What does string::npos mean? [duplicate]

Question

what does the following statement mean?

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

Answer 1

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

Answer 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.