Inheritance: Child uses parent virtual function instead of its own

Question

I'm having this issue with inheritance I just cannot wrap my head around. I have three classes, with a parental relationship between them. All of them has the function calcForces().

class Object {
public:
    virtual void calcForces();
};


class Boat : public Object {
public:
    virtual void calcForces();
};


class Sailboat : public Boat {
public:
    void calcForces(Wind wind);
};

Now my problem is that I I have a object created as a Sailboat (and saved it in an Object-pointer), but when I call calcForces() I end up inside Boat::calcForces(), not Sailboat::calcForces(). What am I doing wrong?

Here is my call to the function:

(*this->object_ptr_arr[i]).calcForces();    //'object_ptr_arr' is of type 'Object**'

You need to call through a pointer for polymorphic behavior. Try: `this->object_ptr_arr[i]->calcForces();` — Galik, Oct 16 '17 at 12:35
To avoid these sort of issues, see http://en.cppreference.com/w/cpp/language/override — stefaanv, Oct 16 '17 at 12:39

acraig5075 · Accepted Answer · 2017-10-16T13:19:13.237

11

The declarations of the overrides must match. Your declaration for Sailboat::calcForces takes a different parameter (a Wind instance) and therefore is not an override.

As commented by @stefaanv and @skypjack you can avoid this issue by embracing the use of the override function specifier, which would've caught the error at compile time.

edited Oct 16 '17 at 13:19

answered Oct 16 '17 at 12:35

acraig5075

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I would mention the `override` keyword that serves the purpose of match checking it at compile-time and avoids the error. – skypjack Oct 16 '17 at 13:06
@skypjack Done! – acraig5075 Oct 16 '17 at 13:19

score 2 · Answer 2 · answered Oct 16 '17 at 12:37

If you want an calcForces() method in your sailboat you should add it there. At the moment you only have a calcForces(Wind wind) method in your sailboat (and the calcForces() that is inherited from Boat)

So when you are calling calcForces without a parameter it will end up in the inherited method. To fix this either add it in Sailboat like described above or call it with an variable of type Wind

score 2 · Answer 3 · answered Oct 16 '17 at 12:41

Suggestion - boats and sailboats exist in an environment. The boat knows how it is affected by the environment.

class Environment
{
public:
  Wind wind;
};

class Object {
public:
    virtual void calcForces(Environment const&);
};


class Boat : public Object {
public:
    virtual void calcForces(Environment const& env); // can ignore the wind
};


class Sailboat : public Boat {
public:
    void calcForces(Environment const& env) override;  // will use env.wind in calculation
};

Now you are able to modify the environment and ask all objects in the environment to calculate their forces. Whether they're boats, sailboats or dolphins.

Very good suggestion and definitely preferable because of the object-orientated approach. But that isn't very helpful if he is facing similiar problems in the future because it doesn't describes what he was doing wrong. — Rafiwui, Oct 16 '17 at 12:44

Inheritance: Child uses parent virtual function instead of its own

3 Answers3