christopher diggins wrote:

I may finally have a good motivating example for stdio_filter:

Consider the roman_numerals.cpp example from the Boost spirit library at http://tinyurl.com/3qw4k

If I make a couple of minor modifications exclusively to the main, such as remove the superflous user prompting and rename it to something legal, i.e.:

int RomanNumeralParse() { string str; while (getline(cin, str)) { if (str[0] == 'q' || str[0] == 'Q') break;

unsigned n = 0; roman roman_p(n); if (parse(str.c_str(), roman_p).full) { cout << n << endl; } else { cerr << "parsing failure" << endl; return 1; } } return 0; }

Using my current version of my filters library (not available online yet) I can now reuse the entire program from another, such as the following:

void RomanNumeralParserTest() { stringstream sIn("MCMLXXIV"), sOut; sIn > SimpleMainFilter(RomanNumeralParse) > sOut; int n; sOut >> n; TEST(n == 1974); }

Nice example! This definitely answers my challenge to find a pre-existing program which can be easily transformed into a stdio_filter. It also answers the question why you might want to deal directly with streams rather than generic sources, since formatted i/o is used.

Many examples from the Spirit examples, can be refactored in this manner, to become reusable with relatively little effort. I suppose one of the calculators would be even more interesting.

This satisfies my "preferably more than one" qualification. ;-)

Is this a good motivating example for introducing a stdio_filter into boost::iostreams?

I think so. I'm still a little worried that people will use this style of filter writing for new filters and then discover that their filters are less reusable [because they have higher memory use and can't be used with streams that have no end]. However, I think it's sufficient to include a strong warning. Time will tell whether the benefits outweight the costs. The examples you mention also serve as a good motivating example for another filter type: line-oriented filters. E.g.: struct roman_numeral_parser : line_filter<char> { void filter(const std::string line, std::ostream& out) { unsigned n = 0; roman roman_p(n); if (parse(line.c_str(), roman_p).full) { out << n << "\n"; } else { throw std::ios::failure("bad roman numeral"); } } };

Christopher Diggins Object Oriented Template Library (OOTL) http://www.ootl.org

Jonathan