Simple inheritance problem in C++

Question

I am trying to understand inheritance in C++ properly.

Firstly, is it ok to have a class that returns instantiations of itself?

class Class1 {
public:   
   Class1 foo();
}

Basically, I have a class that derives equations so it takes an equation and returns an equation.

If I wanted some subclasses of Class1 that also return instantiations of themselves:

class Child : public Class1 {
public:
   Child bar();
}

and I wanted to use some of the functions of Class1 but instead of returning Class1's, I would want to return Child objects, would this be possible with inheritance?

Thank you, hopefully this question isn't too dumb.

Make a more complete example that shows what you want to do. — Kerrek SB
– Kerrek SB, Commented Jul 4, 2011 at 15:37
Why don't you try it out, plug the code into your compiler of choice and experiment. — Vinnyq12
– Vinnyq12, Commented Jul 4, 2011 at 15:38
Hey, I have tried the first bit (a class returning instantiations of itself) and that's fine but I wondering whether it's good/bad practice? The second but I don't know how it's done.. I'm currently still playing it.. will post any updates. — ale
– ale, Commented Jul 4, 2011 at 15:42

James Kanze · Accepted Answer · 2011-07-04 15:48:05Z

1

As written, there's no problem, but how do you intend to use it. Return by value involves copy, and copy and polymorphism don't generally go well together. Usually (but there are exceptions), it's preferable to return a pointer to a newly allocated instance. (You'll have to address memory management issues if you do this. If the logical meaning of the class is such that cycles are impossible, then you can use std::shared_ptr; otherwise, you'll have to do something else.)

answered Jul 4, 2011 at 15:48

James Kanze

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J T · Accepted Answer · 2011-07-04 15:51:28Z

Yes,

It looks to me like you are described a well defined problem, known in Desing Pattern parlance, as a Factory.

Consider the following:

class Class1 {
public:   
   static Class1 * getInstance( Equation * eq );

   virtual void foo() = 0;
}

class Child : public Class1 {
public:
   virtual void foo();
}

class OtherChild : public Class1 {
public:
   virtual void foo();
}

You would implement the foo() method differently for both children.

So, for example, you could:

int main(){

   Equation myEquation("x=y/4");

   Class1 * myInstance = Class1::getInstance ( &myEquation );

   myInstance->foo(); //would call the virtual method of the child class. You don't care what subclass, this was figured out by the "getInstance" method.

}

Igor · Accepted Answer · 2011-07-04 17:15:05Z

Firstly, is it ok to have a class that returns instantiations of itself?

Yes, it's OK, and in your case it sounds like a fine design. You can have the design that your proposed, or you can have a different design where your class will return a different object, e.g. class DerivedEquation, or you can also have a class Deriver which will take an Equation and return anEquation. All these designs are fine.

If I wanted some subclasses of Class1 that also return instantiations of themselves and I wanted to use some of the functions of Class1 but instead of returning Class1's, I would want to return Child objects, would this be possible with inheritance?

That's fine too. You can haveClass1::foo() and Child::bar() just as you proposed. Also, if you don't want to have 2 different function names, you can change the definition of foo to Class1 * foo() or Class1 & foo(), and then you will be able to overload it in Child.

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