Question

The task is to read numbers from file to vector. Format is: one number per line. I want to make with STL-style. So, I wrote such code:

::std::deque<size_t> Input(const ::std::string& filename) {
    ::std::deque<size_t> result;
    try {
        ::std::ifstream file(filename.c_str(), ::std::ios::binary);
        file.exceptions(::std::ifstream::failbit |
                       ::std::ifstream::badbit);

        ::std::copy(::std::istreambuf_iterator<size_t>(file),
                    ::std::istreambuf_iterator<size_t>(),
                    ::std::back_inserter(result));

    } catch ( ::std::exception& e ) {
        ::std::cout << e.what() << ::std::endl;
    }
    return result;
}

It works fine, and I get what I want (all numbers from the file), but also I get an exception with failbit at end of file. What’s the problem? I don’t want to use getline() and parse by hand — wanna ask STL do it for me.
UPDATE: I checked — there is no new line at the end of file.

Answer 1

^{TL;DR: If the stream run out of content while trying to read a new value, failbit is set.}

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It’s defined in the standard that using a istream_iterator has the same effect as doing:

*in_stream >> value; /* istream_iterator::operator++ */
return        value; /* istream_iterator::operator*  */

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Which in the case with handling integers (such as size_t) will results in calls to the below (can be read about under 27.6.1.2.2/2 – lib.istream.formatted.arithmetic).

use_facet<numget> (loc).get (*this, 0, *this, err, lval)

...

setstate (err);

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err is passed by reference and will be set to whatever errors the function might run into. Below is a snippet from the standards definition of use_facet<numget> (loc).get:

22.2.2.1.2/8 - [lib.facet.num.get.virtuals]

The in iterator is always left pointing one position beyond the last
character successfully matched. If val is set, then err is set to
str.goodbit; or to str.eofbit if, when seeking another character to
match, it is found that (in == end).

If val is not set, then err is
set to str.failbit; or to (str.failbit|str.eofbit) if the reason for
the failure was that (in == end).

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^{TL;DR: If the stream run out of content while trying to read a new value, failbit is set.}