使用C++20的协程实现自定义异步函数是一种更好的方式，因为它可以使异步代码更易于编写和理解。下面是一个使用Boost.Asio和C++20协程的示例：

#include 
#include 
#include 
#include 
#include 

namespace asio = boost::asio;
using asio::ip::tcp;

asio::awaitable async_read(asio::ip::tcp::socket& socket) {
    try {
        char data[1024];
        asio::mutable_buffer buffer = asio::buffer(data, sizeof(data));
        size_t len = co_await socket.async_read_some(buffer, asio::use_awaitable);
        co_return std::string(data, len);
    } catch (const std::exception& ex) {
        std::cerr << "read error: " << ex.what() << std::endl;
        throw;
    }
}

asio::awaitable async_write(asio::ip::tcp::socket& socket, const std::string& message) {
    try {
        asio::const_buffer buffer = asio::buffer(message.data(), message.size());
        co_await asio::async_write(socket, buffer, asio::use_awaitable);
    } catch (const std::exception& ex) {
        std::cerr << "write error: " << ex.what() << std::endl;
        throw;
    }
}

asio::awaitable async_echo(tcp::socket socket) {
    try {
        while(true) {
            std::string message = co_await async_read(socket);
            co_await async_write(socket, message);
        }
    } catch (const std::exception& ex) {
        std::cerr << "error: " << ex.what() << std::endl;
    }
}

int main() {
    asio::io_context io_context(1);
    tcp::acceptor acceptor(io_context, tcp::endpoint(tcp::v4(), 1234