Re: Virtual operator overloads don't seem to work?
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Stuart Brockman writes:
On 22 Oct, 23:29, Sam <s...@email-scan.com> wrote:
Stuart Brockman writes:
Hi,
I don't quite get what is going on in this code example:
---------------------------
#include <iostream>
using namespace std;
class Base{
public:
virtual void Test(){
cout << "Base" << endl;
}
virtual bool operator==(const Base &other){
cout << "Base Comparison" << endl;
return false;
}
};
class Derived : public Base{
public:
void Test(){
cout << "Derived" << endl;
}
bool operator==(const Derived &other){
cout << "Derived Comparison" << endl;
return true;
}
};
int main(int argc, char** argv) {
Base a; //Create a base ob=
ject
a.Test(); //Outputs "Base" as exp=
ected
Derived b, c; //Create two derived objects
b.Test(); //Outputs "Derived" as =
expected
if(b==c) cout << "True" << endl; //Do=
es derived comparison and
returns true as expected.
Base *d=&b, *e=&c; //Create two base pointer=
s to derived objects
d->Test(); //Outputs "Derived" as =
expected
if(*d==*e) cout << "True" << endl; //Does ba=
se comparison and
returns false!?
return 0;
}
----------------------------
The output is:
Base
Derived
Derived Comparison
True
Derived
Base Comparison
Notice, that the line "d->Test()" works correctly, but the compariso=
n
on the next line does not. The compiler (g++ (GCC) 4.2.3 (Ubuntu
4.2.3-2ubuntu7) ) seems to be ignoring the virtual-ness of
Base::operator== .
That's because "bool operator==(const Derived &)" does not polymor=
phically
overload "bool operator==(const Base &)". In a derived class, the =
function's
signature must match the base class's virtual function, in order for i=
t to
be polymorphically overloaded (the function parameters must match). In =
your
case, above, you have two different functions, no different that void =
foo()
and void bar(). One does not overload the other.
=
Is this correct?
Hav=
e I made a mistake?
Yes, but a very natural one.
application_pgp-signature_part
< 1KViewDownload
Ahh... I see... So what should I change Derived::operator== to?
Clearly it would have to have a signature of bool
Derived::operator==(const Base &other), but how do I make sure that =
I
get correct behavior when doing a "Derived==Base" (I would prefer t=
hat
Base::operator== was called in this case)?
It depends on what your real intentions are. There are several ways of do=
ing
that. You can define both operator==() functions in the derived class=
, and
provide the appropriate logic, optionally define operator==(const Der=
ived &)
as a virtual function in the base class also. And, you can always use
dynamic_cast<>() to determine whether your const Base & object is actuall=
y
Derived. There are several ways of doing this, depending on what exactly =
you
need to do.
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