How to use boost to put a stream into a buffer - c++

Question

I am new to Streams and Buffers implementation and I do not really know how to fix this:
#include <boost/beast/core.hpp>
#include <boost/beast/websocket.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <string>
#include <thread>
#include <boost/property_tree/json_parser.hpp>
#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/stream.hpp>
using tcp = boost::asio::ip::tcp;
namespace websocket = boost::beast::websocket;
namespace pt = boost::property_tree;
struct DefaultMessage {
std::string command;
int value;
};
void defaultMessage(DefaultMessage *dm ,pt::ptree *root) {
root->put("command", dm->command);
root->put("value", dm->value);
}
// Echoes back all received WebSocket messages
void do_session(tcp::socket &socket) {
try {
// Construct the stream by moving in the socket
websocket::stream<tcp::socket> ws{std::move(socket)};
// Accept the websocket handshake
ws.accept();
for (;;) {
// Read a message into the buffer
boost::beast::multi_buffer buffer;
ws.read(buffer);
// Make string from buffer
auto s = boost::beast::buffers_to_string(buffer.data());
// Create ptree root
pt::ptree root;
// Create array source from s
boost::iostreams::array_source array_source(&s[0], s.size());
// Create input stream from array source
boost::iostreams::stream<boost::iostreams::array_source> input_stream(array_source);
// Read the json an populate ptree root
pt::read_json(input_stream, root);
// Discard all in buffer
buffer.consume(buffer.size());
// Construct a default message
auto message = DefaultMessage{
root.get<std::string>("command"),
root.get<int>("value")
};
defaultMessage(&message, &root);
// **This won't compile.**
pt::write_json(buffer, root);
// Echo the message back
ws.text(ws.got_text());
ws.write(buffer.data());
}
}
catch (boost::system::system_error const &se) {
// This indicates that the session was closed
if (se.code() != websocket::error::closed) {
std::cerr << "Error: " << se.code().message() << std::endl;
}
}
catch (std::exception const &e) {
std::cerr << "Error: " << e.what() << std::endl;
}
}
I want to respond with the message struct. So I guess I need to put it back in the buffer. This part is what is do not know how to do.

Answer 1

If efficiency is not your top concern, I'd suggest
std::ostringstream oss;
pt::write_json(oss, root);
to write to a string, and
// Echo the message back
ws.text(ws.got_text());
ws.write(boost::asio::buffer(oss.str()));
to write that buffer out.
Caveats:
this does more allocations than strictly required, probably
Boost PropertyTree is NOT a JSON library