ostream_iterator for Binary Output

Question

I want to be able to use an ostream_iterator to stream to a binary file. But the ostream_iterator uses a FormattedOuputFunction so it will write ASCII, not binary:<

Answer 1

It works, but you will have to explicitely use an ostream_iterator.

Example (includes omitted for brievety):

int main(int argc, char **argv) {
    std::vector arr;

    std::ofstream fd("foo.txt", std::ios::binary | std::ios::out);

    for (int i=0; i<256; i++) arr.push_back(i);

    std::ostream_iterator oi(fd);
    std::copy(arr.begin(), arr.end(), oi);
    fd.close();
    return 0;
}

will write the 256 bytes for 0 to 255 in foo.txt.

---

Above assumed that you wanted to write directly the value of the int as a char to the file. From your comment, you want to write the int value as a 4 bytes value (assuming int32_t) in native host endianness. I would use an auxilliary class to do the job:

class bint {
private:
    char c[sizeof(int)];

public:
    bint(const int i) { // allows bint b = 5;
        ::memcpy(c, &i, sizeof(c));
    }
    operator int() const {  // allows : bint b = 5; int i=b => gives i=5
        int i;
        ::memcpy(&i, c, sizeof(int));
        return i;
    }
    const char *getBytes() const { // gives public read-only access to the bytes
        return c;
    }
};

std::ostream& operator <<(std::ostream& out, const bint& b) {
    out.write(b.getBytes(), sizeof(int));
    return out;
}

int main(int argc, char **argv) {
    std::vector arr;

    std::ofstream fd("foo.txt", std::ios::binary | std::ios::out);

    for (int i=0; i<256; i++) arr.push_back(i);

    std::ostream_iterator oi(fd);
    std::copy(arr.begin(), arr.end(), oi);
    fd.close();
    return 0;
}

This one writes 1024 bytes (for integer of size 4) containing the representations of the 256 first integers. It would automatically adapts to other sizes of int.