C++ Sorting Custom Objects in a list

Question

I am having trouble sorting a list of custom class pointers. The class I need to sort are events. These get assigned a random time and I need to do them in the right order.

Answer 1

Since the list contains pointers, rather than objects, you'll have to provide a custom comparator to compare the objects they point to. And since you're using a list, you have to use its own sort method: the generic std::sort algorithm only works on random-access sequences.

EventList.sort([](Event * lhs, Event * rhs) {return lhs->time < rhs->time;});

or, if you're stuck in the past and can't use lambdas:

struct CompareEventTime {
    bool operator()(Event * lhs, Event * rhs) {return lhs->time < rhs->time;}
};

EventList.sort(CompareEventTime());

If the list contained objects (as it probably should), then it might make sense to provide a comparison operator instead:

bool operator<(Event const & lhs, Event const & rhs) {return lhs.time < rhs.time;}

std::list<Event> EventList;
//...
EventList.sort();

Answer 2

You should to that with std::sort. You can either make a custom comparator function that you pass as third argument to the std::sort function, or you can make a < operator overload for your class and std::sort will work naturally.