Re: inheritance, list of objects, polymorphism

James Kanze <>
Wed, 16 Dec 2009 08:08:29 -0800 (PST)
On Dec 16, 11:16 am, Vladimir Jovic <> wrote:

James Kanze wrote:

  > General rule: assignment and external copy don't work well with

inheritance. (In my own code, I've gradually been introducing a
PolymorphicObject base class, with a virtual destructor and a
private copy constructor and assignment operator. With the rule
that classes designed to be used polymorphically should inherit from

I do not understand why you said that "assignment and external copy
don't work well with inheritance."

In the case of polymorphic objects, the "value" of an object includes
its type, and you cannot change the type of a declared object. Given
something like:

    Base* p1 = new Derived1;
    Base* p2 = new Derived2;
    *p1 = *p2;

What should this mean? The same thing occurs with external copy: you
have to specify the name of the constructed type, something like:

    Base o( *p1 );

This constructs a Base, not a Derived1.

Can you provide a simple example how your code looks like using

    class Expression : public PolymorphicObject
        // ...

That's all that's necessary.

James Kanze

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