boost asio 學習(二)瞭解boost::bind

2.瞭解boost::bind
使用boost::bind封裝一個函數，考慮如下例子
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;
}

　　

運行代碼無輸出，這是由於咱們建立一個函數觸發對象，可是沒有實際調用。咱們須要使用()操做符調用函數.
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;
}

　　

如今運行示例，將輸出文本。下面示例介紹如何傳輸參數
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;
}

　　

運行程序將輸出預期的文本。

下個示例顯示bind類成員函數
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;
}

　　

咱們必須傳遞類對象的地址以便調用。若是是在類內部調用，則調用this指針或者shared_from_this().
在多線程中，io_service做爲全局對象。在實際應用中，這種作法是不推薦的。若是咱們嘗試應用bind io_service對象，則會發生錯誤，由於io_service不能被拷貝，因此咱們須要使用
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;
}

　　

異步程序中，須要確認全局和共享數據的同步訪問。下列示例示範了mutex對象的使用方法。
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;
}

　　此類mutex對象不可遞歸鎖定。若是遞歸鎖定將形成死鎖。