C++ string class and its applications

Last Updated : 14 Feb, 2023

In C++ we can store string by one of the two ways –

C style strings
string class (discussed in this post)

In this post, the second method is discussed. string class is part of C++ library that supports a lot much functionality over C style strings. C++ string class internally uses char array to store character but all memory management, allocation, and null termination is handled by string class itself that is why it is easy to use. The length of the C++ string can be changed at runtime because of dynamic allocation of memory similar to vectors. As string class is a container class, we can iterate over all its characters using an iterator similar to other containers like vector, set and maps, but generally, we use a simple for loop for iterating over the characters and index them using the [] operator. C++ string class has a lot of functions to handle string easily. Most useful of them are demonstrated in below code.

CPP

// C++ program to demonstrate various function string class
#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    // various constructor of string class
 
    // initialization by raw string
    string str1("first string");
 
    // initialization by another string
    string str2(str1);
 
    // initialization by character with number of occurrence
    string str3(5, '#');
 
    // initialization by part of another string
    string str4(str1, 6, 6); //    from 6th index (second parameter)
                             // 6 characters (third parameter)
 
    // initialization by part of another string : iterator version
    string str5(str2.begin(), str2.begin() + 5);
 
    cout << str1 << endl;
    cout << str2 << endl;
    cout << str3 << endl;
    cout << str4 << endl;
    cout << str5 << endl;
 
    //  assignment operator
    string str6 = str4;
 
    // clear function deletes all character from string
    str4.clear();
 
    //  both size() and length() return length of string and
    //  they work as synonyms
    int len = str6.length(); // Same as "len = str6.size();"
 
    cout << "Length of string is : " << len << endl;
 
    // a particular character can be accessed using at /
    // [] operator
    char ch = str6.at(2); //  Same as "ch = str6[2];"
 
 
    cout << "third character of string is : " << ch << endl;
 
    //  front return first character and back returns last character
    //  of string
 
    char ch_f = str6.front();  // Same as "ch_f = str6[0];"
    char ch_b = str6.back();   // Same as below
                               // "ch_b = str6[str6.length() - 1];"
 
    cout << "First char is : " << ch_f << ", Last char is : "
         << ch_b << endl;
 
    // c_str returns null terminated char array version of string
    const char* charstr = str6.c_str();
    printf("%s\n", charstr);
 
    // append add the argument string at the end
    str6.append(" extension");
    //  same as str6 += " extension"
 
    // another version of append, which appends part of other
    // string
    str4.append(str6, 0, 6);  // at 0th position 6 character
 
    cout << str6 << endl;
    cout << str4 << endl;
 
    //  find returns index where pattern is found.
    //  If pattern is not there it returns predefined
    //  constant npos whose value is -1
 
    if (str6.find(str4) != string::npos)
        cout << "str4 found in str6 at " << str6.find(str4)
             << " pos" << endl;
    else
        cout << "str4 not found in str6" << endl;
 
    //  substr(a, b) function returns a substring of b length
    //  starting from index a
    cout << str6.substr(7, 3) << endl;
 
    //  if second argument is not passed, string till end is
    // taken as substring
    cout << str6.substr(7) << endl;
 
    //  erase(a, b) deletes b characters at index a
    str6.erase(7, 4);
    cout << str6 << endl;
 
    //  iterator version of erase
    str6.erase(str6.begin() + 5, str6.end() - 3);
    cout << str6 << endl;
 
    str6 = "This is a examples";
 
    //  replace(a, b, str)  replaces b characters from a index by str
    str6.replace(2, 7, "ese are test");
 
    cout << str6 << endl;
 
    return 0;
}

Output :

first string
first string
#####
string
first
Length of string is : 6
third character of string is : r
First char is : s, Last char is : g
string
string extension
string
str4 found in str6 at 0 pos
ext
extension
string nsion
strinion
These are examples

As seen in the above code, we can get the length of the string by size() as well as length() but length() is preferred for strings. We can concat a string to another string by += or by append(), but += is slightly slower than append() because each time + is called a new string (creation of new buffer) is made which is returned that is a bit overhead in case of many append operation.
Applications : On basis of above string function some application are written below :

CPP

// C++ program to demonstrate uses of some string function
#include <bits/stdc++.h>
using namespace std;
 
// this function returns floating point part of a number-string
string returnFloatingPart(string str)
{
    int pos = str.find(".");
    if (pos == string::npos)
        return "";
    else
        return str.substr(pos + 1);
}
 
// This function checks whether a string contains all digit or not
bool containsOnlyDigit(string str)
{
    int l = str.length();
    for (int i = 0; i < l; i++)
    {
        if (str.at(i) < '0' || str.at(i) > '9')
            return false;
    }
    //  if we reach here all character are digits
    return true;
}
 
// this function replaces all single space by %20
// Used in URLS
string replaceBlankWith20(string str)
{
    string replaceby = "%20";
    int n = 0;
 
    // loop till all space are replaced
    while ((n = str.find(" ", n)) != string::npos )
    {
        str.replace(n, 1, replaceby);
        n += replaceby.length();
    }
    return str;
}
 
// driver function to check above methods
int main()
{
    string fnum = "23.342";
    cout << "Floating part is : " << returnFloatingPart(fnum) 
         << endl;
 
    string num = "3452";
    if (containsOnlyDigit(num))
        cout << "string contains only digit" << endl;
 
    string urlex = "google com in";
    cout << replaceBlankWith20(urlex) << endl;
 
    return 0;      
}

Output :

Floating part is : 342
string contains only digit
google%20com%20in

Time Complexity : O(N), here N is length of string .

Space Complexity : O(1) , since we not used any extra space.

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