C++ equivalent of algebraic datatype?

Question

Let\'s say I have this Haskell code:

data RigidBody = RigidBody Vector3 Vector3 Float Shape -- position, velocity, mass an         


        

Answer 1

The canonical way to do this in C++ is the inheritance-based solution given in Justin Wood's answer. Canonically, you endow the Shape with virtual functions that each kind of Shape

However, C++ also has union types. You can do "tagged unions" instead:

struct Ball { /* ... */ };
struct Square { /* ... */ };
struct Shape {
  int tag;
  union {
    Ball b;
    Square s;
    /* ... */
  }
};

You use the tag member to say whether the Shape is a Ball or a Square or whatever else. You can switch on the tag member and whatnot.

This has the disadvantage that a Shape is one int larger than the biggest of Ball and Square and the others; objects in OCaml and whatnot do not have this problem.

Which technique you use will depend on how you're using the Shapes.