Re: Virtual operator overloads don't seem to work?

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++
Date:
Thu, 23 Oct 2008 18:49:10 +0200
Message-ID:
<6mbo84Ffru76U1@mid.individual.net>
Stuart Brockman wrote:

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 object
a.Test(); //Outputs "Base" as expected
Derived b, c; //Create two derived objects
b.Test(); //Outputs "Derived" as expected
if(b==c) cout << "True" << endl; //Does derived comparison and
returns true as expected.
Base *d=&b, *e=&c; //Create two base pointers to derived objects
d->Test(); //Outputs "Derived" as expected
if(*d==*e) cout << "True" << endl; //Does base 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
comparison 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
polymorphically overload "bool operator==(const Base &)". In a
derived class, the function's signature must match the base
class's virtual function, in order for it 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?


Yes.

Have 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
that Base::operator== was called in this case)?


You can do that with a free function:

bool operator==(const Derived& Left, const Base& Right)
{ return Right.operator==(Left); }

Bo Persson

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