Question

I am working on code which implements a Multilevel Feedback Queue Scheduler. There is something not clear in a part of the code:

void Scheduler_MFQS :: fill_queue(int clk) {
    list<Process>::iterator itr;
    for(itr = processes.begin(); itr != processes.end(); itr++) {
            if((itr -> has_arrived(clk)) && (!queues[0].contains(*itr))) {
                    Process tmp (*itr);
                    queues[0].add_process(tmp); 
                    remove(processes.begin(), processes.end(), *itr);
             }
    }       
}

What this basically does is just put the process into the base queues under some condition. But i don’t know what Process tmp (*itr); means? However, it compiles legally. Does that mean create a Process object called tmp? But what is the next, iterator (*itr) mean in c++?

Answer 1

"Process tmp (*itr);" mean?

It calls Process(const Process& &) copy constructor to create tmp object;

what is the next, iterator (*itr) mean in c++?

itr is std::list::iterator type, it’s a pointer to current list node. *itr is getting the content of itr, which is a Process.

Your code can enhance a bit, demo as below:

   // list<Process>::iterator itr; // move this into for loop, narrow variable scope and lifetime
    /*auto if C++11*/
     processes.unique();  // you actually only want unique processes from list
    for(list<Process>::iterator itr = processes.begin(); itr != processes.end(); ++itr) 
                                                         ^^ call preincrement, faster
    {
        if((itr -> has_arrived(clk)) /*&& (!queues[0].contains(*itr))*/) {
                                     ^^ process list contains unique item only, no need to compare
         //Process tmp (*itr); comment out this line, save one object copy
         queues[0].add_process(*itr);              
         //remove(processes.begin(), processes.end(), *itr);   
         // You don't need to clear item in the loop              
        }
    }

   processes.clear();   // or swap with an empty list
   // std::list<Process> p2;
   // p2.swap(ps);