istream_iterator behavior misunderstanding

Question

The goal is to read 16 bit signed integers from a binary file. First, I open the file as an ifstream, then I would like to copy each numbers into a vector using istream_iterator and the copy algorithm. I dont' understand what's wrong with this snippet:

int main(int argc, char *argv[]) {
    std::string filename("test.bin");

    std::ifstream is(filename);
    if (!is) {
        std::cerr << "Error while opening input file\n";
        return EXIT_FAILURE;
    }
    
    std::noskipws(is);
    std::vector<int16_t> v;
    std::copy(
        std::istream_iterator<int16_t>(is),
        std::istream_iterator<int16_t>(),
        std::back_inserter(v)
    );

    //v is still empty
}

This code produces no error but the vector remains empty after the call to std::copy. Since I'm opening the file in the standard input mode ("textual" mode), I was expecting istream_iterator to work even if the file is binary. Of course there's something I'm missing about the behavior of this class.

Answer 1

First off, to read a binary file with ifstream, you need to open the file in binary mode, not text mode (the default). Otherwise, read operations may mis-interpret linebreak bytes and translate them between platform encodings (ie, CRLF->LF, or vice versa), thus corrupting your binary data.

Second, istream_iterator uses operator>>, which reads and parses formatted text by default, which is not what you want when reading a binary file. You need to use istream::read() instead. However, there is no iterator wrapper for that (but you can write your own if needed).

Try this instead:

int main(int argc, char *argv[]) {
    std::string filename = "test.bin";

    std::ifstream is(filename, std::ifstream::binary);
    if (!is) {
        std::cerr << "Error while opening input file\n";
        return EXIT_FAILURE;
    }

    std::vector<int16_t> vec;
    int16_t value;

    while (is.read(reinterpret_cast<char*>(&value), sizeof(value))) {
        // swap value's endian, if needed...
        vec.push_back(value);
    }

    // use vec as needed...

    return 0;
}

That being said, if you really want to use istream_iterator for a binary file, then you would have to write a custom class/struct to wrap int16_t, and then define an operator>> for that type to call read(), eg:

struct myInt16_t {
    int16_t value; 
    operator int16_t() const { return value; }
};

std::istream& operator>>(std::istream &is, myInt16_t &v) {
    if (is.read(reinterpret_cast<char*>(&v.value), sizeof(v.value))) {
        // swap v.value's endian, if needed...
    }
    return is;
}

int main(int argc, char *argv[]) {
    std::string filename = "test.bin";

    std::ifstream is(filename, std::ifstream::binary);
    if (!is) {
        std::cerr << "Error while opening input file\n";
        return EXIT_FAILURE;
    }

    std::noskipws(is);
    std::vector<int16_t> vec;
    std::copy(
        std::istream_iterator<myInt16_t>(is),
        std::istream_iterator<myInt16_t>(),
        std::back_inserter(vec)
    );

    // use vec as needed...

    return 0;
}