Why does std::copy (from istream to ostream) raises an ios::failure exception?

Question

The following code should copy data from an wifstream to wcout. After the content is copied, the program throws a ios::failure exception.

#include <string>
#include <iostream>
#include <sstream>
#include <fstream>
#include <locale>
#include <iterator>
#include <algorithm>


int main(void)
{
    std::locale::global(std::locale(""));

    std::wifstream is;
    is.exceptions( std::ios::failbit | std::ios::badbit );
    is.open("test.ts", std::ios::binary);

    is >> std::noskipws;

    std::istream_iterator<wchar_t, wchar_t> in(is);
    std::istream_iterator<wchar_t, wchar_t> end;

    std::copy(in, end,
              std::ostream_iterator<wchar_t, wchar_t>(std::wcout));

    return 0;
} 

The stream should only throw an exception (see exception mask) if anything goes bad, but not on EOF.

Answer 1

To avoid skipping white space use the std::istreambuf_iterator

std::copy(std::istreambuf_iterator<wchar_t, wchar_t>(is),
          std::istreambuf_iterator<wchar_t, wchar_t>(),
          std::ostream_iterator<wchar_t, wchar_t>(std::wcout));

The exception:

The local may be using codecvt facet that is failing.
Try commenting out the locale line see what happens.

Have you tried to print what the exceptions is?

try
{
    // do work
}
catch(std::exception const& e)
{
    std::cout << e.what() << "\n";
}

Answer 2

Because you're using std::istream_iterator, the attempt to read a character past the end of the stream sets both eofbit and failbit (and only after some error bits are set, does the iterator become equal to the end iterator)

Stripping to bare essentials and reverting to char to make it even simpler, program is equivalent to:

#include <iostream>
#include <fstream>
int main()
{
    std::ifstream is("test.txt", std::ios::binary);
    is.exceptions(std::ios::failbit); // failbit only because that's what you get
    is >> std::noskipws;
    if(is)
        for(char c; is >> c;) // will throw!
            std::cout << c;
}