Unexpected results with std::ofstream binary write

Question

I'm new to C++ std::stream and I'm making some tests. I have this simple code:

int i = 10;
char c = 'c';
float f = 30.40f;

std::ofstream out("test.txt", std::ios::binary | std::ios::out);
if(out.is_open())
{
    out<<i<<c<<f;
    out.close();
}

As the stream is opened as std::ios::binary I expect in the test.txt file to have the binary representation of i, c and f, but instead I have 10c30.4.

Can you please tell me what I'm doing wrong?

From cplusplus.com/reference/fstream/ofstream/ofstream binary (binary) Consider stream as binary rather than text. — Mircea Ispas
– Mircea Ispas, Commented Feb 8, 2013 at 7:45
Okay, on to the POSIX definitions of text file streams vs. binary file streams then :) (Hint: why would writing text to a binary file result in something else than text?) — sehe
– sehe, Commented Feb 8, 2013 at 7:46
@Felics: the standard does not specify what "binary" actually means. In fact on some platforms it has no effect (e.g. on POSIX). — Yakov Galka
– Yakov Galka, Commented Feb 8, 2013 at 7:47
To be perfectly clear: +1 for this question. I think it is interesting to find that documentation on this is very sketchy. It is apparently "assumed" to be obvious. More people might be confused, which makes this a perfect Stack Overflow candidate. — sehe
– sehe, Commented Feb 8, 2013 at 7:48
@Felics: your last sentence makes no sense. ISO says that opening an fstream with binary|out is equivalent to fopen with "wb" mode. Now, when you say "I have the expected result" it means that you called fwrite, which corresponds to ostream::write. But your code here uses operator <<, which corresponds to, e.g. fprintf(out, "%d", i) for integers. — Yakov Galka
– Yakov Galka, Commented Feb 8, 2013 at 8:00

sehe · Accepted Answer · 2015-08-26 08:12:44Z

18

std::ios::binary promises to not do any line-end conversions on the stream (and some other small behavioral differences with text streams).

You could look at

Boost Serialization http://www.boost.org/doc/libs/1_53_0/libs/serialization/doc/index.html
Boost Spirit binary generators http://www.boost.org/doc/libs/1_53_0/libs/spirit/doc/html/spirit/karma/reference/binary/
Using ofstream::write(...) to manually write the bytes

Here's an example using Boost Spirit Karma (assuming Big-Endian byte ordering):

#include <boost/spirit/include/karma.hpp>
namespace karma = boost::spirit::karma;

int main()
{
    int i = 10;
    char c = 'c';
    float f = 30.40f;

    std::ostringstream oss(std::ios::binary);
    oss << karma::format(
            karma::big_dword << karma::big_word << karma::big_bin_float, 
            i, c, f);

    for (auto ch : oss.str())
        std::cout << std::hex << "0x" << (int) (unsigned char) ch << " ";
    std::cout << "\n";
}

This prints

0x0 0x0 0x0 0xa 0x0 0x63 0x41 0xf3 0x33 0x33

edited Aug 26, 2015 at 8:12

answered Feb 8, 2013 at 7:44

sehe

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2 Comments

sehe Over a year ago

I've added a demo based on Karma, see also here: http://liveworkspace.org/code/4B5okj

M.M Over a year ago

That's not quite "end of story", there are various possible transformations that can be performed on text files, including that the data read back may differ from the data written (e.g. trim trailing space), and some file operations such as fseek behave differently for each type.

mistapink · Accepted Answer · 2013-02-08 10:16:10Z

18

In order to write raw binary data you have to use ostream::write. It does not work with the output operators.

Also make sure if you want to read from a binary file not to use operator>> but instead istream::read.

The links also provide examples how you can handle binary data.

So for your example:

int i = 10;
char c = 'c';
float f = 30.40f;

std::ofstream out("test.txt", std::ios::binary | std::ios::out);
if(out.is_open())
{
    out.write(reinterpret_cast<const char*>(&i), sizeof(i));
    out.write(&c, sizeof(c));
    out.write(reinterpret_cast<const char*>(&f), sizeof(f));
    out.close();
}

edited Feb 8, 2013 at 10:16

answered Feb 8, 2013 at 7:55

mistapink

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6 Comments

sehe Over a year ago

beware of UB and unportability when doing this. I'd suggest using well tested libraries to handle this for you, and account for endianness (Boost Spirit, Serialization, protobuf etc.)

mistapink Over a year ago

Yes, you are right. But I just wanted to point to an answer with plain C++ without any boost or other libraries.

sehe Over a year ago

E.g. the sample technically requires reinterpret_cast<const char*> to not invoke Undefined Behaviour, I think. (PS. I did point to the answer in plain C++ too :))

Konrad Rudolph Over a year ago

@sehe That (= the code not invoking UB) is guaranteed by the standard, actually. The result of doing this, now that is unspecified.

molbdnilo Over a year ago

I think you're missing a & or three. And a couple of sizeofs.

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