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File Based Streams

Copying a File

So you want to copy a file quickly and easily, and most important, completely portably. And since this is C++, you have an open ifstream (call it IN) and an open ofstream (call it OUT):

   #include <fstream>

   std::ifstream  IN ("input_file");
   std::ofstream  OUT ("output_file"); 

Here's the easiest way to get it completely wrong:

   OUT << IN;

For those of you who don't already know why this doesn't work (probably from having done it before), I invite you to quickly create a simple text file called "input_file" containing the sentence

      The quick brown fox jumped over the lazy dog.

surrounded by blank lines. Code it up and try it. The contents of "output_file" may surprise you.

Seriously, go do it. Get surprised, then come back. It's worth it.

The thing to remember is that the basic_[io]stream classes handle formatting, nothing else. In particular, they break up on whitespace. The actual reading, writing, and storing of data is handled by the basic_streambuf family. Fortunately, the operator<< is overloaded to take an ostream and a pointer-to-streambuf, in order to help with just this kind of "dump the data verbatim" situation.

Why a pointer to streambuf and not just a streambuf? Well, the [io]streams hold pointers (or references, depending on the implementation) to their buffers, not the actual buffers. This allows polymorphic behavior on the chapter of the buffers as well as the streams themselves. The pointer is easily retrieved using the rdbuf() member function. Therefore, the easiest way to copy the file is:

   OUT << IN.rdbuf();

So what was happening with OUT<<IN? Undefined behavior, since that particular << isn't defined by the Standard. I have seen instances where it is implemented, but the character extraction process removes all the whitespace, leaving you with no blank lines and only "Thequickbrownfox...". With libraries that do not define that operator, IN (or one of IN's member pointers) sometimes gets converted to a void*, and the output file then contains a perfect text representation of a hexadecimal address (quite a big surprise). Others don't compile at all.

Also note that none of this is specific to o*f*streams. The operators shown above are all defined in the parent basic_ostream class and are therefore available with all possible descendants.

Binary Input and Output

The first and most important thing to remember about binary I/O is that opening a file with ios::binary is not, repeat not, the only thing you have to do. It is not a silver bullet, and will not allow you to use the <</>> operators of the normal fstreams to do binary I/O.

Sorry. Them's the breaks.

This isn't going to try and be a complete tutorial on reading and writing binary files (because "binary" covers a lot of ground), but we will try and clear up a couple of misconceptions and common errors.

First, ios::binary has exactly one defined effect, no more and no less. Normal text mode has to be concerned with the newline characters, and the runtime system will translate between (for example) '\n' and the appropriate end-of-line sequence (LF on Unix, CRLF on DOS, CR on Macintosh, etc). (There are other things that normal mode does, but that's the most obvious.) Opening a file in binary mode disables this conversion, so reading a CRLF sequence under Windows won't accidentally get mapped to a '\n' character, etc. Binary mode is not supposed to suddenly give you a bitstream, and if it is doing so in your program then you've discovered a bug in your vendor's compiler (or some other chapter of the C++ implementation, possibly the runtime system).

Second, using << to write and >> to read isn't going to work with the standard file stream classes, even if you use skipws during reading. Why not? Because ifstream and ofstream exist for the purpose of formatting, not reading and writing. Their job is to interpret the data into text characters, and that's exactly what you don't want to happen during binary I/O.

Third, using the get() and put()/write() member functions still aren't guaranteed to help you. These are "unformatted" I/O functions, but still character-based. (This may or may not be what you want, see below.)

Notice how all the problems here are due to the inappropriate use of formatting functions and classes to perform something which requires that formatting not be done? There are a seemingly infinite number of solutions, and a few are listed here:

  • Derive your own fstream-type classes and write your own <</>> operators to do binary I/O on whatever data types you're using.

    This is a Bad Thing, because while the compiler would probably be just fine with it, other humans are going to be confused. The overloaded bitshift operators have a well-defined meaning (formatting), and this breaks it.

  • Build the file structure in memory, then mmap() the file and copy the structure.

    Well, this is easy to make work, and easy to break, and is pretty equivalent to using ::read() and ::write() directly, and makes no use of the iostream library at all...

  • Use streambufs, that's what they're there for.

    While not trivial for the beginner, this is the best of all solutions. The streambuf/filebuf layer is the layer that is responsible for actual I/O. If you want to use the C++ library for binary I/O, this is where you start.

How to go about using streambufs is a bit beyond the scope of this document (at least for now), but while streambufs go a long way, they still leave a couple of things up to you, the programmer. As an example, byte ordering is completely between you and the operating system, and you have to handle it yourself.

Deriving a streambuf or filebuf class from the standard ones, one that is specific to your data types (or an abstraction thereof) is probably a good idea, and lots of examples exist in journals and on Usenet. Using the standard filebufs directly (either by declaring your own or by using the pointer returned from an fstream's rdbuf()) is certainly feasible as well.

One area that causes problems is trying to do bit-by-bit operations with filebufs. C++ is no different from C in this respect: I/O must be done at the byte level. If you're trying to read or write a few bits at a time, you're going about it the wrong way. You must read/write an integral number of bytes and then process the bytes. (For example, the streambuf functions take and return variables of type int_type.)

Another area of problems is opening text files in binary mode. Generally, binary mode is intended for binary files, and opening text files in binary mode means that you now have to deal with all of those end-of-line and end-of-file problems that we mentioned before.

An instructive thread from comp.lang.c++.moderated delved off into this topic starting more or less at this post and continuing to the end of the thread. (The subject heading is "binary iostreams" on both comp.std.c++ and comp.lang.c++.moderated.) Take special note of the replies by James Kanze and Dietmar Kühl.

Briefly, the problems of byte ordering and type sizes mean that the unformatted functions like ostream::put() and istream::get() cannot safely be used to communicate between arbitrary programs, or across a network, or from one invocation of a program to another invocation of the same program on a different platform, etc.