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String Class

  • Describe the string class and its member functions

"Use vector and string instead of arrays." Sutter, Alexandruscu (2005)

The C++ Standard Library defines the string class as the object-oriented alternative to null-terminated character strings of the C language. The member functions of this class that are specialized for direct operations on strings.

Class Definition​

The string class is defined within the std namespace. The <string> header file contains the class definition.

Constructors​

The constructors of the string class can initialize an object using:

  1. a C-style null-terminated string
  2. a C-style null-terminated substring
  3. another string object
  4. a substring of another string object
  5. a sequence of characters

That is,

string a = "Hello";           // a C-style string
string b("Good Bye", 4);      // the 1st 4 characters of C-style string
string c = a;                 // another string object
string d(b, 5, 3);            // "Bye" - a substring of a string object
string e(a.begin(), a.end()); // a sequence of characters

Member Functions and Operations​

Member functions and operations of the string class include:

  • string& operator=(const string& s) - assign string s to the current object
  • string& operator+=(const string& s) - append string s to the current object
  • string& operator+=(const char* s) - append C-style string s to the current object
  • string& operator+=(char c) - append character c to the current object
  • size_t size() const - returns number of bytes stored in the current object
  • size_t length() const - returns number of bytes stored in the current object
  • bool empty() const - returns whether the current object is empty
  • char& operator[](size_t i) - returns a reference to the character at index i (unchecked)
  • const char& operator[](size_t i) const - read the character at index i (unchecked)
  • at(size_t i) const - read the character at index i (checked)
  • char& front() - returns a modifiable reference to the first character
  • const char& front() const - read the first character
  • char& back() - returns a modifiable reference to the last character
  • const char& back() const - read the last character
  • const char* c_str() const - return the address of the C-style null-terminated string equivalent
  • size_t find_first_of(const string& str) const - returns the position of the first character that matches a character in str; if no match, returns string::npos
  • size_t find_last_of(const string& str) const - returns the position of the last character that matches a character in str; if no match, returns string::npos
  • string substr(size_t pos, size_t len = npos) - returns a string that contains the portion of the current string starting at character position pos and spanning len characters (or to the end of the current string)
  • int compare(const string& str) const - compares the current object to str and returns 0 if the strings are equal, != 0 otherwise
  • void clear() - erases the contents of the current object

npos is a class member constant with the greatest possible value for position within the string (unsigned(-1)).

Example​

// String Class
// string.cpp

#include <string>
#include <iostream>

int main()
{
  std::string str("Good ");

  str += "day";
  str += '!';

  str[5] = 'D';
  std::cout << str.length() << ' ' << str.c_str() << std::endl;
}
9 Good Day!

Helper Functions​

Helper functions for the string class include:

  • std::istream& getline(std::istream&is, string& str, char delimiter) - extracts characters from is until delimiter; stores the characters in str and discards the delimiter
  • string operator+(const string& lhs, const string& rhs) - concatenates lhs to rhs and returns the concatenated string
  • istream& operator>>(istream& is, string& s) - extracts s from is and returns a reference to is
  • ostream& operator<<(ostream& os, const string& s) - inserts s into os and returns a reference to os

The string class is the default instantiation of the basic_string class template. The basic_string class template is similar to the vector class template. The string class provides iterators and can be used with the standard algorithms of the standard template library. Pointer/array equivalence does not hold: if s is an instance of the string class, then &s[0] is not the same as s.