问题
I'm surveying c++ libraries for portable, blocking I/O access to the filesystem and network. It looks like boost::filesystem, boost::iostreams and boost::asio will, between the three of them, do the job.
To be clear, I'm not currently interested in the asynchronous aspects of boost::asio; I just want a portable, blocking interface to the network.
Digging in, I see boost::iostreams has a notion of Devices, each of which has an associated mode concept. The bidirectional mode specifically seems hand-tailored for streaming access to a full-duplex TCP connection. Awesome.
boost::iostreams does not seem to offer support for actually opening TCP connections (unlike the local filesystem.) That's fine, surely boost::asio will let me open the connection, appropriately model it as a bidirectional Device, and wrap it in a boost::iostreams::stream.
..except it won't? I see boost::asio::ip::tcp::iostream, which would replace the boost::iostreams::stream, but presumably not act as a Device.
I understand the tcp::iostream would act similarly, but I would still prefer to learn and code against just one interface, not two. Specifically, dealing with two error handling regimes & exception hierarchies is not very palatable.
So, the question: am I blind? Maybe an adapter between the two libraries exists, that I missed googling around. Or perhaps someone has already released such an adapter as a 3rd-party component I could drop in?
回答1:
I'm not aware of a direct mapping. However, if you were interested, writing such a device is fairly straightforward. This version throws boost::system::system_error for non-EOF errors, but you could choose to do something else.
#include <iosfwd>
#include <boost/asio/io_service.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/buffer.hpp>
#include <boost/iostreams/categories.hpp>
#include <boost/system/system_error.hpp>
class asio_stream_device
{
public:
typedef char char_type;
typedef boost::iostreams::bidirectional_device_tag category;
explicit asio_stream_device(boost::asio::ip::tcp::socket& sock) : socket_(sock)
{
}
std::streamsize read(char* s, std::streamsize n)
{
// Read up to n characters from the underlying data source
// into the buffer s, returning the number of characters
// read; return -1 to indicate EOF
boost::system::error_code ec;
std::size_t rval = socket_.read_some(boost::asio::buffer(s, n), ec);
if (!ec)
{
return rval;
}
else if (ec == boost::asio::error::eof)
{
return -1;
}
else
{
throw boost::system::system_error(ec,"read_some");
}
}
std::streamsize write(const char* s, std::streamsize n)
{
// Write up to n characters to the underlying
// data sink into the buffer s, returning the
// number of characters written
boost::system::error_code ec;
std::size_t rval = socket_.write_some(boost::asio::buffer(s, n), ec);
if (!ec)
{
return rval;
}
else if (ec == boost::asio::error::eof)
{
return -1;
}
else
{
throw boost::system::system_error(ec,"write_some");
}
}
private:
boost::asio::ip::tcp::socket& socket_;
};
Basically, open/connect the socket as normal, then pass it to the constructor. The example simply reads and outputs to the screen.
void test
{
namespace asio = boost::asio;
namespace io = boost::iostreams;
asio::io_service service;
asio::ip::tcp::socket socket(service);
asio::ip::tcp::endpoint remote - ...; ////
socket.connect(remote);
io::stream<asio_stream_device> str(socket);
std::string line;
while (std::getline(str, line)) {
std::cout << line << std::endl;
}
}
来源：https://stackoverflow.com/questions/12023166/is-there-a-boostiostreams-bidirectional-device-for-a-blocking-boostasio-tc