CODEWITHSUNDEEP
c++pointerspolymorphism
aglasscage
aglasscage

Reputation: 25

c++ double pointer polymorphism

I'm trying to create an array of pointers using polymorphism. I will have the array of the superclass point to multiple subclasses. Is there anyway of doing this and still using a method from the subclass? Here's a sample code:

#include <iostream>


class Test1
{
public:
    Test1()
        : x(1)
    {}
    int get_x()
    {
        return x;
    }
private:
    int x;
};

class Test2 : public Test1
{
public:
    Test2()
        : y(2)
    {}
    void print()
    {
        std::cout << get_x() << ' ' << y << std::endl;
    }
private:
    int y;
};

class Test3 : public Test1
{
public:
    Test3()
        : y(3)
    {}
    void print()
    {
        std::cout << get_x() << ' ' << y << std::endl;
    }
private:
    int y;
};

int main()
{
    Test1** test = new Test1*[2];
    for (int i = 0; i < 2; i++)
    {
        if (i % 2 == 0)
        {
            test[i] = NULL;
            test[i] = new Test2;
        }
        else
        {
            test[i] = NULL;
            test[i] = new Test3;
        }
    }

    test[0]->print(); // ERROR. Is this even possible?
    test[1]->print(); // ERROR. Is this even possible?


    return 0;
}

Thank you I've only been coding for about 8 months.

Upvotes: 2

Views: 886

Answers (2)

James Adkison
James Adkison

Reputation: 9602

test[0]->print(); // ERROR. Is this even possible?

In general, yes. However, not with your code.

  1. If Test1 is going to be your base class and you're going to use new then it must have a virtual destructor (e.g., virtual ~Test1() {})
    • This is necessary for delete to work correctly when deleting a derived type via a pointer to the base class
  2. Any function you want to call using a pointer to the base class must exist on the base class (i.e., you need a print function in Test1)
  3. If you want the derived classes to have their own implementation of print then it must be declared virtual in the base class (e.g., virtual void print();)
    • If it dosn't make sense for Test1 to have an implementation of the print function then it can declare it pure virtual and not provide an implementation (e.g., virtual void print() = 0;) thus making it an abstract class

Upvotes: 2

Barry
Barry

Reputation: 302817

It would be if Test1 had such a member function named print. Your code doesn't compile as-is because Test::print doesn't exist. So at the very least, you will have to define that member function. For it to be truly polymorphic, as the intent of the question suggests, you should make Test::print a virtual member function so that Test2's and Test3's implementations of that function would get called instead:

class Test1 { 
    ...
    virtual void print() {
        std::cout << "Test1" << std::endl;
    }
    ...
};

For more information, see this tutorial on virtual functions.

Upvotes: 1

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