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Apart from allowing us to create c-style strings by using char arrays, C++ provides us a string class using which we can easily create string objects. String is a nothing but a sequence of characters within double quotes. For example - "Hello World" is a string literal, using it we can create a string object of string class.

Constructors of string class

There are about three constructors of string class, providing it multiple ways by which we could create string objects. Let us see these constructors.

Constructor	Description
string()	This constructor is used to create an empty string object.
string(const string & str)	This constructor is used to create a string object, by passing a string object to it. The new string object is initialized with the content of the passed string object.
string(const char *st)	This constructor is used to create a string object, by passing a char array to it. The new string object is initialized with the content of the passed char array.

How a String object is created?

In C++, a string object can be constructed in various ways as explained below -:

• string  s1();

This line of code creates a new empty string object by calling the constructor of string class, which does not take a string object. This string object is referred by a reference variable or name s1.




• string  s2("Hello There");

This line of code creates a new string object by calling the constructor of string class, which takes a string object. The new string object is initialized with a string literal value Hello There. This string object is referred by a reference variable or name s2.




• string s1("Hello There");
  string s2(s1);

This line of code creates a new string object by calling the constructor of string class, which takes a string object. This string object initialize it with a value Hello There. This string object is referred by a reference variable or name s1. This string object is passed to the constructor of string class to create another string object which is assigned to a reference variable s2.




• char arr[] = {'W', 'e', 'l', 'c', 'o', 'm', 'e', '\0'};  // line 1
  string s(arr); 	  // line 2

In the code above, line 1 creates a c-style string by using a char array, which is passed as an argument to the constructor of string class in line2, which creates a string object, referred by reference variable s. , initialized to a value Welcome.

All the three ways described above create a string object on the heap portion of memory.







Note :


In C++, the string objects can be added by using + operator, which is overloaded to allow the addition of objects of string class, just as we add two primitive variables.




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Creating and initializing String objects.


Here in program below, we have creating and initialized five string objects, each in a different way.



//Initializing strings in different ways. 

#include<iostream>
#include<string>

using namespace std;

int main()
{
//Creating first string object by passing a string literal to constructor
string str1("New York");    


//Creating second string object, initializing from another string
string str2(str1);	    


 //Creating a third string empty object.
string str3;		   
str3 ="London";


//Creating fourth string object by assigning a string literal
string str4 = "Poland";	    
cout<<"String 4 : " << str4 << "\n";

cout<<"String 1 : " << str1 << "\n";
cout<<"String 2 : " << str2 << "\n";
cout<<"String 3 : " << str3 << "\n";


//Creating fifth string object by adding two string objects by + operator
string str5 = str1+str2;    
cout<<"String 4 : " << str5;

}





Output is :




String 4 : Poland
String 1 : New York
String 2 : New York
String 3 : London
String 4 : New YorkNew York





Program Analysis



• The first string object is created by calling the constructor of string class and passing it a string literal New York.
• The second string object is created by passing the first string object to the constructor of string class.
• The third string object is created empty but it is initialized by assigning it a string value London in the next line.
• The fourth string object is created empty by assigning a string literal value Poland in the same line.
• The fifth string object is created by adding the first two string objects using + operator, which gives a new string value New YorkNew Delhi.






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