Using keyword with virtual inheritance in cpp

Question

I want to resolve an inheritance problem of my Paladin class. Here is a resume of the inheritance between my classes :

class Peasant
{
    virtual int attack();
}

class Knight : virtual public Peasant
{
    int attack() override;
}

class Enchanter : public Peasant
{
    int attack() override;
}

class Priest : virtual public Enchanter

class Paladin : public Knight, public Priest
{
    using Knight::attack();
}

This way I got the following error :

Paladin.hpp:16:7: error: no unique final redefinition « virtual int Peasant::attack() » in « Paladin »
   16 | class Paladin : public Knight, public Priest
      |       ^~~~~~~

Words may differ in the official error message as I traduce this error from another language to english.

I solved my problem this way :

class Peasant
{
    int attack();
}

class Knight : virtual public Peasant
{
    int attack();
}

class Enchanter : public Peasant
{
    int attack();
}

class Priest : virtual public Enchanter

class Paladin : public Knight, public Priest
{
    using Knight::attack();
}

But I don't really like this solution. I prefer using virtual and override when I override the base's methods in derived classes. I know I could do :

class Peasant
{
    virtual int attack();
}

class Knight : virtual public Peasant
{
    virtual int attack() override;
}

class Enchanter : public Peasant
{
    virtual int attack() override;
}

class Priest : virtual public Enchanter

class Paladin : public Knight, public Priest
{
    int attack() override {return Knight::attack();};
}

But in the case I'm just re-using a method, I think it's more elegant to use using.

So I would like to know if there is a way to use both virtual/override and using in the same syntax.

Thank you in advance :D

What would be the purpose of adding an override to a using? The using does not override anything — UnholySheep
– UnholySheep, Commented Jan 22, 2024 at 14:34
And it makes no sense to inherit the same class both virtually and on-virtually. The result is utter confusion. Perhaps rethink the relationship between the various classes in this hierarchy? — Sam Varshavchik
– Sam Varshavchik, Commented Jan 22, 2024 at 14:36
Also none of the examples actually compile due to various syntax and access specifier errors, making it very hard to tell what your original problem was — UnholySheep
– UnholySheep, Commented Jan 22, 2024 at 14:36
Rule of thumb: virtual inheritance everywhere or nowhere. in Enchanter, Peasant should be a virtual base. — Pete Becker
– Pete Becker, Commented Jan 22, 2024 at 14:47
"I solved my problem this way :...." its not just your personal preference, but overriding a method does require the method to be virtual. You cannot override a non virtual method and in that middle code nothing is being overridden — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Jan 22, 2024 at 15:40

Caleth · Accepted Answer · 2024-01-22 15:26:43Z

2

Your second option is the correct way to go. using doesn't define a function, it brings a name into scope.

You also have another error, attack is private in each of these classes, so you can't return Knight::attack().

Not a syntax error, but you are missing virtual off your inheritance at least in Enchanter, and probably in Paladin.

struct Peasant
{
    virtual int attack();
};

struct Knight : virtual Peasant
{
    int attack() override;
};

struct Enchanter : virtual Peasant
{
    int attack() override;
};

struct Priest : virtual Enchanter
{};

struct Paladin : virtual Knight, virtual Priest
{
    int attack() override { return Knight::attack(); };
};

However, trying to embed the rules of a game into subclasses within an inheritance hierarchy can be like fitting a round peg in a square hole.

answered Jan 22, 2024 at 15:26

Caleth

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