Question

Once again I find myself failing at some really simple task in C++. Sometimes I wish I could de-learn all I know from OO in java, since my problems usually start by thinking like Java.

Anyways, I have a std::list<BaseObject*> that I want to sort. Let’s say that BaseObject is:

class BaseObject {
protected:
    int id;
public: 
    BaseObject(int i) : id(i) {};
    virtual ~BaseObject() {};
};

I can sort the list of pointer to BaseObject with a comparator struct:

struct Comparator {
    bool operator()(const BaseObject* o1, const BaseObject* o2) const {
        return o1->id < o2->id;
    }
};

And it would look like this:

std::list<BaseObject*> mylist;
mylist.push_back(new BaseObject(1));
mylist.push_back(new BaseObject(2));
// ...

mylist.sort(Comparator()); 

// intentionally omitted deletes and exception handling

Until here, everything is a-ok. However, I introduced some derived classes:

class Child : public BaseObject {
    protected:
    int var;
    public: 
    Child(int id1, int n) : BaseObject(id1), var(n) {};
    virtual ~Child() {};
};

class GrandChild : public Child {
    public:
    GrandChild(int id1, int n) : Child(id1,n) {};
    virtual ~GrandChild() {};
};

So now I would like to sort following the following rules:

1. For any Child object c and BaseObject b, b<c
2. To compare BaseObject objects, use its ids, as before.
3. To compare Child objects, compare its vars. If they are equal, fallback to rule 2.
4. GrandChild objects should fallback to the Child behavior (rule 3).

I initially thought that I could probably do some casts in Comparator. However, this casts away constness. Then I thought that probably I could compare typeids, but then everything looked messy and it is not even correct.

How could I implement this sort, still using list<BaseObject*>::sort ?

Thank you

Answer 1

You are looking at doing double dispatch – that is calling a virtual function depending on the types of two objects rather than one. Take a look at this wikipedia article for a heads-up http://en.wikipedia.org/wiki/Double_dispatch. I have to say that whenever I find myself in this situation, I try to change direction 🙂

And can I make a couple of observations about your code. There is nothing precisely wrong with it but:

• in C++, the std::list is the container of last resort – you should normally default to using a std:;vector, unless you specifically need a feature that only list provides:

• protected data is always a bad idea