为io_service添加任务，并且区分dispatch与post的区别.如果说io_service是asio库的大脑，那么post与dispatch就是asio库的手和脚。先看看示例1
#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(); } size_t fib( size_t n ) { if ( n <= 1 ) { return n; } boost::this_thread::sleep( boost::posix_time::milliseconds( 1000 ) ); return fib( n - 1 ) + fib( n - 2); } void CalculateFib( size_t n ) { global_stream_lock.lock(); std::cout << "[" << boost::this_thread::get_id() << "] Now calculating fib( " << n << " ) " << std::endl; global_stream_lock.unlock(); size_t f = fib( n ); global_stream_lock.lock(); std::cout << "[" << boost::this_thread::get_id() << "] fib( " << n << " ) = " << f << 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() << "] The program will exit when all work has finished." << std::endl; global_stream_lock.unlock(); boost::thread_group worker_threads; for( int x = 0; x < 2; ++x ) { worker_threads.create_thread( boost::bind( &WorkerThread, io_service ) ); } io_service->post( boost::bind( CalculateFib, 3 ) ); io_service->post( boost::bind( CalculateFib, 4 ) ); io_service->post( boost::bind( CalculateFib, 5 ) ); work.reset(); worker_threads.join_all(); return 0; }代码使用智能指针控制io_service，使用mutex控制各个进程间的输出互斥。work类维持io_service的生命周期, 然后使用post添加执行任务。
在此基础上我们再查看post与dispatch的区别：
post 优先将任务排进处理队列，然后返回，任务会在某个时机被完成。
dispatch会即时请求io_service去调用指定的任务。
#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(); } void Dispatch( int x ) { global_stream_lock.lock(); std::cout << "[" << boost::this_thread::get_id() << "] " << __FUNCTION__ << " x = " << x << std::endl; global_stream_lock.unlock(); } void Post( int x ) { global_stream_lock.lock(); std::cout << "[" << boost::this_thread::get_id() << "] " << __FUNCTION__ << " x = " << x << std::endl; global_stream_lock.unlock(); } void Run3( boost::shared_ptr< boost::asio::io_service > io_service ) { for( int x = 0; x < 3; ++x ) { io_service->dispatch( boost::bind( &Dispatch, x * 2 ) ); io_service->post( boost::bind( &Post, x * 2 + 1 ) ); boost::this_thread::sleep( boost::posix_time::milliseconds( 1000 ) ); } } 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() << "] The program will exit when all work has finished." << std::endl; global_stream_lock.unlock(); boost::thread_group worker_threads; for( int x = 0; x < 1; ++x ) { worker_threads.create_thread( boost::bind( &WorkerThread, io_service ) ); } io_service->post( boost::bind( &Run3, io_service ) ); work.reset(); worker_threads.join_all(); return 0; }我们可以看到结果是先显示dispatch的结果然后才显示post结果，与预想的是一致的.
