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From: =?UTF-8?Q?Arne_Vajh=c3=b8j?= <arne@vajhoej.dk>
Newsgroups: comp.lang.java.programmer
Subject: Re: How to assign an object to another in it method?
Date: Thu, 13 Jul 2017 21:17:57 -0400
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On 7/13/2017 1:19 PM, Andreas Leitgeb wrote:
> 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.

You are correct that even virtual call change behavior
depending on type of the pointer when default values are
applied.

I would code the example a bit different:

#include <iostream>

using namespace std;

class P
{
public:
     void virtual foo(int v = 123) { cout << v << endl; }
};

class C : public P
{
public:
     void virtual foo(int v = 456) { P::foo(v); }
     void virtual bar() { cout << "Hi" << endl; }
};

int main()
{
     // old code
     P *o1 = new C();
     o1->foo();
     // new code
     C *o2 = new C();
     o2->bar();
     o2->foo();
     //
     return 0;
}

But it shows the same thing.

P *o1 = new C(); vs C *o2 = new C(); changes the behavior of the foo 
call. Aka non-polymorphic behavior for a virtual method.

But:
- you would get the same effect without default values for non-virtual 
methods
- how do you know that the method is virtual without noticing the 
default values?

Arne