C++ strings

Strings are sequence of characters. C++ supports two ways of representing strings :
1) C style string ( Array of characters )
2) Standard string class present in C++ library

C style strings
C language supports string representation with array of characters terminated by a null character ‘ \0 ‘. This feature remained a part of C++ language as well.
Consider the following statements to initialize a string (character array) :

char str [ ] = { ‘W’, ‘O’, ‘R’, ‘L’, ‘D’, ‘\0’ }; // Initializing a NULL terminated string ‘WORLD’
char str [ ] = ” WORLD “ ; // Another way of initializing

Please note that the second statement doesn’t contain ‘ \0 ‘ since in this case, NULL character is automatically appended by the C++ compiler.

Standard string manipulation function
The header file < cstring > provides a number of standard functions which can be used for string manipulation. Some of the most commonly used functions are :

Function	Purpose
strlen ( str1 )	Returns the length of string str1
strcpy ( str1, str2 )	Copies string str1 into str2
strcat ( str1, str2 )	Appends string str2 at the end of string str1
strcmp ( str1, str2 )	Compares string str1 and str2 character by character and returns 0 if they are equal, else returns the difference of ascii values of the first mismatching characters of the two strings.

There are a lot of standard functions offered by the library. Following program demonstrates the use of some string manipulation functions :

#include<iostream>
#include<cstring>
using namespace std;

int main() {
   char str1[10] = { 'G', 'o', 'o', 'd', '\0' };
   char str2[] = "Bye";
   char str3[10]; // declare a string to store maximum 10 characters
   int len = strlen(str1); // get the length of str1
   cout << "Length of str1 is " << len << endl;
   cout << "str1 : " << str1 << endl;
   cout << "str2 : " << str2 << endl;
   strcpy(str3, str2); // copy str2 into str3
   cout << "str3 : " << str3 << endl;
   if (strcmp(str2, str3) == 0) { // compare two strings
      cout << "Two strings are equal" << endl;
   }
   else {
      cout << "Strings are not equal" << endl;
   }
   strcat(str1, str2); // concatenate/append str2 to str1
   cout << "After concatenating, str1 : " << str1 << endl;
   return 0;
}

Taking string as user input
Strings can be taken as user input from console using following ways :

cin >> str;	Stops taking the input when it sees a space character. Also it is dangerous since there is no length check.
gets(str);	Dangerous to use this because it doesn’t check for length of the input string. This may lead to buffer overflow ( Suppose the string is declared as char str [ 10 ] and user enters a string of length > 10 ).
fgets(str, 10, stdin);	Safer form of gets as it contains a length parameter.
cin.getline(str, 10);	Yet another way to input a string in C++.

Following program demonstrates the safe techniques to take string as input from console :

#include<iostream>
#include<cstdio>
using namespace std;

int main() {
   char str1[10], str2[10]; // can store strings of max length 10
   cout << "Enter a string : ";
   fgets(str1, 10, stdin); // input a string of size 10
   cout << "Enter another string : ";
   cin.getline(str2, 10);  // another way of taking input
   cout << "str1 : " << str1;
   cout << "str2 : " << str2 << endl;
   return 0;
}

string class in C++
The C++ library < string > provides a rich set of string manipulation functions. Now, we can use string as a data type like int, float, etc. and we can declare a variable of type string. Following program demonstrates the use of string data type and some of the member functions of string class :

#include<iostream>
#include<string>
using namespace std;

int main() {
   string str1 = "civic";
   string str2 = "subject";
   string str3;
   str3 = str2; // copy str2 to str3
   str2 = str1 + str3; // concatenate str1 and str3
   cout << "str1 : " << str1 << endl;
   cout << "str2 : " << str2 << endl;
   cout << "str3 : " << str3 << endl;
   str1.push_back('s'); // append a character to a string
   cout << "After Append str1 : " << str1 << endl;
   int len = str2.length(); // get the length of str2
   cout << "Length of str2 : " << len << endl;
   str3.append(str1); // append string str1 at the end of str3
   cout << "After append str3 : " << str3 << endl;
   string str4 = str3.substr(3); // get the substring starting from index 3
   cout << "Substring str4 : " << str4 << endl;
   return 0;
}