double dispatch infinite loop

Question

I'm using double dispatch to obtain the distance between 2 objects of 2 classes(B, C) that are subclasses of another (A). I think that the methods in class A should be pure virtual, but them are used in a test elsewhere, so the class A must be instantiable, so I can't make them pure virtual, right?

Another thing is that I'm not sure if I'm using double dispatch in a good way, since sometimes generates a infinite loop, in the definition of Distance(A *a) in the base class.

In the class A, I have the methods:

virtual double Distance(A *a)  {return a->Distance(this);}
virtual double DistanceB(B *b) {return std::numeric_limits<double>::max();}
virtual double DistanceB(C *c) {return std::numeric_limits<double>::max();}
virtual double DistanceC(B *b) {return std::numeric_limits<double>::max();}
virtual double DistanceC(C *c) {return std::numeric_limits<double>::max();}

In the class B:

double B::Distance(A *a)  { return a->DistanceB(this); }
double B::DistanceB(B *b) { /*calculate distance*/ }
double B::DistanceC(C *c) { return c->DistanceB(this); }

In the class C:

double C::Distance(A *a)  { return a->DistanceC(this); }
double C::DistanceB(B *b) { /*calculate distance*/ }
double C::DistanceC(C *c) { /*calculate distance*/ }

Pretty please with strawberries and a cherry on top, format code as code. — Angew is no longer proud of SO, Feb 18 '14 at 14:05
It would help people if you describe what triggers the infinite loop (show which method you're calling and how, i.e. with which arguments). — TypeIA, Feb 18 '14 at 14:10
`double A::Distance(A *a) {return a->Distance(this);}` looks suspicious. — laune, Feb 18 '14 at 14:30
Following may help if you can use C++11 : https://ideone.com/8VxALs — Jarod42, Feb 18 '14 at 15:00

score 0 · Answer 1 · answered Feb 18 '14 at 14:32

0

virtual double Distance(A *a)
{return a->Distance(this);}

So if I have:

A* a1, a2;

And I call:

a1->Distance(a2);

The aforementioned implementation will call

a2->Distance(a1);

Which will call:

a1->Distance(a2);

...And that's an infinite loop.

answered Feb 18 '14 at 14:32

PaF

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score 0 · Answer 2 · answered Feb 18 '14 at 14:40

Since A is not pure abstract, you have to provide also :

virtual double DistanceA(A *a);
virtual double DistanceA(B *b);
virtual double DistanceA(C *c);
virtual double DistanceB(A *a);
virtual double DistanceC(A *a);

And fixing

double A::Distance(A *a) {return a->DistanceA(this); }

double dispatch infinite loop

2 Answers2