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I am trying to create a BaseClass array that stores SubClass objects. Below are the follow steps I use to do this:

  1. I create a BaseClass array.
  2. I create a new SubClass object to be stored in the array.
  3. I call the printSelf() function of that object in the array.
  4. The method incorrectly calls the BaseClass function, not the SubClass function.

The error here is that the object stored in the BaseClass array is of a BaseClass object. I need it so that the BaseClass array stores an object of type SubClass.

The problem is designed such that if there were to be multiple SubClasses (i.e. SubClassA, SubClassB, SubClassC...) they would all be stored in a single array.

I've tried various other solutions on StackOverFlow. None so far have provided a solution as to why creating a SubClass object will not store its class type properly in a BaseClass array.

class BaseClass {
    public: void printSelf() { cout << "This element is a BaseClass." << endl; };
};

class SubClass : public BaseClass {
    public: void printSelf() { cout << "This element is a SubClass." << endl; };
};

class House {
    public: House();
    private: BaseClass* subClassArray[1];
};

House::House() {
    subClassArray[0] = new SubClass;
    subClassArray[0]->printSelf();
}

int main() {
    House houseMain;
}

I expect the output to be "This element is a SubClass."

Instead, the output I am receiving is "This element is a BaseClass."

Raedwald
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Ryan Kruse
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  • C++ does not work this way. This is called "object slicing". This is how objects might work in Java, but C++ is not Java. See the duplicate question for more information. You assign a derived class, a subclass, to a base class, the value in the array. This slices away the derived class, leaving you with the base class in the array. Same thing would happen if you simply assign a plain derived class object to a plain base class object. – Sam Varshavchik Aug 12 '19 at 01:10
  • Possible duplicate of [What is object slicing?](https://stackoverflow.com/questions/274626/what-is-object-slicing) – Raedwald Aug 12 '19 at 07:51

2 Answers2

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You need to declare printSelf as virtual. In addition you almost certainly want to add a virtual destructor to your class.

class BaseClass {
    public: virtual void printSelf() const { cout << "This element is a BaseClass." << endl; };
            ////// <- must           ///// <- recommended
            virtual ~BaseClass() = default; // <-- HIGHLY recommended
};

class SubClass : public BaseClass {
    public: void printSelf() const override { cout << "This element is a SubClass." << endl; };
                                   //////// <- recommended
};

The following line is twice problematic.

private: BaseClass* subClassArray[1];

First, it is using raw C-style pointers. Second, it uses a raw C-style array. Both are not recommended in today's C++ development. You want to read about smart pointers, std::vector and std::array.

Do NOT use any casts here, this is a non-option.

n. m. could be an AI
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  • After speaking with my professor, this was the confirmed solution. Apparently the array format an error on the assignment, but the virtual functionality is what was being tested. – Ryan Kruse Aug 13 '19 at 02:33
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The issue is that you are hiding the printSelf method, rather than overriding it. Hence, the base class' version of printSelf is used when you have a pointer to the base class type.

There are two options to get around this, either you cast subClassArray[0] to the subclass type (not recommended unless you don't own the base class) or you mark printSelf as virtual and override it in the subclass.

Scroog1
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