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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.java.programmer
Subject: Re: How to assign an object to another in it method?
Date: Thu, 13 Jul 2017 17:19:47 -0000 (UTC)
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Jerry Stuckle <jstucklex@attglobal.net> wrote:
> On 7/13/2017 6:08 AM, Andreas Leitgeb wrote:
>> I brought up the problem(imho) of overriding methods with
>> changed default values for arguments, which in C++ are static
>> type matter (non-polymorphic).
> Incorrect.  Methods with default values are also polymorphic.

#include <stdio.h>

struct A { virtual void foo(int i = 42) { printf("A: %d\n", i); } };
struct B : public A { void foo(int i = 54) { printf("B: %d\n", i); } };

int main() {
      A a;  a.foo();
      B b;  b.foo();
      A *pa = &a;  pa->foo();
      pa = &b;     pa->foo();
}

Compiling and running it yields:
A: 42
B: 54
A: 42
B: 42

The last one polymorphically(because of the 'virtual') calls B::foo(int),
but passss it A::foo's default value, as the pointer is A-typed.

So, I still consider my claim as correct.

>> Ideally, a subclass should be allowed to "remove" methods in a way that the
>> compiler could preempt some mistakes: "If you already know you're dealing
>> with a Square, then rather use Square's setDimension(int length) instead of
>> the inherited setDimension(int,int)".  Obviously the compiler couldn't catch
>> cases like ((Rectangle)square).setDimensions(1,2), so not all runtime
>> errors can be prevented at compile time, but at least some.
>
> So what happens when you pass the derived object to a method expecting a
> base object?

Then it would end up running that "disabled" method, which will do whatever
is coded into it:  e.g. always throw an error, or spend some effort to
determine still-sensible calls, for which to do something sensible.

In C++ you can declare an override as private - if it was virtual within
the super class, it can still be called through super class, but not through
its own type.

In the example above, add a "private:" into B's body before void foo...,
and since the b.foo() call becomes invalid, comment it out. The last call
will still invoke the now private B::foo (and still with 42 as default).

I don't like the non-polymorphic default values of C++, but I do like that
in C++ a subclass *can* get rid of its inherited baggage.