I'm using boost asio a lot lately and I find that I'm working with std::strings and asio::streambufs quite a bit. I find that I'm trying to get data back and forth between streambufs and strings a lot as part of parsing network data. In general, I don't want to mess around with 'formatted io', so iostreams aren't very useful. I've found that while ostream::operator<<(), in spite of the official documentation, seems to relay my strings into streambufs unmolested, istream::operator>>() mangles the contents of my streambufs (as you would expect given that it's 'formatted').
It really seems to me like the standard library is missing a whole lot of iterators and stream objects for dealing with streambufs and strings and unformatted io. For example, if I want to get a substring of a string into a streambuf, how do I do that without creating a copy of the string? A basic all-in-all-out transfer can be accomplished like:
// Get a whole string into a streambuf, and then get the whole streambuf back
// into another string
{
boost::asio::streambuf sbuf;
iostream os(&sbuf);
string message("abcdefghijk lmnopqrs tuvwxyz");
cout << "message=" << message << endl;
os << message;
std::istreambuf_iterator<char> sbit(&sbuf);
std::istreambuf_iterator<char> end;
std::string sbuf_it_wholestr(sbit, end);
cout << "sbuf_it_wholestr=" << sbuf_it_wholestr << endl;
}
prints:
message=abcdefghijk lmnopqrs tuvwxyz
sbuf_it_wholestr=abcdefghijk lmnopqrs tuvwxyz
If I want to get just part of a streambuf into a string, that seems really hard, because istreambuf_iterator isn't a random access iterator and doesn't support arithmetic:
// Get a whole string into a streambuf, and then get part of the streambuf back
// into another string. We can't do this because istreambuf_iterator isn't a
// random access iterator!
{
boost::asio::streambuf sbuf;
iostream os(&sbuf);
string message("abcdefghijk lmnopqrs tuvwxyz");
cout << "message=" << message << endl;
os << message;
std::istreambuf_iterator<char> sbit(&sbuf);
// This doesn't work
//std::istreambuf_iterator<char> end = sbit + 7; // Not random access!
//std::string sbuf_it_partstr(sbit, end);
//cout << "sbuf_it_partstr=" << sbuf_it_partstr << endl;
}
And there doesn't seem to be any way of directly using string::iterators to dump part of a string into a streambuf:
// istreambuf_iterator doesn't work in std::copy either
{
boost::asio::streambuf sbuf;
iostream os(&sbuf);
string message("abcdefghijk lmnopqrs tuvwxyz");
cout << "message=" << message << endl;
std::istreambuf_iterator<char> sbit(&sbuf);
//std::copy(message.begin(), message.begin()+7, sbit); // Doesn't work here
}
I can always pull partial strings out of a streambuf if I don't mind formatted io, but I do - formatted io is almost never what I want:
// Get a whole string into a streambuf, and then pull it out using an ostream
// using formatted output
{
boost::asio::streambuf sbuf;
iostream os(&sbuf);
string message("abcdefghijk lmnopqrs tuvwxyz");
cout << "message=" << message << endl;
string part1, part2;
os << message;
os >> part1;
os >> part2;
cout << "part1=" << part1 << endl;
cout << "part2=" << part2 << endl;
}
prints:
message=abcdefghijk lmnopqrs tuvwxyz
part1=abcdefghijk
part2=lmnopqrs
If I'm ok with an ugly copy, I can generate a substring, of course - std::string::iterator is random access...
// Get a partial string into a streambuf, and then pull it out using an
// istreambuf_iterator
{
boost::asio::streambuf sbuf;
iostream os(&sbuf);
string message("abcdefghijk lmnopqrs tuvwxyz");
cout << "message=" << message << endl;
string part_message(message.begin(), message.begin()+7);
os << part_message;
cout << "part_message=" << part_message << endl;
std::istreambuf_iterator<char> sbit(&sbuf);
std::istreambuf_iterator<char> end;
std::string sbuf_it_wholestr(sbit, end);
cout << "sbuf_it_wholestr=" << sbuf_it_wholestr << endl;
}
prints:
message=abcdefghijk lmnopqrs tuvwxyz
part_message=abcdefg
sbuf_it_wholestr=abcdefg
The stdlib also has the curiously stand-alone std::getline(), which lets you pull individual lines out of an ostream:
// If getting lines at a time was what I wanted, that can be accomplished too...
{
boost::asio::streambuf sbuf;
iostream os(&sbuf);
string message("abcdefghijk lmnopqrs tuvwxyz\n1234 5678\n");
cout << "message=" << message << endl;
os << message;
string line1, line2;
std::getline(os, line1);
std::getline(os, line2);
cout << "line1=" << line1 << endl;
cout << "line2=" << line2 << endl;
}
prints: message=abcdefghijk lmnopqrs tuvwxyz 1234 5678
line1=abcdefghijk lmnopqrs tuvwxyz
line2=1234 5678
I feel like there's some Rosetta Stone that I've missed and that dealing with std::string and asio::streambuf would be so much easier if I discovered it. Should a just abandon the std::streambuf interface and make use of asio::mutable_buffer, which I can get out of asio::streambuf::prepare()?
istream::operator>>() mangles the contents of my streambufs (as you would expect given that it's 'formatted').
Open your input stream with std::ios::binary flag and manipulate it with is >> std::noskipws
For example, if I want to get a substring of a string into a streambuf, how do I do that without creating a copy of the string? A basic all-in-all-out transfer can be accomplished like
Try like
outstream.write(s.begin()+start, length);
Or use boost::string_ref:
outstream << boost::string_ref(s).instr(start, length);
And there doesn't seem to be any way of directly using string::iterators to dump part of a string into a streambuf:
std::copy(it1, it2, ostreambuf_iterator<char>(os));
Re. parsing the message lines:
You can split into iterator ranges with iter_split.
You can parse an embedded grammar on the fly with boost::spirit::istream_iterator