Note: The string class is based on a template class named basic_string.
Some of the member function declarations below may be a little confusing
to those new to C++, even though they have been simplified somewhat.
Fortunately, these functions are quite easy to use in practice.

Initialization (constructor) A string object may defined without an initializing value, in which case its initial value is an empty string (zero length, no characters):
string str1;

A string object may also be initialized with

  • a string expression:
    string str2 = str1;
string str3 = str1 + str2;
string str4 (str2);  // Alternate form
  • a character string literal:
    string str4 = "Hello there";
string str5 ("Goodbye");  // Alternate form
  • a single character
    Unfortunately, the expected methods don't work:
    string str6 = 'A';  // Incorrect
string str7 ('A');  // Also incorrect

    Instead, we must use a special form with two values:

    string str7 (1,'A'); // Correct

    The two values are the desired length of the string and a character to fill the string with. In this case, we are asking for a string of length one, filled with the character A.

  • a substring of another string object:
    string str8 = "ABCDEFGHIJKL";
// Initialize str9 as "CDEFG"
// Starts at character 2 ('C')
// with a length of 5
// (or the rest of the string, if shorter)
string str9 (str8,2,5);
length

size

 size_type length() const;
size_type size() const;
Both of these functions return the length (number of characters) of the string. The size_type return type is an unsigned integral type. (The type name usually must be scoped, as in string::size_type.)
string str = "Hello";
string::size_type len;
len = str.length(); // len == 5 
len = str.size();   // len == 5
c_str const char* c_str() const;
For compatibility with "older" code, including some C++ library routines, it is sometimes necessary to convert a string object into a character array ("C-style string"). This function does the conversion. For example, you might open a file stream with a user-specified file name:
string filename;
cout << "Enter file name: ";
cin >> filename;
ofstream outfile (filename.c_str());
outfile << "Data" << endl;
insert string& insert(size_type pos, const string& str);
Inserts a string into the current string, starting at the specified position.
string str11 = "abcdefghi";
string str12 = "0123";
str11.insert (3,str12);
cout << str11 << endl; // "abc0123defghi"
str12.insert (1,"XYZ");
cout << str12 << endl; // "0XYZ123"
erase string& erase(size_type pos, size_type n);
Delete a substring from the current string.
string str13 = "abcdefghi";
str12.erase (5,3);
cout << str12 << endl; // "abcdei"
replace string& replace(size_type pos, size_type n, const string& str);
Delete a substring from the current string, and replace it with another string.
string str14 = "abcdefghi";
string str15 = "XYZ";
str14.replace (4,2,str15);
cout << str14 << endl; // "abcdXYZghi"
find

rfind

size_type find (const string& str, size_type pos);
Search for the first occurrence of the substring str in the current string, starting at position pos. If found, return the position of the first character. If not, return a special value (called string::npos). The member function rfind does the same thing, but returns the position of the last occurrence of the specified string.
string str16 = "abcdefghi";
string str17 = "def";
string::size_type pos = str16.find (str17,0);
cout << pos << endl; // 3
pos = str16.find ("AB",0);
if (pos == string::npos) cout << "Not found" << endl;
substr string substr (size_type pos, size_type n);
Returns a substring of the current string, starting at position pos and of length n:
string str18 = "abcdefghi"
string str19 = str18.substr (6,2);
cout << str19 << endl; // "gh"

Non-member functions

In addition to member functions of the string class, some non-member functions are designed to work with strings; the most common of these is:

getline istream& getline (istream& is, string& str, char delim = '\n');
Reads characters from an input stream into a string, stopping when one of the following things happens:
  • An end-of-file condition occurs on the input stream
  • When the maximum number of characters that can fit into a string have been read
  • When a character read in from the string is equal to the specified delimiter (newline is the default delimiter); the delimiter character is removed from the input stream, but not appended to the string.

The return value is a reference to the input stream. If the stream is tested as a logical value (as in an if or while), it is equivalent to true if the read was successful and false otherwise (e.g., end of file).

The most common use of this function is to do "line by line" reads from a file. Remember that the normal extraction operator (>>) stops on white space, not necessarily the end of an input line. The getline function can read lines of text with embedded spaces.

vector<string> vec1;
string line;
vec1.clear();
ifstream infile ("stl2in.txt");
while (getline(infile,line,'\n'))
{
  vec1.push_back (line);
}

Operators

A number of C++ operators also work with string objects:

= The assignment operator may be used in several ways:
  • Assigning one string object's value to another string object
    string string_one = "Hello";
string string_two;
string_two = string_one;
  • Assigning a C++ string literal to a string object
    string string_three;
string_three = "Goodbye";
  • Assigning a single character (char) to a string object
    string string_four;
char ch = 'A';
string_four = ch;
string_four = 'Z';
+ The "plus" operator concatenates:
  • two string objects
    string str1 = "Hello ";
string str2 = "there";
string str3 = str1 + str2; // "Hello there"
  • a string object and a character string literal
    string str1 = "Hello ";
string str4 = str1 + "there";
  • a string object and a single character
    string str5 = "The End";
string str6 = str5 + '!';
+= The "+=" operator combines the above assignment and concatenation operations in the way that you would expect, with a string object, a string literal, or a single character as the value on the right-hand side of the operator.

string str1 = "Hello ";
str1 += "there";
==
!=
<
>
<=
>=		 The comparison operators return a Boolean (true/false) value indicating whether the specified relationship exists between the two operands. The operands may be:
  • two string objects
  • a string object and a character string literal
<< The insertion operator writes the value of a string object to an output stream (e.g., cout).

string str1 = "Hello there";
cout << str1 << endl;
>> The extraction operator reads a character string from an input stream and assigns the value to a string object.

string str1;
cin >> str1;
[]

(subscript)

The subscript operator accesses one character in a string:

string str10 = "abcdefghi";
char ch = str10[3];
cout << ch << endl; // 'd'
str10[5] = 'X';
cout << str10 << endl; // "abcdeXghi"

This document was written by Dr. Mark J. Sebern (See stl) and last updated on April 16, 1999