What does string::npos mean? [duplicate]

This question already has answers here: What does string::npos mean in this code? (12 answers) 开发者_如何学Python Closed 9 years ago.

what does the following statement mean?

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

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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

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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.

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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.