Mark P wrote:[color=blue]
> I've written a class IntervalSet which is meant to contain intervals
> (a,b) on the x-axis and which supports insertion, deletion, and
> various types of range queries. Now I'd like to templatize this
> class so that it can hold (pointers to) arbitrary objects as long as
> there's "some
> way" to compute an interval based on the object. For example the
> class might hold polygons located in a plane and the corresponding
> interval would be their shadow on the x-axis produced by a separate light
> source object.
>
> The problem I'm struggling with is, what's the most general and
> flexible way to take into account that the assoication of object to
> an interval may be performed by a 3rd object (i.e., not the IntervalSet
> class and
> not the contained object class).
>
> The usage I envision might be something like this:
>
> class Master
> {
> public:
> void doStuff()
> {
> // perform operations on myISet;
> }
>
> // convert w to (x1,x2) interval - modify x1 and x2
> void computeInterval(const Widget& w, int& x1, int& x2);
>
> private:
> IntervalSet<Widget*> myISet;
> vector<Widget> allMyWidgets;
> };
>
> The idea is that there should be some way to get myISet to make use of
> the function computeInterval so that when I try to insert a Widget*
> into myISet, it will call computeInterval to figure out the
> corresponding x-range.
>
> One thought I had was to have Master derive from IntervalSet and
> override a function from IntervalSet:
>
> template <class T>
> void computeInterval(const T& obj, int& x1, int& x2)
>
> But I'm not sure inheritance is really the right idea here and I think
> I'd rather have the interface be that IntervalSet is given an object
> of an arbitrary 3rd class which knows how to compute x1 and x2. I'm not
> sure how to implement this idea though. Would this be some sort of
> functor?[/color]
A functor has to be an object. Here is an example of using a static
member function of a type:
template<class T, class Computer = T>
class IntervalSet
{
map<T*, pair<int,int> > myset;
public:
IntervalSet();
void add(T* obj)
{
pair<int,int> i = Computer::compute(obj);
map[obj] = i;
}
};
class Widget
{
public:
static pair<int,int> compute(Widget* ptr)
{
ptr->compute();
}
virtual pair<int,int> compute(); // so you can derive
// and change it
};
class Polygon {};
class PolygonProjector
{
public:
static pair<int,int> compute(Polygon*); // whatever
};
int main()
{
IntervalSet<Widget> iw;
iw.add(new Widget);
IntervalSet<Polygon, PolygonProjector> ipp;
ipp.add(new Polygon);
}
Here is an example of using a functor:
template<class T, class Computer>
class IntervalSet
{
Computer puter;
map<T*, pair<int,int> > myset;
public:
explicit IntervalSet(Computer const& c) : puter(c) {}
void add(T* obj)
{
pair<int,int> i = puter(obj);
map[obj] = i;
}
};
class Polygon {};
class PolygonProjector
{
public:
pair<int,int> operator()(Polygon*); // whatever
};
int main()
{
PolygonProjector pp; // here is the functor
IntervalSet<Polygon, PolygonProjector> ipp(pp);
ipp.add(new Polygon);
}
V
design question [long] 
