Why doesn't std::noskipws work, or what is it supposed to do?

Question

First off my understanding is that

cin >> std::noskipws >> str;

should stick a whole line from cin like "i have spaces" into str. However this only puts "i" into str. This could be a false assumption in which case what does std::noskipws do?

I know there is a function std::getline and that does work but simply for educational purposes I decided I would try to get std::noskipws to work for me. I have tried in the past and it just never works so I normally move on and use std::getline.

What I think I have found so far is that std::noskipws technically just unsets std::skipws which internally to the basic_iostream just calls

ios_base::unsetf(std::ios::skipws);

or

ios_base::unsetf(ios_base::skipws);

So I tried inheriting my own stream form basic_iostream and setting those flags (unsetting) them manually. Still no dice.

So, am I just totally off base or is there a way to make this work?

Maybe you want to [change the delimiter](http://stackoverflow.com/a/7304184/3484570), but I doubt it. — nwp, Aug 11 '15 at 14:35

score 13 · Accepted Answer · answered Apr 13 '11 at 19:48

13

std::noskipws tells the istream to not skip any leading white space when attempting to read a type. When there is no leading white space, then the flag has no impact.

answered Apr 13 '11 at 19:48

Howard Hinnant

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ahh, fair enough, i take it there is no flag to tell it that whitespace is not a delimiter? – Digital Powers Apr 13 '11 at 20:28
@Digital: You can redefine whitespace however you like using the `std::ctype` formatting facet. If you are willing to wade into facets. – Potatoswatter Apr 13 '11 at 20:49
Or you can create a line class that has its own operator >> that reads a line. – Martin York Apr 13 '11 at 21:15

score 3 · Answer 2 · answered Apr 14 '11 at 09:44

std::skipws works as follows: std::istream always keeps a current read position. If std::skipws is set, before operator>> is called the current read position is advanced to the first non-space character.

The behavior you're seeing (stop at the first space after 'i') is caused by operator>> for std::string (and std::wstring). That operator doesn't take the std::istream flags into account. An operator<< for another type may decide otherwise and continue even across spaces.

Why doesn't std::noskipws work, or what is it supposed to do?

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