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Copys of different pointer types to a single object

cppstudent (7)
Hi, I wanted to have an array of base class pointers to access all the derived classes' member functions. Here is what I've typed

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#include <iostream>
using namespace std;

class Base {
public:
	Base() { };
	virtual void print() { cout<<"Base\n"; };
};

class A : public Base {
public:
	A() { };
	void print() { cout<<"A\n"; };
	void funInA() { };
};

class B : public Base {
public:
	B() { };
	void print() { cout<<"B\n"; };
	void funInB() { };
};

int main() {
	Base* arrayOfBasePointers[10] = {};
	for (int i = 0; i < 5; i++) {
		arrayOfBasePointers[i] = new A;
		arrayOfBasePointers[i+5] = new B;
	} //[0]..[4] is of type A, [5]..[9] is of type B

	for (int i = 0; i < 10; i++) {
		arrayOfBasePointers[i]->print();
	} // this outputs nicely, A A A A A B B B B B

//	arrayOfBasePointers[3]->funInA(); //error
//	arrayOfBasePointers[8]->funInB(); //error
	// Question! How do you make it works?

	// I have tried this
	((A*)arrayOfBasePointers[3])->funInA(); //OK
	((A*)arrayOfBasePointers[8])->funInA(); //NOT supposes to be OK

	// and this
	A* arrayOfAObjects[5] = {};
	B* arrayOfBObjects[5] = {};
	for (int i = 0; i < 5; i++) {
		arrayOfAObjects[i] = (A*)arrayOfBasePointers[i];
		arrayOfBObjects[i] = (B*)arrayOfBasePointers[i+5];
	}
	arrayOfAObjects[3]->funInA();	//OK
	arrayOfBObjects[3]->funInB();	//OK

	//but then I cannot have a good feature like
	//for (int i = 0; i < 10; i++) {
	//	arrayOfBasePointers[i]->print();
	//}
}


At the end of the day, I ended up having both arrayOfAObjects and arrayOfBasePointers pointing to the same object. If I delete either one, the other one will have memory access violation, which I think is not a good way to achieve my goal.

Please share your opinion, thank you!
Last edited on
dfarmer (8)
Have you considered adding another virtual function to your Base class?
virtual void fun() = 0;

This will make Base an abstract base class, requiring subclasses (A and B) to implement their own version of fun().

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class A : public Base {
public:
    A() {}
    void print() { cout << "A\n"; }
    void fun() { cout << "fun in A\n"; }
};

class B : public Base {
public:
    B() {}
    void print() { cout << "B\n"; }
    void fun() { cout << "fun in B\n"; }
};


Now your loop need not be concerned with the specifics of the type.

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...
    for (int i = 0; i < 10; ++i) {
        arrayOfBasePointers[i]->fun();
    }
cppstudent (7)
Yes thank you. This will be a quick fix but in a long run, class A or B may have new functions being implemented while the Base class is not always available for amendment (i.e. adding in new virtual functions).
Duthomhas (13206)
I believe this indicates a failure in your design pattern.

You should not be accessing derived class functions not present in the base class.
dfarmer (8)
I'm in agreement with Duoas; however, if you just want to see if it can be done, look into dynamic_cast. http://cplusplus.com/doc/tutorial/typecasting/
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