Why are C++ STL iostreams not "exception friendly"?

1. C++ wasn't built with exceptions from day one. "C with classes" started in 1979, and exceptions were added in 1989. Meanwhile, the streams library was written as early as 1984 (later becomes iostreams in 1989 (later reimplemented by GNU in 1991)), it just cannot use exception handling in the beginning.

   Ref:

   • Bjarne Stroustrup, A History of C++: 1979−1991
   • C++ Libraries

2. You can enable exceptions with the .exceptions method.

// ios::exceptions
#include <iostream>
#include <fstream>
#include <string>

int main () {
    std::ifstream file;
    file.exceptions(ifstream::failbit | ifstream::badbit);
    try {
        file.open ("test.txt");
        std::string buf;
        while (std::getline(file, buf))
            std::cout << "Read> " << buf << "\n";
    }
    catch (ifstream::failure& e) {
        std::cout << "Exception opening/reading file\n";
    }
}

OK, it's "Answer my own question" time...

First, thanks to KennyTM for the history. As he says, C++ was NOT designed with exceptions from day one, so it's unsurprising that iostreams 'exception' handling was bolted on afterwards.

Second, as Neil B points out, having exceptions on input format conversion errors would be a significant pain. This surprised me, because I was considering iostreams as a simple filesystem wrapper layer, and I hadn't considered that case at all.

Third, it appears BOOST does bring something to the party: Boost.IOStreams. If I understand correctly, these handle the low-level I/O and buffering aspect of streams, leaving the regular c++ IOStreams library to handle conversion issues. Boost.IOStreams does use exceptions in the way I'd expect. If I understand it correctly, Kenny's example could also look like this:

#include <ostream>
#include <boost/iostreams/device/file.hpp>
#include <boost/iostreams/stream.hpp>

int main () {
  boost::iostreams::stream_buffer <boost::iostreams::file_source> buf("test.txt");
  std::istream file(&buf);

  try {
    std::string buf;
    while (std::getline(file, buf))
      std::cout << "Read> " << buf << "\n";
  }
  catch (std::ios_base::failure::failure e) {
    std::cout << "Exception opening/reading file\n";
  }
  std::cout.flush();

  file.close();

  return 0;
}

I think with this version, things like "file not found" should throw, but 'istream' errors will be reported by badbit/failbit.