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c++extractistream

Contents of the string after failed extraction from istream

If I do this:

ifstream stream("somefilewhichopenssuccesfully.txt");
string token;
if( stream >> token )
    cout << token;
else
    cout << token;

Is the output in the second case guaranteed to be an empty string? I can't seem to find the answer to this on cplusplus.com.

Thanks!

Solution

  • Is the output in the second case guaranteed to be an empty string?

    The answer is : no, because it depends, as described below.

    Since else block will be executed only if an attempt to read from the stream fails, and that can occur anytime in the course of reading.

    • If it fails at the very first attempt, then there is no character extraction from the stream, and hence token will be empty (as it was).

    • If it fails after few reads, then token will not be empty. It will contain the characters successfully read so far from the stream.

    The section §21.3.7.9 from the Standard says,

    Begins by constructing a sentry object k as if k were constructed by typename basic_istream::sentry k(is). If bool(k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:

    — n characters are stored;

    — end-of-file occurs on the input sequence;

    — isspace(c,is.getloc()) is true for the next available input character c.

    After the last character (if any) is extracted, is.width(0) is called and the sentry object k is destroyed.

    If the function extracts no characters, it calls is.setstate(ios::failbit), which may throw ios_base::failure (27.4.4.3).

    Also note that the section §21.3.1/2 from the Standard guarantees that the default constructed string will be empty. The Standard says its size will be zero, that means, empty.