c++ - How to call a function when a work item is finished in Boost.Asio? -

i implement command queue handles incoming commands concurrently thread pool (so queue grows temporarily when threads working). post callback callers when command worker started , finished. implementation based on this example asio website.

is there way hook these events , signal somehow? avoid command functors knowing callbacks (since call callbacks inside command functors).

pseudocode illustrate (initialization , error handling omitted brevity):

class commandqueue { public:     void handle_command(cmdid id, int param)     {         io_service.post(boost::bind(&(dispatch_map[id]), param));          // pseudocode:         // when 1 of worker threads start item, want call          callback_site.cmd_started(id, param);          // when command functor returns , thread finished         callback_site.cmd_finished(id, param);     }  private:     boost::asio::io_service io_service;     asio::io_service::work work;     std::map<cmdid, commandhandler> dispatch_map; // commandhandler functor taking int parameter     callbacksite callback_site; }; 

is there way without having command functors depend on callbacksite?

so want build in happens when 1 of run() commands starts process command, , on return.

personally, wrapping function call:

    class commandqueue     {     public:         void handle_command(cmdid id, int param)         {             io_service.post(boost::bind(&commandqueue::dispatchcommand, this,id,param));         }      private:         boost::asio::io_service io_service;         asio::io_service::work work;         std::map<cmdid, commandhandler> dispatch_map; // commandhandler functor taking int parameter         callbacksite callback_site;         void dispatchcommand(cmdid id, int param)         {           // when 1 of worker threads start item, want call            callback_site.cmd_started(id, param);           dispatch_map[id](param);           // when command functor returns , thread finished           callback_site.cmd_finished(id, param);         }     }; 

this pattern use when want handle exceptions in dispatched commands. can post different events instead of running them inline.