std::set sorted

Question

I wonder how is the most convenient way to have a sorted set, a set of pointers to objects, like

std::set<myClass*> mySet;

I want this set to be sorted by myClass::someProperty (say, an int).

Should I overload operator < in myClass? I'm not sure if it will work, because it's not a set of myClass, but a set of pointers.

How can I define a compare function?

Thank you very much.

Dupe of http://stackoverflow.com/questions/2620862/using-custom-stdset-comparator? — user508633, May 11 '17 at 01:40

score 4 · Accepted Answer · answered May 11 '17 at 02:44

You need to define a type (or a function) that dereferences the pointers and compares the attributes of the objects they point at, something on this general order:

class myClass {
    int value;
public:
    myClass(int i = 0) : value(i) {}

    struct cmp {
        bool operator()(myClass *const &a, myClass *const &b) const {
            return a->value < b->value;
        }
    };
};

We they define the set something like this:

std::set<myClass*, myClass::cmp> mySet;

My advice, however, would be to store objects instead of pointers (if possible).

Thank you very much. In my case, it's not possible to store objects. — Alejandro Silvestri, May 11 '17 at 11:01

score 3 · Answer 2 · answered May 11 '17 at 03:57

You can also specialize std::less for your myClass* as given below and then no need to pass comparator while creating set:

namespace std {
template<>
struct less<myClass*>
{
   bool operator()(const myClass* k1, const myClass* k2) const
   {
      // Some code ...
   }
};
}

std::set sorted

2 Answers2