What does string::npos mean?

Question

what does the following statement mean?

    string s="Joe Alan Smith"";
    cout << (s.find("Allen") == string::npos) << endl;

Nawaz · Answer 1 · 2011-02-24T01:26:24.957

15

Actually string::find() returns the position of the found string, but if it doesn't find the given string, it returns string::npos, where npos means no position.

npos is an unsigned integral value, Standard defines it to be -1 (signed representation) which denotes no position.

//npos is unsigned, that is why cast is needed to make it signed!
cout << (signed int) string::npos <<endl;

Output:

-1

See at Ideone : http://www.ideone.com/VRHUj

edited Feb 24 '11 at 01:26

answered Feb 23 '11 at 20:01

Nawaz

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The value isn't implementation-defined; the standard specifies that it is `string::size_type(-1)`. – Mike Seymour Feb 23 '11 at 20:40
@Mike: Thanks. Mentioned this in my answer! – Nawaz Feb 24 '11 at 01:27

score 2 · Answer 2 · answered Feb 23 '11 at 19:49

2

http://www.cplusplus.com/reference/string/string/npos/

As a return value it is usually used to indicate failure.

In other words, print out if the string 'Allen' was not found in the given string s.

answered Feb 23 '11 at 19:49

Amber

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score 2 · Answer 3 · answered Feb 23 '11 at 19:51

The .find() method returns string::npos if it did not find the target string within the searched string. It is an integer whose value cannot represent a "found" index value, usually -1. A valid index value is an integer >= 0 which represents the index at which the target string was found.

What does string::npos mean?

3 Answers3