Strange effect of passing parameter as `const`

Question

Consider the following classes :

Bullet class

    class Bullet : public sf::Drawable {
    public:
        Bullet(const sf::Vector2f& pos, const sf::Vector2f& dir, 
            const float& speed, const float& time, const float& life_time);
        ~Bullet();
        
        bool collides(const Wall &wall);
    private:
        ...
}

and Wall class

    class Wall : public sf::Drawable {
    public:
        Wall(const sf::Vector2f & endpoint1, const sf::Vector2f& endpoint2);
        void sample();
        ~Wall();
    private:
          ...
}

For some reason, that I can't entirely comprehend, I can not call any methods for the wall parameter of the bool collides(const Wall &wall) method, when the const is present, e.g. if I remove the const, everything works just fine.

I think it might have something to do with inheriting the sf::Drawable, but I am not that experienced with SFML yet.

Can somebody clarify what should I look into to find what is causing this? Thank you in advance.

You can only call `const` methods on `const` objects. If `collides` doesn't change internal state of `Bullet`, you should declare (and define) it like this: `bool collides(const Wall &wall) const;` I suppose there is a duplicate somewhere, will try to find something — Yksisarvinen, Jul 02 '20 at 09:50
@Yksisarvinen Thank you very much, completely forgot about that. — naomipappe, Jul 02 '20 at 09:56

score 2 · Accepted Answer · answered Jul 02 '20 at 09:52

You cannot call an non-const member function on a const object or a reference to a const object, simple as that.

class Wall : public sf::Drawable {
public:
    void sample() const; // <---- you need this
};

Now it's up to you, either you make member functions that don't change the state be const, or get rid of constness of the parameter of collides.

Strange effect of passing parameter as `const`

1 Answers1