CODEWITHSUNDEEP
c++classpolymorphism
user6105649
user6105649

Reputation:

'Overriding' class member

quick question: I have a base class with a member variable and I'm trying to 'override' this value in a derived class. How do I do this exactly?

I tried:

class A
{
public:
    double i = 1;
};

class B : public A
{
public:
    double i = 2;
};


int main(int argc, char** argv)
{
    B* b = new B();
    A* a = b;
    std::cout << b->i << '\t' << a->i << std::endl;  // output 2    1
}

But the desired output is '2 2'. What am I doing wrong?

Thanks,

Niklas

Upvotes: 0

Views: 663

Answers (2)

some
some

Reputation: 363

There is even a simplier solution than Pete proposed.

Just define your class B like this:

class B : public A
{
public:
    B() { i = 2; }
}

So that class B will reuse the member i defined in its base (A) and its default value will be "overriden" to 2 so your test code will return "2 2".

That's how you 1) don't change the base class A (don't add that unnecessary int argument with a default value, some people even say that function parameters with default values is a bad practice in c++) and 2) don't add an overhead of virtual functions to your classes (which is also unnecessary in your simple code).

Upvotes: 0

Pete Becker
Pete Becker

Reputation: 76305

You can override virtual functions; you can't override data members. This class hierarchy has two different members named i, one in class A and one in class B.

If you want the derived class to change the value of i that's in the base class, just do it in the constructor: B() { i = 2; }. But a better approach would be to have a constructor for A that sets the value of i, and to call that from B:

A::A(int ii = 1) : i(ii) {}
B::B() : A(2) {}

Upvotes: 1

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