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

From:
Sam <sam@email-scan.com>
Newsgroups:
comp.lang.c++
Date:
Wed, 22 Oct 2008 17:29:47 -0500
Message-ID:
<cone.1224714586.787762.25797.500@commodore.email-scan.com>
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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.

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