Order of tasks (handlers) putting to taskqueue via post and dispatch

The output is:

Handler A
Handler B
Handler D
Handler E
Handler C

Given,

• post() puts the handler into the taskqueue and returns immediately

• dispatch() can run the task immediately if the main thread already calls run() (which is the case)

then,

• why "Handler E" wasn't run before B and D ? It was dispatch(), and main thread already runs the io_context after all.

• why "Handler C" was run last ? It kinds of make sense as it was post within post. But still the order of the tasks being put to the taskqueue isn't very self-explained.

int main()
{
    boost::asio::io_service io_service;

    io_service.dispatch( [](){ cout << "Handler A\n"; } );

    io_service.post(
        [&io_service]() {
            cout << "Handler B\n";
            io_service.post(
                [](){
                    cout << "Handler C\n";
                }
            );
        }
    );

    io_service.post( []() {
            cout << "Handler D\n";
        }
    );

    io_service.dispatch( [](){ cout << "Handler E\n"; } );

    cout << "Running io_service\n";
    io_service.run();

    return 0;
}