How to solve this C++ multiple inheritance similar issue - c++

Question

I'm wondering what is the recommended way to avoid the issue below.
I have a class rigidBody3D that has a pointer to another rigidBody structure, and a class box3D that inherits rigidBody3D.
class rigidBody3D {
public:
void setPosition(float x, float y, float z);
rigidBody* rb;
};
class box3D : public rigidBody3D {
public:
box3D(float w float h, float l);
..other box functions..
};
I then have 2D classes that use these 3D classes.
class rigidBody2D {
public:
rigidBody2D();
void setPosition(float x, float y);
rigidBody3D body;
};
class box2D : public rigidBody2D {
public:
box2D(float w, float h);
box3D box;
};
For example rigidBody2D's setPosition calls the 3D setPosition of its rigidBody3D.
void rigidBody2D::setPosition(float x, float y)
{
body.setPosition(x,y,0);
}
THE ISSUE:
As is, creating a box2D object creates two rigidBody pointers. One because box2D inherits rigidBody2D which has a rigidBody3D. The other because box2D has a box3D object that inherits rigidBody3D.
I only want one rigidBody pointer. I also want to be able to call the 2D setPosition for 2D classes like box2D, but also call their 3D specific functions, like box2D calling box3D.
SOLUTION:
I used virtual inheritance to solve the problem, and also inherited the 3D classes instead of having objects of them.
class rigidBody3D {
public:
void setPosition(float x, float y, float z);
rigidBody* rb;
};
class box3D : public virtual rigidBody3D {
public:
box3D(float w float h, float l);
..other box functions..
};
class rigidBody2D : private virtual rigidBody3D {
public:
rigidBody2D();
void setPosition(float x, float y);
};
class box2D : public rigidBody2D, private box3D {
public:
box2D(float w, float h);
};
void rigidBody2D::setPosition(float x, float y)
{
rigidBody3D::setPosition(x,y,0);
}

Answer 1

Your comment:
I want (...) 2D objects, which are 3D objects that are drawn in 2D and have their z
components set to 0
strongly suggest that a rigidBody2D is a rigidBody3D, and that a box2D is a box3D. Hence it would seem natural to prefer inheritance over compostion:
Alternative 1: simple inheritance
Here we'll consider that the root of all these 2D and 3D objects is the rigidBody2D.
One question remains: is a box2D more a kind of box3D or more a rididBody2D ?
class rigidBody2D : public rigidBody3D {
public:
rigidBody2D();
void setPosition(float x, float y);
// no body anymore: it's inherited from rigidBody3D
};
class box2D : public box3D { // is it more like a box3d or more like a rigiBbody2D ? You decide !
public:
box2D(float w, float h);
// no box anymore: it's inherited from box3D
};
box2D::box2D (float w, float h) : box3D (w, h, 0) { /*...*/ } // how to create a box2D as a special case of box3D
void rigidBody2D::setPosition(float x, float y) // call the setpos of the parent.
{
rigidBody3D::setPosition(x, y, 0); // call parent's setpos
}
All therse objects inherit from rigidBody3D and have then a pointer to a rigidBody.
A further question is in fact to know whether a rigidBody3D should not himself inherit from a rigidBody, or if there are strong arguments against (for ex: if the lifespan of the two are different).
Alternative 2: multiple inheritance
Here it's about the design of box2D. Is it more a box3D, having the same kind of member functions but than a box3D, but taking into account that third dimension is 0 ? Or is it more a rigidBody2D, because it should have and use all the member functions provided by this object ?
If you can easily decide, go back to alternative 1.
If you can't decide, because it's a little bit both, then you could consider multiple inheritance:
class box2D : public box3D, public rigidBody2D {
public:
box2D(float w, float h);
// no box anymore: it's inherited from box3D
};
THen you would inherit member (function and data of both). Hower multiple inheritance ain't easy. You'd have here 2 rigidBody pointers: those who are inherited from the box, and those who are inherited from the body.
This issue could be solved in your case by making inheritance of rigidBody3D virtual:
class rigidBody2D : public virtual rigidBody3D { ... };
class box3D : public virtual rigidBody3D { ... };
in this case, if an object inherits several time rigidBody3D, only one of such subobject is created.

Answer 2

it seems to me that 2d and 3d objects are related only by the fact that they are implemented in terms of a rigidbody*. The semantics of setPosition are different for the two concepts.
Another way to say this is that a 2d body is not a kind-of 3d body, it's a different concept so it seems that there should be no inheritance relationship between them.
so I would start by thinking along these lines:
struct rigid_body_3d_concept {
virtual ~rigid_body_3d_concept();
void set_position(double x, double y, double z) {
_impl->really_set_the_position(x, y, z);
}
private:
rigidbody* _impl;
};
struct rigid_body_2d_concept {
virtual ~rigid_body_2d_concept();
void set_position(double x, double y) {
_impl->really_set_the_position(x, y, 0);
}
private:
rigidbody* _impl;
};
struct box3d
: public rigid_body_3d_concept
{
box3d(double w, double h, double l);
};
struct box2d
: public rigid_body_2d_concept
{
box2d(double w, double h);
};