std::string class in C++

• Difficulty Level : Easy
• Last Updated : 23 Aug, 2021

C++ has in its definition a way to represent sequence of characters as an object of class. This class is called std:: string. String class stores the characters as a sequence of bytes with a functionality of allowing access to single byte character. 
 

std:: string vs Character Array

• A character array is simply an array of characters can terminated by a null character. A string is a class which defines objects that be represented as stream of characters.

• Size of the character array has to allocated statically, more memory cannot be allocated at run time if required. Unused allocated memory is wasted in case of character array. In case of strings, memory is allocated dynamically. More memory can be allocated at run time on demand. As no memory is preallocated, no memory is wasted.

• There is a threat of array decay in case of character array. As strings are represented as objects, no array decay occurs.

• Implementation of character array is faster than std:: string. Strings are slower when compared to implementation than character array.

• Character array do not offer much inbuilt functions to manipulate strings. String class defines a number of functionalities which allow manifold operations on strings.

Operations on strings

Input Functions
1. getline() :- This function is used to store a stream of characters as entered by the user in the object memory.
2. push_back() :- This function is used to input a character at the end of the string.
3. pop_back() :- Introduced from C++11(for strings), this function is used to delete the last character from the string.
 

• CPP

// C++ code to demonstrate the working of

// getline(), push_back() and pop_back()

#include<iostream>

#include<string> // for string class

using namespace std;

int main()

{

    // Declaring string

    string str;

    // Taking string input using getline()

    // "geeksforgeek" in giving output

    getline(cin,str);

    // Displaying string

    cout << "The initial string is : ";

    cout << str << endl;

    // Using push_back() to insert a character

    // at end

    // pushes 's' in this case

    str.push_back('s');

    // Displaying string

    cout << "The string after push_back operation is : ";

    cout << str << endl;

    // Using pop_back() to delete a character

    // from end

    // pops 's' in this case

    str.pop_back();

    // Displaying string

    cout << "The string after pop_back operation is : ";

    cout << str << endl;

    return 0;

}

Input: 
 

geeksforgeek

Output: 
 

The initial string is : geeksforgeekThe string after push_back operation is : geeksforgeeksThe string after pop_back operation is : geeksforgeek

 
Capacity Functions
4. capacity() :- This function returns the capacity allocated to the string, which can be equal to or more than the size of the string. Additional space is allocated so that when the new characters are added to the string, the operations can be done efficiently.
5. resize() :- This function changes the size of string, the size can be increased or decreased.
6.length():-This function finds the length of the string
7.shrink_to_fit() :- This function decreases the capacity of the string and makes it equal to the minimum capacity of the string. This operation is useful to save additional memory if we are sure that no further addition of characters have to be made.
 

• CPP

// C++ code to demonstrate the working of

// capacity(), resize() and shrink_to_fit()

#include<iostream>

#include<string> // for string class

using namespace std;

int main()

{

    // Initializing string

    string str = "geeksforgeeks is for geeks";

    // Displaying string

    cout << "The initial string is : ";

    cout << str << endl;

    // Resizing string using resize()

    str.resize(13);

    // Displaying string

    cout << "The string after resize operation is : ";

    cout << str << endl;

    // Displaying capacity of string

    cout << "The capacity of string is : ";

    cout << str.capacity() << endl;

    //Displaying length of the string

    cout<<"The length of the string is :"<<str.length()<<endl;

    // Decreasing the capacity of string

    // using shrink_to_fit()

    str.shrink_to_fit();

    // Displaying string

    cout << "The new capacity after shrinking is : ";

    cout << str.capacity() << endl;

    return 0;

}

Output

The initial string is : geeksforgeeks is for geeksThe string after resize operation is : geeksforgeeksThe capacity of string is : 26The length of the string is :13The new capacity after shrinking is : 15

 
Iterator Functions
8. begin() :- This function returns an iterator to beginning of the string.
9. end() :- This function returns an iterator to end of the string.
10. rbegin() :- This function returns a reverse iterator pointing at the end of string.
11. rend() :- This function returns a reverse iterator pointing at beginning of string.
 

• CPP

// C++ code to demonstrate the working of

// begin(), end(), rbegin(), rend()

#include<iostream>

#include<string> // for string class

using namespace std;

int main()

{

    // Initializing string`

    string str = "geeksforgeeks";

    // Declaring iterator

    std::string::iterator it;

    // Declaring reverse iterator

    std::string::reverse_iterator it1;

    // Displaying string

    cout << "The string using forward iterators is : ";

    for (it=str.begin(); it!=str.end(); it++)

    cout << *it;

    cout << endl;

    // Displaying reverse string

    cout << "The reverse string using reverse iterators is : ";

    for (it1=str.rbegin(); it1!=str.rend(); it1++)

    cout << *it1;

    cout << endl;

    return 0;

}

Output

The string using forward iterators is : geeksforgeeksThe reverse string using reverse iterators is : skeegrofskeeg

 
Manipulating Functions
12. copy(“char array”, len, pos) :- This function copies the substring in target character array mentioned in its arguments. It takes 3 arguments, target char array, length to be copied and starting position in string to start copying.
13. swap() :- This function swaps one string with other.
 

• CPP

// C++ code to demonstrate the working of

// copy() and swap()

#include<iostream>

#include<string> // for string class

using namespace std;

int main()

{

    // Initializing 1st string

    string str1 = "geeksforgeeks is for geeks";

    // Declaring 2nd string

    string str2 = "geeksforgeeks rocks";

    // Declaring character array

    char ch[80];

    // using copy() to copy elements into char array

    // copies "geeksforgeeks"

    str1.copy(ch,13,0);

    // Displaying char array

    cout << "The new copied character array is : ";

    cout << ch << endl << endl;

    // Displaying strings before swapping

    cout << "The 1st string before swapping is : ";

    cout << str1 << endl;

    cout << "The 2nd string before swapping is : ";

    cout << str2 << endl;

    // using swap() to swap string content

    str1.swap(str2);

    // Displaying strings after swapping

    cout << "The 1st string after swapping is : ";

    cout << str1 << endl;

    cout << "The 2nd string after swapping is : ";

    cout << str2 << endl;

    return 0;

}

Output: 
 

The new copied character array is : geeksforgeeksThe 1st string before swapping is : geeksforgeeks is for geeksThe 2nd string before swapping is : geeksforgeeks rocksThe 1st string after swapping is : geeksforgeeks rocksThe 2nd string after swapping is : geeksforgeeks is for geeks

• CPP

// C++ code to demonstrate the working of

// copy() and swap()

#include<iostream>

#include<string> // for string class

using namespace std;

int main()

{

    // Initializing 1st string

    string str1 = "geeksforgeeks is for geeks";

    // Declaring 2nd string

    string str2 = "geeksforgeeks rocks";

    // Declaring character array

    char ch[80];

    // using copy() to copy elements into char array

    // copies "geeksforgeeks"

    str1.copy(ch,13,0);

    // Displaying char array

    cout << "The new copied character array is : ";

    cout << ch << endl << endl;

    // Displaying strings before swapping

    cout << "The 1st string before swapping is : ";

    cout << str1 << endl;

    cout << "The 2nd string before swapping is : ";

    cout << str2 << endl;

    // using swap() to swap string content

    str1.swap(str2);

    // Displaying strings after swapping

    cout << "The 1st string after swapping is : ";

    cout << str1 << endl;

    cout << "The 2nd string after swapping is : ";

    cout << str2 << endl;

    return 0;

}

The 2nd string after swapping is : geeksforgeeks is for geeks