Boost.Asio学习之Tutorial

左手的ㄟ右手 2022-07-13 11:15 137阅读 0赞

boost的文档很全，两天时间看完tutorial，熟悉asio编程的基本套路。

所有代码可以参考[http://www.boost.org/doc/libs/1\_61\_0/doc/html/boost\_asio/tutorial.html][http_www.boost.org_doc_libs_1_61_0_doc_html_boost_asio_tutorial.html]  
或者[https://github.com/NearXdu/AsioLearn][https_github.com_NearXdu_AsioLearn]

有几个注意的地方：

## 1.shared\_ptr来管理Tcp\_Connection对象 ##

在tutorial\_daytime3中有如下语句

boost::asio::async_write(socket_,boost::asio::buffer(message_),
                boost::bind(&tcp_connection::handle_write,
                shared_from_this(),//ensure the life tiem of the obj
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));

首先shared\_from\_this()要保证当前的class继承于`enable_shared_from_this`  
shared\_ptr使得内存管理变得简单。  
通过将`shared_ptr`绑定到异步操作中，首先保证对象的生命周期可以延长到异步调用结束；并且当io\_service析构时，所有的shared\_ptr引用将被销毁。

参考：[http://stackoverflow.com/questions/3629557/boost-shared-from-this][http_stackoverflow.com_questions_3629557_boost-shared-from-this]

the boost::asio::io\_service destructor documentation explains it fairly well

> The destruction sequence described above permits programs to simplify their resource management by using shared\_ptr<>. Where an object’s lifetime is tied to the lifetime of a connection (or some other sequence of asynchronous operations), a shared\_ptr to the object would be bound into the handlers for all asynchronous operations associated with it. This works as follows:
> 
> When a single connection ends, all associated asynchronous operations complete. The corresponding handler objects are destroyed, and all shared\_ptr references to the objects are destroyed.  
> To shut down the whole program, the io\_service function stop() is called to terminate any run() calls as soon as possible. The io\_service destructor defined above destroys all handlers, causing all shared\_ptr references to all connection objects to be destroyed.

Typically your objects will chain asynchronous operations where the handlers are bound to member functions using boost::bind and boost::shared\_from\_this(). There are some examples that use this concept.

## 2. 序列化异步操作 ##

详见tutorial\_timer5

我们知道io\_service是 proactor的抽象，用来调用Asynchronous Event Demultiplexer，以及分派与事件相关的处理程序。

首先，在一个线程中至少有一个io\_service对象，并且异步事件的调用是在`io_service::run`所在的线程，除此之外，我们的程序还可以有如下可能：  
1.单线程但io\_service对象  
2.多线程单io\_service对象  
3.多线程多io\_service对象

当出现多线程单io\_service对象时，可以通过strand+wrap使得异步操作序列化，例如：

timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
    timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
    
    //其中异步完成回调
        void print1()
        {
            if (count_ < 10)
            {
    
            //std::cout << "Timer 1: " << count_ << std::endl;
            std::cout<<"Timer 1 "<<"myThreadId is "<<getThreadId()<<std::endl;
            ++count_;
    
            timer1_.expires_at(timer1_.expires_at() + boost::posix_time::seconds(1));
            //timer1_.async_wait(boost::bind(&printer::print1, this));
    
            timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
            }
        }
    
        void print2()
        {
            if (count_ < 10)
            {
            //std::cout << "Timer 2: " << count_ << std::endl;
    
            std::cout<<"Timer 2 "<<"myThreadId is "<<getThreadId()<<std::endl;
            ++count_;
    
            timer2_.expires_at(timer2_.expires_at() + boost::posix_time::seconds(1));
    
            //  timer2_.async_wait(boost::bind(&printer::print2, this));
            timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
            }
        }

运行程序，将看到，分别在不同线程调用的io\_service::run会先完成time1和time2两个异步事件再进入另一个线程：  
![这里写图片描述][SouthEast]

如果不使用会出现如下情况：  
![这里写图片描述][SouthEast 1]

线程就切来切去了，在boost文档中，用chain这个单词，表示异步操作被串起来依次执行。

## 3.参考 ##

1.[http://www.boost.org/doc/libs/1\_61\_0/doc/html/boost\_asio/tutorial.html][http_www.boost.org_doc_libs_1_61_0_doc_html_boost_asio_tutorial.html]  
2.[http://blog.csdn.net/mmoaay/article/details/43759703][http_blog.csdn.net_mmoaay_article_details_43759703]

[http_www.boost.org_doc_libs_1_61_0_doc_html_boost_asio_tutorial.html]: http://www.boost.org/doc/libs/1_61_0/doc/html/boost_asio/tutorial.html
[https_github.com_NearXdu_AsioLearn]: https://github.com/NearXdu/AsioLearn
[http_stackoverflow.com_questions_3629557_boost-shared-from-this]: http://stackoverflow.com/questions/3629557/boost-shared-from-this
[SouthEast]: /images/20220713/31b1433dc7bb4cf4b51713ee378fcffa.png
[SouthEast 1]: /images/20220713/be88b62e0828415886a90c71bb8604b5.png
[http_blog.csdn.net_mmoaay_article_details_43759703]: http://blog.csdn.net/mmoaay/article/details/43759703