Some clarification needed about synchronous versus asynchronous asio operations

Question

As far as I know, the main difference between synchronous and asynchronous operations. I.e. write() or read() vs async_write() and

Answer 1

synchronous is easy to control the program flow.

asynchronous has better performance since it need not save/restore registers for fiber tasks.

asynchronous uses callback and hard to programmer. We can try promise-cpp to make the asynchronous flow like synchronous --

Example of http client --

//<1> Resolve the host
async_resolve(session->resolver_, host, port)

.then([=](tcp::resolver::results_type &results) {
    //<2> Connect to the host
    return async_connect(session->socket_, results);

}).then([=]() {
    //<3> Write the request
    return async_write(session->socket_, session->req_);

}).then([=](std::size_t bytes_transferred) {
    boost::ignore_unused(bytes_transferred);
    //<4> Read the response
    return async_read(session->socket_, session->buffer_, session->res_);

}).then([=](std::size_t bytes_transferred) {
    boost::ignore_unused(bytes_transferred);
    //<5> Write the message to standard out
    std::cout << session->res_ << std::endl;

}).then([]() {
    //<6> success, return default error_code
    return boost::system::error_code();
}, [](const boost::system::error_code err) {
    //<6> failed, return the error_code
    return err;

}).then([=](boost::system::error_code &err) {
    //<7> Gracefully close the socket
    std::cout << "shutdown..." << std::endl;
    session->socket_.shutdown(tcp::socket::shutdown_both, err);
});

Full code here