boost asio 学习(二)了解boost::bind

2.了解boost::bind
使用boost::bind封装一个函数，考虑以下例子
示例2a

#include <iostream>
#include <boost/bind.hpp>

void F1()
{
	std::cout << __FUNCTION__ << std::endl;
}

int main( int argc, char * argv[] )
{
	boost::bind( &F1 );
	return 0;
}

　　

运行代码无输出，这是因为我们创建一个函数触发对象，但是没有实际调用。我们需要使用()操作符调用函数.
示例2b

#include <iostream>
#include <boost/bind.hpp>

void F1()
{
	std::cout << __FUNCTION__ << std::endl;
}

int main( int argc, char * argv[] )
{
	boost::bind( &F1 )();
	return 0;
}

　　

现在运行示例，将输出文本。下面示例介绍如何传输参数
示例2C

#include <iostream>
#include <boost/bind.hpp>

void F2( int i, float f )
{
	std::cout << "i: " << i << std::endl;
	std::cout << "f: " << f << std::endl;
}

int main( int argc, char * argv[] )
{
	boost::bind( &F2, 42, 3.14f )();
	return 0;
}

　　

运行程序将输出预期的文本。

下个示例显示bind类成员函数
示例2d

#include <iostream>
#include <boost/bind.hpp>

class MyClass
{
public:
	void F3( int i, float f )
	{
		std::cout << "i: " << i << std::endl;
		std::cout << "f: " << f << std::endl;
	}
};

int main( int argc, char * argv[] )
{
	MyClass c;
	boost::bind( &MyClass::F3, &c, 42, 3.14f )();
	return 0;
}

　　

我们必须传递类对象的地址以便调用。如果是在类内部调用，则调用this指针或者shared_from_this().
在多线程中，io_service作为全局对象。在实际应用中，这种做法是不推荐的。如果我们尝试应用bind io_service对象，则会发生错误，因为io_service不能被拷贝，所以我们需要使用
shred_ptr.
示例2e

#include <boost/asio.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <iostream>

void WorkerThread( boost::shared_ptr< boost::asio::io_service > io_service )
{
	std::cout << "Thread Start\n";
	io_service->run();
	std::cout << "Thread Finish\n";
}

int main( int argc, char * argv[] )
{
	boost::shared_ptr< boost::asio::io_service > io_service(
		new boost::asio::io_service
		);
	boost::shared_ptr< boost::asio::io_service::work > work(
		new boost::asio::io_service::work( *io_service )
		);

	std::cout << "Press [return] to exit." << std::endl;

	boost::thread_group worker_threads;
	for( int x = 0; x < 4; ++x )
	{
		worker_threads.create_thread( boost::bind( &WorkerThread, io_service ) );
	}

	std::cin.get();

	io_service->stop();

	worker_threads.join_all();

	return 0;
}

　　

异步程序中，需要确认全局和共享数据的同步访问。下列示例示范了mutex对象的使用方法。
示例2f

#include <boost/asio.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/thread.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/bind.hpp>
#include <iostream>

boost::mutex global_stream_lock;

void WorkerThread( boost::shared_ptr< boost::asio::io_service > io_service )
{
	global_stream_lock.lock();
	std::cout << "[" << boost::this_thread::get_id() << 
		"] Thread Start" << std::endl;
	global_stream_lock.unlock();

	io_service->run();

	global_stream_lock.lock();
	std::cout << "[" << boost::this_thread::get_id() <<
		"] Thread Finish" << std::endl;
	global_stream_lock.unlock();
}

int main( int argc, char * argv[] )
{
	boost::shared_ptr< boost::asio::io_service > io_service(
		new boost::asio::io_service
		);
	boost::shared_ptr< boost::asio::io_service::work > work(
		new boost::asio::io_service::work( *io_service )
		);

	global_stream_lock.lock();
	std::cout << "[" << boost::this_thread::get_id() 
		<< "] Press [return] to exit." << std::endl;
	global_stream_lock.unlock();

	boost::thread_group worker_threads;
	for( int x = 0; x < 4; ++x )
	{
		worker_threads.create_thread( boost::bind( &WorkerThread, io_service ) );
	}

	std::cin.get();

	io_service->stop();

	worker_threads.join_all();

	return 0;
}

　　此类mutex对象不可递归锁定。如果递归锁定将造成死锁。