【Boost】Asio库学习（一）

Boost::Asio

同步使用计时器

• 所有使用asio的程序都需要至少有一个I/O执行上下文，例如io_context或thread_pool对象。I/O执行上下文提供对I/O功能的访问。

#include <iostream>
#include <boost/asio.hpp>int main()
{boost::asio::io_context io;boost::asio::steady_timer t(io, boost::asio::chrono::seconds(5));t.wait();std::cout << "Hello, world!" << std::endl;return 0;
}

• boost::asio::steady_timer::wait() 在timer过期后返回，也就是在timer创建5s后，而不是在wait()调用5s后。timer只有两种状态，过期和未过期。如果wait()是一个过期的timer调用，那么会立即返回。

异步使用计时器

• timer不实用阻塞wait()，调用steady_timer::async_wait()实现异步，被调用函数签名void(const boost::system::error_code&)，最后必须调用boost::asio::io_context::run()成员函数。

#include <iostream>
#include <boost/asio.hpp>void print(const boost::system::error_code& /*e*/)
{std::cout << "Hello, world!" << std::endl;
}int main()
{boost::asio::io_context io;boost::asio::steady_timer t(io, boost::asio::chrono::seconds(5));t.async_wait(&print);io.run();return 0;
}

• boost::asio::io_context没有任务之后才会停止，io_context绑定了timer，也就是timer过期以及print函数返回前，会一直执行。

绑定参数

• 实现一个重复的计时器，还需要传递两个参数到处理函数，一个是timer的指针，还有一个是停止条件参数。 std::bind()函数可以绑定参数并将函数正确转化为void(const boost::system::error_code&)签名的函数，需要确认参数列表对应。

#include <functional>
#include <iostream>
#include <boost/asio.hpp>void print(const boost::system::error_code& /*e*/,boost::asio::steady_timer* t, int* count)
{if (*count < 5){std::cout << *count << std::endl;++(*count);t->expires_at(t->expiry() + boost::asio::chrono::seconds(1));t->async_wait(std::bind(print,boost::asio::placeholders::error, t, count));}
}int main()
{boost::asio::io_context io;int count = 0;boost::asio::steady_timer t(io, boost::asio::chrono::seconds(1));t.async_wait(std::bind(print,boost::asio::placeholders::error, &t, &count));io.run();std::cout << "Final count is " << count << std::endl;return 0;
}

将成员函数用作处理函数

• 创建一个类，这个类有一个boost::asio::io_context对象引用，当初始化timer时使用。非静态函数有一个隐性指针this，需要绑定到成员函数，std::bind()函数将成员函数转换为一个可以被调用的函数对象，函数签名void(const boost::system::error_code&)。

#include <functional>
#include <iostream>
#include <boost/asio.hpp>class printer
{
public:printer(boost::asio::io_context& io): timer_(io, boost::asio::chrono::seconds(1)),count_(0){timer_.async_wait(std::bind(&printer::print, this));}~printer(){std::cout << "Final count is " << count_ << std::endl;}void print(){if (count_ < 5){std::cout << count_ << std::endl;++count_;timer_.expires_at(timer_.expiry() + boost::asio::chrono::seconds(1));timer_.async_wait(std::bind(&printer::print, this));}}private:boost::asio::steady_timer timer_;int count_;
};int main()
{boost::asio::io_context io;printer p(io);io.run();return 0;
}

多线程程序中同步

• 使用strand类模版，除了初始化boost::asio::steady_timer成员，也要初始化strand成员，boost::asio::strand<boost::asio::io_context::executor_type>对象，strand保证它分配的执行器，一个正在执行的可以在下一个处理之前完成，而不用管调用boost::asio::io_context的线程数量。将程序绑定到同一个strand，可以保证程序不并发执行。

#include <functional>
#include <iostream>
#include <thread>
#include <boost/asio.hpp>class printer
{
public:printer(boost::asio::io_context& io): strand_(boost::asio::make_strand(io)),timer1_(io, boost::asio::chrono::seconds(1)),timer2_(io, boost::asio::chrono::seconds(1)),count_(0){timer1_.async_wait(boost::asio::bind_executor(strand_,std::bind(&printer::print1, this)));timer2_.async_wait(boost::asio::bind_executor(strand_,std::bind(&printer::print2, this)));}~printer(){std::cout << "Final count is " << count_ << std::endl;}void print1(){if (count_ < 10){std::cout << "Timer 1: " << count_ << std::endl;++count_;timer1_.expires_at(timer1_.expiry() + boost::asio::chrono::seconds(1));timer1_.async_wait(boost::asio::bind_executor(strand_,std::bind(&printer::print1, this)));}}void print2(){if (count_ < 10){std::cout << "Timer 2: " << count_ << std::endl;++count_;timer2_.expires_at(timer2_.expiry() + boost::asio::chrono::seconds(1));timer2_.async_wait(boost::asio::bind_executor(strand_,std::bind(&printer::print2, this)));}}private:boost::asio::strand<boost::asio::io_context::executor_type> strand_;boost::asio::steady_timer timer1_;boost::asio::steady_timer timer2_;int count_;
};int main()
{boost::asio::io_context io;printer p(io);std::thread t([&]{ io.run(); });io.run();t.join();return 0;
}

可能的输出：

Timer 2: 0
Timer 1: 1
Timer 2: 2
Timer 1: 3
Timer 2: 4
Timer 1: 5
Timer 2: 6
Timer 1: 7
Timer 2: 8
Timer 1: 9