Question

I have two structs (part of the assignment). A list of one — Activity, contained in the other — Process. Then, several of the parent Process struct are contained in a priority queue.

struct Activity {
public:
    int time;
    string type;
    Activity(int newTime, string newType):
    time(newTime),type(newType){}
};

struct Process {
public:
    string PID;
    int arrivalTime;
    int priority;
    list<Activity> activityQueue;
    Process( string newPID, int newTime, int newPriority, list<Activity>
         newActivityQueue):
    PID(newPID),arrivalTime(newTime), priority(newPriority), 
         activityQueue(newActivityQueue){}

};

I get the following error…

main.cpp:206:61: error: passing ‘const std::list<Activity>’ as ‘this’ argument of
‘void std::list<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = Activity,
_Alloc = std::allocator<Activity>, std::list<_Tp, _Alloc>::value_type = 
Activity]’ discards qualifiers [-fpermissive]

…when I attempt to push_back an Activity to a Process’s activityQueue.

Activity currentActivity = cpuQueue.top().activityQueue.back();
currentActivity.time--
cpuQueue.top().activityQueue.push_back(currentActivity);

Answer 1

std::priority_queue<T>::top() returns a const reference to the top item: this is so you can’t mutate it in-place and break the ordering constraints.

If you’re happy that the activity list is an implementation detail that won’t affect the position of a process in the cpu queue, you could just make Process::activityQueue mutable.

Otherwise, you should pull the process out of the queue, modify it, and re-add.