使用异步操作时为什么没有堆栈溢出？(Why there is no stack overflow while using asynchronous operations?)

 我正在使用Boost.Asio来学习异步操作。 在阅读了很多关于这个概念的文章之后，我仍然感到困惑的是为什么这段代码是Asio Docs的C ++ 11例子的一部分并没有使堆栈溢出？ 我无法想象代码流在这个地方。 它看起来很递归，因为do_accept(); 一次又一次地召唤自己...我可以想象这个堆栈适用于20个客户端，但是对于2 000个客户端？ 我认为对于异步操作，将acceptor_.async_accept()置于循环中并且不在其中进行递归调用更为明确。 它会一样吗？ 
 
 我添加了类成员call_nr用于测试目的。 
 
 代码： 
 
class server
{
public:
    server(boost::asio::io_service& io_service, short port)
        : acceptor_(io_service, tcp::endpoint(tcp::v4(), port)),
        socket_(io_service), call_nr(0)
    {
        do_accept();
    }

private:
    void do_accept()
    {
        acceptor_.async_accept(socket_,
            [this](boost::system::error_code ec) // lambda equation
        {
            using namespace std;
            cout << "Call nr " << (++call_nr) << endl;
            if (!ec)
            {
                std::make_shared<session>(std::move(socket_))->start();
            }

            do_accept();
        });
    }

    int call_nr;
    tcp::acceptor acceptor_;
    tcp::socket socket_;
};

I'm using Boost.Asio for learning asynchronic operations. After reading many articles about this concept I'm still confused why this piece of code which is part of C++11 Examples from Asio Docs isn't making stack overflow? I just can't imagine the flow of the code in this place. It looks recursive so much, because do_accept(); calls itself again and again... I can imagine that the stack works for 20 clients, but for 2 000 clients? I thought that for Asynchronous operations it is more clear to place acceptor_.async_accept() within a loop and without recursive call inside it. Would it work the same?
 
Class member call_nr is added by me for testing purposes.
 
The code:
 
class server
{
public:
    server(boost::asio::io_service& io_service, short port)
        : acceptor_(io_service, tcp::endpoint(tcp::v4(), port)),
        socket_(io_service), call_nr(0)
    {
        do_accept();
    }

private:
    void do_accept()
    {
        acceptor_.async_accept(socket_,
            [this](boost::system::error_code ec) // lambda equation
        {
            using namespace std;
            cout << "Call nr " << (++call_nr) << endl;
            if (!ec)
            {
                std::make_shared<session>(std::move(socket_))->start();
            }

            do_accept();
        });
    }

    int call_nr;
    tcp::acceptor acceptor_;
    tcp::socket socket_;
};

原文：https://stackoverflow.com/questions/35727482