Good practice for downcasting return type of inherited functions - objective-c

Question

I have got a Matrix-Class from which a Vector-Class is derived from and for extra functionality and better usage I've got a Vector3-Class which is derived from the Vector class. My problem is now that the Vector-class implements a function for instance +normalizeVector: which returns a new allocated Vector-instance. The subclass Vector3 should inherit these two functions but an inheritance leads to the function-prototypes that return a Vector-instance and not a Vector3-instance. This is just how inheritance works but is there a good practice how to solve that problem? A naive solution is to create Vector3 als a new class which subclasses NSObject but I want that Vector- and Vector3-instances can interact easily.
Here an code-example:
#interface Vector : NSObject {
....
}
+(Vector*) normalizeVector:(Vector*)v; //returns a new allocated Vector-instance
-(Vector*) normalize; //normalizes itself and returns itself
-(Vector*) otherFunction;
#end
#interface Vector3 : Vector {
}
-(Vector3*) specialFunction;
#end
usage of that code:
Vector3 *v3 = ...;
[[v3 normalize] specialFunction]; //Compiler gives me a warning because Vector has no specialFunction. Cast would help
[[Vector3 normalizeVector:v3] specialFunction]; //Compiler gives me a warning and during runtime it will crash because a `Vector` doesn't implement specialFunction
a cast to Vector3 would help but is not nice to work with and that also fails with the static function +normalizeVector: because in that static function a Vector-instance is allocated and a pointer-cast doesn't help.
any ideas? or other approaches / other modeling ?
edit: Code for my static function normalizeVector which gets inherited by Vector3:
#implementation Vector
...
+(Vector*) normalizeVector:(Vector *)v
{
unsigned int dim = vector_max(v.cols, v.rows);
Vector *res = [[[Vector alloc]initAsColumnVectorWithDim:dim] autorelease];
[Vector normalizeVector:v destination:res]; // this does only the logic: calc length and divide each component by the len and store at the vector passed to destination
return res;
}
#end

Answer 1

You will notice that -init methods always return type id -(id)init {..} exactly becauase of this.
Also, instead of [Vector alloc] - as you have noticed you don't actually know what Class you are in at runtime (it could be a subclass), so instead just use [self alloc] where self is the current Class because you are in a Class method. So, if you do [Vector3 normalizeVector:v] self is Vector3 and if you do [Vector normalizeVector:v] self is Vector.
Try adjusting your +normailzeVector: method to
+ (id)normalizeVector:(Vector *)v {
unsigned int dim = vector_max(v.cols, v.rows);
id res = [[[self alloc] initAsColumnVectorWithDim:dim] autorelease];
[self normalizeVector:v destination:res];
return res;
}
Just a note, + (id)normalizeVector: is not a function and definitely not a static function. It is a class method, it just helps to get the terms right.

Answer 2

In this case I would make normaliseVector an instance method. So instead of
Vector *newV = [Vector normalizeVector:v];
call
Vector *newV = [v normalizeVector];
Then you can produce a different normalizeVector for Vector and Vector3
EDIT:
For [[v3 normalize] specialFunction]; there is a problem in that normalize can sometimes return an object that specialFunction does not work on - ie it only works if v3 is a Vector3. So in this case there is extra information you have so a cast would be needed or that Vector3 normailze differs from Vector's. In this case I would produce a cover method on Vector3 to call normalize] specialFunction] so that the cast is in Vector3 specific code.