Arduino C++ Inheritance and function declaration problem

Question

class Entity {
  public:
    virtual void applyCollisionBehaviorTo(Entity &entity) { }
    
    virtual void onCollision(Entity &entity) { }
};

class Ball : public Entity {
  public:
    void applyCollisionBehaviorTo(Entity entity) override {
      
    }

    void onCollision(Entity entity) override {
      entity.applyCollisionBehaviorTo(this); // error: no matching function for call to 'Entity::applyCollisionBehaviorTo(Ball*)'
    }
};

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  // put your main code here, to run repeatedly:

}

I come from a C# background so I'm getting my head around C++ inheritance and polymorphism.

Because it has not been declared at that point. You only declare it later. — spectras
– spectras, Commented Feb 17, 2021 at 0:51
Also, the base classes shouldn't know about the derived ones! — Jack Lilhammers
– Jack Lilhammers, Commented Feb 17, 2021 at 0:53
You don't. It defeats the whole point using inheritance in the first place. Are you maybe trying to implement some kind of double dispatch? — spectras
– spectras, Commented Feb 17, 2021 at 0:53
In order to pass Ball by value you need to have a full definition of it prior to the use. But you can pass a pointer or reference to it using just a forward declaration. — Mark Ransom
– Mark Ransom, Commented Feb 17, 2021 at 1:02
I've updated the question to reflect more of what the problem is. — gotnull
– gotnull, Commented Feb 17, 2021 at 1:08

gotnull · Accepted Answer · 2023-03-05 22:48:49Z

1

Your class Entity should be like this:

class Entity
{
 public:
   virtual void applyCollisionBehaviorTo(Entity &entity) = 0;    
   virtual void onCollision(Entity &collidingEntity) = 0;
};

You can't refer to object of Ball class inside Entity, in fact entities don't even know about the existence of balls.

OTOH balls "know" that they are entities

edited Mar 5, 2023 at 22:48

gotnull

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answered Feb 17, 2021 at 1:01

Jack Lilhammers

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5 Comments

gotnull Over a year ago

Yeah, see I tried that but I get: no matching function for call to 'Entity::applyCollisionBehaviorTo(Ball*)' in the Ball class when passing this.

Jack Lilhammers Over a year ago

Because you must pass *this

Jack Lilhammers Over a year ago

this is a pointer. If you use a reference or a value, you must dereference it

gotnull Over a year ago

Okay great, *this seemed to fix it on my end :) Thank you.

Jack Lilhammers Over a year ago

You're welcome, but keep in mind that you still have to match the methods signatures in Ball

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