string::npos in C++ with Examples

• 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.

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.