C++ fstream << and >> operators with binary data

Question

I've always read and been told that when dealing with binary files that one should use read() and write() as opposed to the << and >> operators as they are meant for use with formatted data. I've also read that it is possible to use them, but it is an advanced topic, which I can't find where anyone dives into and discusses.

I recently saw some code which did the following:

 std::ifstream file1("x", ios_base::in | ios_base::binary);
 std::ofstream file2("y", ios_base::app | ios_base::binary);

 file1 << file2.rdbuf();

When I pointed out the use of the << operator with the binary file, I was told that the rdbuf() call returns a streambuf * and that << overloads the streambuf* and does a direct copy with no formatting and is thus safe.

Is this true and also safe? How about efficiency? Any gotchas? Details would be much appreciated.

Thanks!

Answer 1

Yes (see 27.6.2.5.3/6 where the overload of << for streambuf is described).

Answer 2

It's entirely safe and a reasonable way to copy streams.

Note that it also allows stuff like:

std::ifstream file_in1("x1", ios_base::in | ios_base::binary);
std::ifstream file_in2("x2", ios_base::in | ios_base::binary);
std::ofstream file_out("y", ios_base::app | ios_base::binary);

file_out << file_in1.rdbuf() << "\nand\n" << file_in2.rdbuf();

Answer 3

In § 27.7.3.6.3 of the C++ standard, it's mentioned that
basic_ostream<charT,traits>& operator<< (basic_streambuf<charT,traits>* sb);
Effects: Behaves as an unformatted output function (as described in 27.7.3.7, paragraph 1).
§ 27.7.3.7 describes "unformatted input" which is basically a binary copy. This means that "unformatted" ostream functions are safe for binary data. The other "unformatted" functions mentioned in the standard that I can find are put, write and (officially) flush.