Recommendations for a C++ polymorphic, seekable, binary I/O interface
- by Trevor Robinson
I've been using std::istream and ostream as a polymorphic interface for random-access binary I/O in C++, but it seems suboptimal in numerous ways:
64-bit seeks are non-portable and error-prone due to streampos/streamoff limitations; currently using boost/iostreams/positioning.hpp as a workaround, but it requires vigilance
Missing operations such as truncating or extending a file (ala POSIX ftruncate)
Inconsistency between concrete implementations; e.g. stringstream has independent get/put positions whereas filestream does not
Inconsistency between platform implementations; e.g. behavior of seeking pass the end of a file or usage of failbit/badbit on errors
Don't need all the formatting facilities of stream or possibly even the buffering of streambuf
streambuf error reporting (i.e. exceptions vs. returning an error indicator) is supposedly implementation-dependent in practice
I like the simplified interface provided by the Boost.Iostreams Device concept, but it's provided as function templates rather than a polymorphic class. (There is a device class, but it's not polymorphic and is just an implementation helper class not necessarily used by the supplied device implementations.) I'm primarily using large disk files, but I really want polymorphism so I can easily substitute alternate implementations (e.g. use stringstream instead of fstream for unit tests) without all the complexity and compile-time coupling of deep template instantiation.
Does anyone have any recommendations of a standard approach to this? It seems like a common situation, so I don't want to invent my own interfaces unnecessarily. As an example, something like java.nio.FileChannel seems ideal.
My best solution so far is to put a thin polymorphic layer on top of Boost.Iostreams devices. For example:
class my_istream
{
public:
virtual std::streampos seek(stream_offset off, std::ios_base::seekdir way) = 0;
virtual std::streamsize read(char* s, std::streamsize n) = 0;
virtual void close() = 0;
};
template <class T>
class boost_istream : public my_istream
{
public:
boost_istream(const T& device) : m_device(device)
{
}
virtual std::streampos seek(stream_offset off, std::ios_base::seekdir way)
{
return boost::iostreams::seek(m_device, off, way);
}
virtual std::streamsize read(char* s, std::streamsize n)
{
return boost::iostreams::read(m_device, s, n);
}
virtual void close()
{
boost::iostreams::close(m_device);
}
private:
T m_device;
};
