Multiple inheritance and pure virtual functions

Question

The following code:

struct interface_base
{
    virtual void foo() = 0;
};

struct interface : public interface_base
{
    virtual void bar() = 0;
};

struct         


        

Answer 1

For the case of 'solving' the diamond inheritance problem, the solutions offered by bdonlan are valid. Having said that, you can avoid the diamond-problem with design. Why must every instance of a given class be seen as both classes? Are you ever going to pass this same object to a class that says something like:

void ConsumeFood(Food *food);
void ConsumeDrink(Drink *drink);

class NutritionalConsumable {
  float calories() = 0;
  float GetNutritionalValue(NUTRITION_ID nutrition) = 0;
};
class Drink : public NutritionalConsumable {
  void Sip() = 0;
};
class Food : public NutritionalConsumable {
  void Chew() = 0;
};
class Icecream : public Drink, virtual public Food {};

void ConsumeNutrition(NutritionalConsumable *consumable) {
  ConsumeFood(dynamic_cast(food));
  ConsumeDrink(dynamic_cast(drink));
}

// Or moreso
void ConsumeIcecream(Icecream *icecream) {
  ConsumeDrink(icecream);
  ConsumeFood(icecream);
}

Surely it would be better in this case for Icecream to just implement NutritionalConsumable and provide a GetAsDrink() and GetAsFood() method that will return a proxy, purely for the sake of appearing as either food or drink. Otherwise that suggests that there is a method or object that accepts a Food but somehow wants to later see it as a Drink, which can only be achieved with a dynamic_cast, and needn't be the case with a more appropriate design.