Chapter 27: Input/Output

Chapter 27 deals with iostreams and all their subcomponents and extensions. All kinds of fun stuff.


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 part 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 hexidecimal 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 descendents.

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The buffering is screwing up my program!

First, are you sure that you understand buffering? Particularly the fact that C++ may not, in fact, have anything to do with it?

The rules for buffering can be a little odd, but they aren't any different from those of C. (Maybe that's why they can be a bit odd.) Many people think that writing a newline to an output stream automatically flushes the output buffer. This is true only when the output stream is, in fact, a terminal and not a file or some other device -- and that may not even be true since C++ says nothing about files nor terminals. All of that is system-dependant. (The "newline-buffer-flushing only occuring on terminals" thing is mostly true on Unix systems, though.)

Some people also believe that sending endl down an output stream only writes a newline. This is incorrect; after a newline is written, the buffer is also flushed. Perhaps this is the effect you want when writing to a screen -- get the text out as soon as possible, etc -- but the buffering is largely wasted when doing this to a file:

   output << "a line of text" << endl;
   output << some_data_variable << endl;
   output << "another line of text" << endl; 
The proper thing to do in this case to just write the data out and let the libraries and the system worry about the buffering. If you need a newline, just write a newline:
   output << "a line of text\n"
          << some_data_variable << '\n'
          << "another line of text\n"; 
I have also joined the output statements into a single statement. You could make the code prettier by moving the single newline to the start of the quoted text on the thing line, for example.

If you do need to flush the buffer above, you can send an endl if you also need a newline, or just flush the buffer yourself:

   output << ...... << flush;    // can use std::flush manipulator
   output.flush();               // or call a member fn 

On the other hand, there are times when writing to a file should be like writing to standard error; no buffering should be done because the data needs to appear quickly (a prime example is a log file for security-related information). The way to do this is just to turn off the buffering before any I/O operations at all have been done, i.e., as soon as possible after opening:

   std::ofstream    os ("/foo/bar/baz");
   std::ifstream    is ("/qux/quux/quuux");
   int   i;

   os << "this data is written immediately\n";
   is >> i;   // and this will probably cause a disk read 

Since all aspects of buffering are handled by a streambuf-derived member, it is necessary to get at that member with rdbuf(). Then the public version of setbuf can be called. The arguments are the same as those for the Standard C I/O Library function (a buffer area followed by its size).

A great deal of this is implementation-dependant. For example, streambuf does not specify any actions for its own setbuf()-ish functions; the classes derived from streambuf each define behavior that "makes sense" for that class: an argument of (0,0) turns off buffering for filebuf but has undefined behavior for its sibling stringbuf, and specifying anything other than (0,0) has varying effects. Other user-defined class derived from streambuf can do whatever they want.

A last reminder: there are usually more buffers involved than just those at the language/library level. Kernel buffers, disk buffers, and the like will also have an effect. Inspecting and changing those are system-dependant.

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Binary I/O

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 part 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.)

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:

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 article and continuing to the end of the thread. (You'll have to sort through some flames every couple of paragraphs, but the points made are good ones.)

Iostreams class hierarchy diagram

The diagram is in PDF. Rumor has it that once Benjamin Kosnik has been dead for a few decades, this work of his will be hung next to the Mona Lisa in the Musee du Louvre.

What is this <sstream>/stringstreams thing?

Stringstreams (defined in the header <sstream>) are in this author's opinion one of the coolest things since sliced time. An example of their use is in the Received Wisdom section for Chapter 21 (Strings), describing how to format strings.

The quick definition is: they are siblings of ifstream and ofstream, and they do for std::string what their siblings do for files. All that work you put into writing << and >> functions for your classes now pays off again! Need to format a string before passing the string to a function? Send your stuff via << to an ostringstream. You've read a string as input and need to parse it? Initialize an istringstream with that string, and then pull pieces out of it with >>. Have a stringstream and need to get a copy of the string inside? Just call the str() member function.

This only works if you've written your <</>> functions correctly, though, and correctly means that they take istreams and ostreams as parameters, not ifstreams and ofstreams. If they take the latter, then your I/O operators will work fine with file streams, but with nothing else -- including stringstreams.

If you are a user of the strstream classes, you need to update your code. You don't have to explicitly append ends to terminate the C-style character array, you don't have to mess with "freezing" functions, and you don't have to manage the memory yourself. The strstreams have been officially deprecated, which means that 1) future revisions of the C++ Standard won't support them, and 2) if you use them, people will laugh at you.

Comments and suggestions are welcome, and may be sent to Phil Edwards or Gabriel Dos Reis.
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