C++ STL set: Compare object with extrinsic state

Question

This definitions are inside of OuterClass:

struct Compare
{
    bool operator ()(const T&, const T&);
};
typedef set<T, Compare> MySet;

My problem is that the compare function operator () depends on the state of OuterClass. (MySet instances are used during an algorithm for an optimization and they have to sort differently at different stages.)

Is there any way/workaround to access nonstatic members of OuterClass from within the compare function operator ()?

Answer 1

Is there any way/workaround to access nonstatic members of OuterClass from within the compare function operator ()?

There is. Just write a user-defined constructor for Compare that accepts and stores a reference to OuterClass, this way:

struct Compare
{
    Compare(OuterClass& o) : oc(o) { }
    bool operator ()(const T&, const T&)
    {
        // Uses oc somehow...
    }
private:
    OuterClass& oc;
};

Then, when you create your set, you can do something like:

int main()
{
    typedef std::set<T, Compare> MySet;

    OuterClass oc; // <== Construct an object of type Outerclass somehow...

    MySet ms(Compare(oc)); // <== Construct your comparator and pass it
                           //     in input to the constructor of std::set
}

Beware though: the ordering criterion should remain stable. Elements must always compare the same for the same set. Per paragraph 23.2.4/3 of the C++11 Standard:

The phrase “equivalence of keys” means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false. For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value.