Keywords: ISO C++ , library

Chapter 27. File Based Streams
Prev | Part XI. Input and Output | Next

## Chapter 27. 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 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 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.

Prev | Up | Next
Chapter 26. Memory Based Streams | Home | Binary Input and Output