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string::npos in C++ with Examples

Last Updated : 23 Jul, 2025

What is string::npos?

• It is a constant static member value with the highest possible value for an element of type size_t.
• It actually means until the end of the string.
• It is used as the value for a length parameter in the string’s member functions.
• As a return value, it is usually used to indicate no matches.

Syntax:

static const size_t npos = -1;

Where npos is a constant static value with the highest possible value for an element of type size_t and it is defined with -1.

Note: std::string::npos is a constant that holds the largest possible value of size_t type ( 18446744073709551615 on 64-bit systems ), which is an unsigned integer type. Hence, -1 corresponds to the actual value of std::string::npos.

Program 1: Below is the C++ program to illustrate the use of string::npos.

// C++ program to demonstrate the use
// of string::npos
#include <bits/stdc++.h>
using namespace std;

// Function that using string::npos
// to find the index of the occurrence
// of any string in the given string
void fun(string s1, string s2)
{
    // Find position of string s2
    int found = s1.find(s2);

    // Check if position is -1 or not
    if (found != string::npos) {

        cout << "first " << s2 << " found at: " << (found)
             << endl;
    }

    else
        cout << s2 << " is not in"
             << "the string" << endl;
}

// Driver Code
int main()
{
    // Given strings
    string s1 = "geeksforgeeks";
    string s2 = "for";
    string s3 = "no";

    // Function Call
    fun(s1, s2);

    return 0;
}

first for found at: 5

Explanation: In the above program string::npos constant is defined with a value of -1, because size_t is an unsigned integral type, and -1 is the largest possible representable value for this type.

What if the valid position for a substring is not found in a string?

Various member functions of the String class return the default value of std::string::npos if a valid position or index for a substring is not found in the string.

Below are the String Functions that return the value of std::string::npos in the case of failure:

1. find()
2. rfind()
3. find_first_of()
4. find_last_of()
5. substr()
6. erase()

Program 2: C++ Program to Illustrate that Some String Functions return the value of std::string::npos in case of failure.

#include <iostream>
using namespace std;

int main()
{
    std::string str = "Hello, world!";
    // Returns std::string::npos because "abc" is not found.
    size_t position = str.find("abc");

    if (position == std::string::npos)
        cout << "Substring not found";

    else
        cout << position;

    return 0;
}

Substring not found

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