Boost.Asio async_read a string from a socket

I'm trying to write a function async_read_string_n to asynchronously read a string of exactly n bytes from a socket with Boost.Asio 1.78 (and GCC 11.2).

This is how I want to use the function async_read_string_n:

void run() {
  co_spawn (io_context_, [&]() -> awaitable<void> {
    auto executor = io_context_.get_executor();
    tcp::acceptor acceptor(executor, listen_endpoint_);

    auto [ec, socket] = co_await acceptor.async_accept(as_tuple(use_awaitable));
    co_spawn(executor, [&]() -> awaitable<void> {

      auto [ec, header] = co_await async_read_string_n(socket, 6, as_tuple(use_awaitable));
      std::cerr << "received string " << header << "\n";
      co_return;
    }
    , detached);
    co_return;
  }
  , detached);
}

Here is my attempt to write async_read_string_n, following the advice in

(I don't care about memory copying. This isn't supposed to be fast; it's supposed to have a nice API.)

template<class CompletionToken> auto async_read_string_n(tcp::socket& socket, int n, CompletionToken&& token) {
  async_completion<CompletionToken, void(boost::system::error_code, std::string)> init(token);
  asio::streambuf b;
  asio::streambuf::mutable_buffers_type bufs = b.prepare(n);
  auto [ec, bytes_transferred] = co_await asio::async_read(socket, bufs, asio::transfer_exactly(n), as_tuple(use_awaitable));
  b.commit(n);
  std::istream is(&b);
  std::string s;
  is >> s;
  b.consume(n);
  init.completion_handler(ec, s);
  return init.result.get();
}

Edit

(I had a syntax error and I fixed it.) Here is the compiler error in async_read_string_n which I'm stuck on:

GCC error:

error: 'co_await' cannot be used in a function with a deduced return type

How can I write the function async_read_string_n?

Solution

You don't have to use streambuf. Regardless, using the >> extraction will not reliably extract the string (whitespace stops the input).

The bigger problem is that you have to choose whether you want to use

co_await (which requires another kind of signature as your second link correctly shows)
or the async result protocol, which implies that the caller will decide what mechanism to use (a callback, future, group, awaitable etc).

So either make it:

Using Async Result Protocol:

#include <boost/asio.hpp>
#include <boost/asio/awaitable.hpp>
#include <boost/asio/experimental/as_tuple.hpp>
#include <boost/asio/use_awaitable.hpp>
#include <iostream>
#include <iomanip>
namespace net = boost::asio;
using net::ip::tcp;
using boost::system::error_code;

template <typename CompletionToken>
auto async_read_string_n(tcp::socket& socket, int n, CompletionToken&& token)
{
    struct Op {
        net::async_completion<CompletionToken, void(error_code, std::string)>
            init;
        std::string buf;
        Op(CompletionToken token) : init(token) {}
    };
    auto op = std::make_shared<Op>(token);

    net::async_read(socket, net::dynamic_buffer(op->buf),
                    net::transfer_exactly(n), [op](error_code ec, size_t n) {
                        op->init.completion_handler(ec, std::move(op->buf));
                    });
    return op->init.result.get();
}

int main() {
    net::io_context ioc;
    tcp::socket s(ioc);
    s.connect({{}, 8989});

    async_read_string_n(s, 10, [](error_code ec, std::string s) {
        std::cout << "Read " << ec.message() << ": " << std::quoted(s)
                  << std::endl;
    });

    ioc.run();
}

Prints

NOTE This version affords you the calling semantics that you desire in your sample run() function.

OR Use co_await

Analogous to the sample here:

boost::asio::awaitable<void> echo(tcp::socket socket)
{
  char data[1024];
  for (;;)
  {
    auto [ec, n] = co_await socket.async_read_some(boost::asio::buffer(data),
        boost::asio::experimental::as_tuple(boost::asio::use_awaitable));
    if (!ec)
    {
      // success
    }

    // ...
  }
}