[英]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).我正在尝试编写一个 function
async_read_string_n
以从具有 Boost.Asio 1.78(和 GCC 11.2)的套接字中异步读取正好n
个字节的字符串。
This is how I want to use the function async_read_string_n
:这就是我想使用 function
async_read_string_n
的方式:
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这是我按照中的建议编写
async_read_string_n
的尝试
(I don't care about memory copying. This isn't supposed to be fast; it's supposed to have a nice API.) (我不关心 memory 复制。这不应该很快;它应该有一个不错的 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();
}
(I had a syntax error and I fixed it.) Here is the compiler error in async_read_string_n
which I'm stuck on: (我有一个语法错误,我修复了它。)这是我坚持的
async_read_string_n
中的编译器错误:
GCC error: GCC 错误:
error: '
co_await' cannot be used in a function with a deduced return type
How can I write the function async_read_string_n
?如何编写 function
async_read_string_n
?
You don't have to use streambuf
.您不必使用
streambuf
。 Regardless, using the >>
extraction will not reliably extract the string (whitespace stops the input).无论如何,使用
>>
提取不会可靠地提取字符串(空格会停止输入)。
The bigger problem is that you have to choose whether you want to use更大的问题是你必须选择是否要使用
co_await
(which requires another kind of signature as your second link correctly shows) co_await
(需要另一种签名,因为您的第二个链接正确显示) So either make it:所以要么做到:
#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印刷
Analogous to the sample here :类似于此处的示例:
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
}
// ...
}
}
Thank you @sehe for your answer, which gave me the information I needed to write async_read_string_n
which works with co_await
:感谢@sehe 的回答,它为我提供了编写与
async_read_string_n
一起使用的co_await
所需的信息:
asio::awaitable<std::tuple<boost::system::error_code, std::string>> async_read_string_n(tcp::socket& socket, int n) {
std::string buf;
auto [ec, bytes_transferred] = co_await asio::async_read(socket, asio::dynamic_buffer(buf), asio::transfer_exactly(n), as_tuple(use_awaitable));
co_return make_tuple(ec, buf);
}
Use it like this:像这样使用它:
auto [ec, string6] = co_await async_read_string_n(socket, 6);
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