You never create an actual streambuf object, but only objects of classes derived from class streambuf. Examples are filebuf and strstreambuf, which are described in man pages filebuf(3CC4) and ssbuf(3), respectively. Advanced users may want to derive their own classes from streambuf to provide an interface to a special device or to provide other than basic buffering. Man pages sbufpub(3CC4) and sbufprot(3CC4) discuss how to do this.
Apart from creating your own special kind of streambuf, you may want to access the streambuf associated with an iostream to access the public member functions, as described in the man pages referenced above. In addition, each iostream has a defined inserter and extractor which takes a streambuf pointer. When a streambuf is inserted or extracted, the entire stream is copied.
ifstream fromFile("thisFile"); ofstream toFile ("thatFile"); toFile << fromFile.rdbuf();
We open the input and output files as before. Every iostream class has a member function rdbuf that returns a pointer to the streambuf object associated with it. In the case of an fstream, the streambuf object is type filebuf. The entire file associated with fromFile is copied (inserted into) the file associated with toFile. The last line could also be written like this:
fromFile >> toFile.rdbuf();
The source file is then extracted into the destination. The two methods are entirely equivalent.