Strings In C++

Strings are used a lot in programs. C++ programs either use low level C-style strings or string objects. C-Style strings are simple arrays of characters, whereas string objects are objects with useful methods. Although string objects would be considered the more modern OO approach, there is a lot of code out there that uses C-style string so you should be familiar with both approaches.

C-style strings are arrays of characters with the special null characters '\0' denoting the end of the string. That is, the array memory can be larger than the string but the null character denotes where the string ends in the array.

Literal strings like "Saturday" (denoted by double quotes) are string constants and represented as arrays of characters. String constants can be declared as follows.

char * name = "Lou";
char day[] = "Saturday";

or

char name[15];  //notice array is bigger than the string placed in it
day[0] = 'L';
day[1] = 'o';
day[2] = 'u';
day[4] = '\0';   //signifies the end of the characters of the string

String constants are not modifiable. The following code example compiles and runs but crashes at run time because of the attempt to modify an literal string. (Literal strings are stored where the compiler puts static constants.)

#include <iostream>
using namespace std;

void main() {
     char * name = "saturday";
     cout << name << endl;
     cout << name[0] << endl;
     name[0] = 'S'; //ERROR -code crashes at run time
     cout << name << endl;

The follow, more correct, program produces a compile error instead because the literal string is, more correctly, referenced through a pointer to a constant.  

void main() {
     const char * name = "saturday";
     cout << name << endl;
     cout << name[0] << endl;
     name[0] = 'S'; //ERROR -compile time error this time
     cout << name << endl;

<cstring> or <string.h> Standard library

C++ provides the cstring, or older string.h, standard library of manipulating c-style strings. Like iostream, cstring is declared in the namespace std. The following table provides some of its operations that are used throughout these note. 

Using C-style Strings (Example)
 

#include <iostream>
#include <cstring> //for strcmp(), strcpy(), strlen(), strcat()

using namespace std;

int main() {
     char input[100];
     char * myString;
    
     cout << "enter an input string" << endl;
     cin >> input;
     cout << "you said: ";
     cout << input << endl;
     cout << "the length of your string is " << strlen(input) << endl;
    
     myString = new char[strlen(input) + 1];
     strcpy(myString, input);
    
     cout << "the length of myString is " << strlen(myString) << endl;
     cout << myString << endl;
    
     //comparing strings
     cout << strcmp("abba", myString) << endl;
     cout << strcmp(myString, myString) << endl;
     cout << strcmp(myString, "abba") << endl;
   
     return 0;

/* Program output

enter an input string
Saturday
you said: Saturday
the length of your string is 8
the length of myString is 8
Saturday
1
0
-1

*/

Example of Dynamic Memory for C-style string variables in a class
 

Notice in this example the consequences of trying to use a const char * to refer to the name. The idea is that we don't want the name modifiable after the person object is created, but we do want to allow the email address to be changed. This is an illustration of the consequences of referring to the name through a const char *. (Be aware that the name is actually protected though since we did not provide a public "set" method to change the name.)

#include <iostream>
#include <cstring> //for strcmp(), strcpy(), strlen(), strcat()

using namespace std;

class Person {
public:
   Person(char * their_name = "unknown", char * email = "unknown"){
      char * temp = new char[strlen(their_name) + 1];
      strcpy(temp, their_name);
      name = temp;
      email_address = new char[strlen(email) + 1];
      strcpy(email_address, email);
   }

   ~Person(void) {
      delete [] (char *) name;   //notice cast is necessary to allow destruction
      delete [] email_address;
   }

   char * getEmailAddress(){return email_address;}
   const char * getName() {return name;}

   void setEmailAddress(char * new_email_address) {
      if(strlen(new_email_address) > strlen(email_address)){
           //allocate new memory if necessary
           delete [] email_address; //destroy old email address memory
           email_address = new char[strlen(new_email_address) + 1];
      }
      strcpy(email_address, new_email_address);
   }

private:
   const char * name; //characters of name should not be modifiable
   char * email_address;  //characters of email_address should be modifiable
};

void main(){
      Person lou("Lou", "lou@hotmail.com");
      Person sue("Sue", "sue@scs.carleton.ca");
      Person p;
      cout << lou.getName() << " email: " << lou.getEmailAddress() << endl;
      cout << sue.getName() << " email: " << sue.getEmailAddress() << endl;
      cout << p.getName() << " email: " << p.getEmailAddress() << endl;

      lou.setEmailAddress("louis@connect.carleton.ca");
      cout << lou.getName() << " email: " << lou.getEmailAddress() << endl;
}

/* output
Lou email: lou@hotmail.com
Sue email: sue@scs.carleton.ca
unknown email: unknown
Lou email: louis@connect.carleton.ca
*/

String Objects

C++ provides string objects, as opposed to C-style character arrays. String objects are defined in the <string> standard library. Do not confuse <string> with <string.h>. The newer implementation of <string.h> is <cstring>, <string> on the other hand defines the string objects.

The following table gives some of the constructors and methods associated with <string> objects. Unless otherwise indicated, str, str1 and str2 are string objects in the table below.

#include <iostream>
#include <string> //for string objects

using namespace std;

int main() {
   string myString;
   cout << "enter an input string" << endl;
   cin >> myString;
   cout << "you said: ";
   cout << myString << endl;
   cout << "the length of your string is " << myString.length() << endl;

   //comparing strings
   cout << myString.compare("abba") << endl;
   cout << myString.compare(myString) << endl;
|   string abba = "abba";
   cout << abba.compare(myString) << endl;
   return 0;
}

/* Program output
enter an input string
Saturday
you said: Saturday
the length of your string is 8
-1
0
1
*/

Example 2

#include <iostream>
#include <string> //for string object

using namespace std;

class Person {
public:
   Person(char * their_name = "unknown", char * email = "unknown") {
      name = their_name; //this should be done in base member initializer list instead
      email_address = email;
   }
 
~Person(void) {
      //nothing to do here
   }

   const char * getEmailAddress(){return email_address.c_str();}
   const char * getName() {return name.c_str();}
   void setEmailAddress(char * new_email_address) {
      email_address = new_email_address;
   }

private:
   string name;
   string email_address;
};

void main(){
   Person lou("Lou", "lou@hotmail.com");
   Person sue("Sue", "sue@scs.carleton.ca");
   Person p;

   cout << lou.getName() << " email: " << lou.getEmailAddress() << endl;
   cout << sue.getName() << " email: " << sue.getEmailAddress() << endl;
   cout << p.getName() << " email: " << p.getEmailAddress() << endl;
  
   lou.setEmailAddress("louis@connect.carleton.ca");
   cout << lou.getName() << " email: " << lou.getEmailAddress() << endl;
}