Unary operator overload and inheritance

Jonax · #1 (**permalink**) January 1st, 2006, 12:29 AM

Hi guys, and a happy new year...

I'm having some trouble inheriting overloaded operators - I've put together this little piece of code to illustrate my grievance...

It all works fine when I overload the prefix-operator - but if I overload the postfix-operator in a derived class, I get into all kinds of trouble - please give me some pointers!

Code:

/* 
** Overloading and inheritance
** tested using overloads of unary operators
** 051231 Jonax
*/

#include <iostream.h>
#include <conio.h>

class cApple
{
protected:
    int iWormCount;
    float fTotalLengthOfWorms;
public:
    cApple()
    {
        iWormCount = 0;
        fTotalLengthOfWorms = 0;
    }

    cApple(int iWC, float fTLoW)
    {
        iWormCount = iWC;
        fTotalLengthOfWorms = fTLoW;
    }

    cApple operator ++ () 
    {
        return cApple(++iWormCount, ++fTotalLengthOfWorms);
    }

    void put()
    {
        cout << "Number of worms: " << iWormCount << endl;
        cout << "Total length of worms: " << fTotalLengthOfWorms << endl;
    }
};

class cBigApple:public cApple
{
private:
    float fWeightInKilos;
public:
    cBigApple() : cApple()
    {
        fWeightInKilos = 0;
    }

    cBigApple(int iWC, float fTLoW, float fWiK) : cApple(iWC, fTLoW)
    {
        fWeightInKilos = fWiK;
    }

    cBigApple operator -- () 
    {
        return cBigApple(--iWormCount, --fTotalLengthOfWorms, 0.0);
    }
};

class cBigRedApple:public cBigApple
{
private:
public:
    cBigRedApple() : cBigApple()
    {
    }

    cBigRedApple(int iWC, float fTLoW, float fWiK) : cBigApple(iWC, fTLoW, fWiK)
    {
    }

    cBigRedApple operator -- (int)
    {
        return cBigRedApple(iWormCount--, fTotalLengthOfWorms--, 0.0);
    }
};

int main(){
    cApple oMyApple;
    ++oMyApple;
    cout << "oMyApple:" << endl;
    oMyApple.put();

    cBigApple oMyBigApple;
    ++oMyBigApple;
    ++oMyBigApple;
    ++oMyBigApple;
    cout << "\noMyBigApple:" << endl;
    oMyBigApple.put();

    cBigRedApple oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    --oMyBigRedApple;
    oMyBigRedApple--;
    cout << "\noMyBigRedApple:" << endl;
    oMyBigRedApple.put();

    getch();
    return 0;
}

vector · #2 (**permalink**) January 1st, 2006, 02:34 AM

See the change
#include <iostream>

using namespace std;

class cApple
{
protected:
    int iWormCount;
    float fTotalLengthOfWorms;
public:
    cApple()
    {
        iWormCount = 0;
        fTotalLengthOfWorms = 0;
    }

    cApple(int iWC, float fTLoW)
    {
        iWormCount = iWC;
        fTotalLengthOfWorms = fTLoW;
    }

    cApple operator ++ ()
    {
        return cApple(++iWormCount, ++fTotalLengthOfWorms);
    }

    void put()
    {
        cout << "Number of worms: " << iWormCount << endl;
        cout << "Total length of worms: " << fTotalLengthOfWorms << endl;
    }
};

class cBigApple:public cApple
{
private:
    float fWeightInKilos;
public:
    cBigApple() : cApple()
    {
        fWeightInKilos = 0;
    }

    cBigApple(int iWC, float fTLoW, float fWiK) : cApple(iWC, fTLoW)
    {
        fWeightInKilos = fWiK;
    }

    cBigApple operator -- ()
    {
        return cBigApple(--iWormCount, --fTotalLengthOfWorms, 0.0);
    }
};

class cBigRedApple:public cBigApple
{

public:
    cBigRedApple() : cBigApple()
    {
    }

    cBigRedApple(int iWC, float fTLoW, float fWiK) : cBigApple(iWC, fTLoW, fWiK)
    {
    }

    cBigRedApple operator -- ()
    {
        return cBigRedApple(iWormCount--, fTotalLengthOfWorms--, 0.0);
    }
};

int main()
{
    cApple oMyApple;
    ++oMyApple;
    cout << "oMyApple:" << endl;
    oMyApple.put();

    cBigApple oMyBigApple;
    ++oMyBigApple;
    ++oMyBigApple;
    ++oMyBigApple;
    cout << "\noMyBigApple:" << endl;
    oMyBigApple.put();

    cBigRedApple oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    --oMyBigRedApple;
    oMyBigRedApple--;
    cout << "\noMyBigRedApple:" << endl;
    oMyBigRedApple.put();

    return 0;
}

Jonax · #3 (**permalink**) January 1st, 2006, 09:13 AM

Thanks Vector, but it doesn't quite solve my problem.

What I want is to overload both the prefix and the postfix versions of a unary operator. And I want to do it on different levels in a class / subclass hierarchy. The code you've shown me only overloads the prefix version and the compiler gives me the warning:

"Warn : unaryoverloadvector.cpp(94,21):Overloaded prefix 'operator --' used as a postfix operator"

vector · #4 (**permalink**) January 1st, 2006, 12:11 PM

The have both the versions of increment operators in your program...

Jonax · #5 (**permalink**) January 1st, 2006, 02:47 PM

In the code in my first post I had both

Code:

    cBigApple operator -- () 
    {
        return cBigApple(--iWormCount, --fTotalLengthOfWorms, 0.0);
    }

and

Code:

    cBigRedApple operator -- (int)
    {
        return cBigRedApple(iWormCount--, fTotalLengthOfWorms--, 0.0);
    }

But the compiler complains that the line in main reading

Code:

    --oMyBigRedApple;

is an "Error: UnaryOverload.cpp(98,19):Illegal structure operation"

vector · #6 (**permalink**) January 1st, 2006, 10:23 PM

Try making operator-- function of cBigApple a friend function

Jonax · #7 (**permalink**) January 2nd, 2006, 05:07 AM

Thanks Vector - the following seems to work:

Code:

/* Overloading and inheritance
** tested using overloads of unary operators
** 051231 Jonax
*/

#include <iostream.h>    // cout,cin,endl
#include <conio.h>        // getche()
#include <iomanip.h>    // setw()
#include <string.h>        // strlen()

class cApple
{
protected:
    int iWormCount;
    float fTotalLengthOfWorms;
public:
    cApple()
    {
        iWormCount = 0;
        fTotalLengthOfWorms = 0;
    }

    cApple(int iWC, float fTLoW)
    {
        iWormCount = iWC;
        fTotalLengthOfWorms = fTLoW;
    }

    cApple operator ++ () 
    {
        return cApple(++iWormCount, ++fTotalLengthOfWorms);
    }

    void put()
    {
        cout << "Antal orme: " << iWormCount << endl;
        cout << "Længde: " << fTotalLengthOfWorms << endl;
    }
};

class cBigApple:public cApple
{
private:
    float fWeightInKilos;
public:
    cBigApple() : cApple()
    {
        fWeightInKilos = 0;
    }

    cBigApple(int iWC, float fTLoW, float fWiK) : cApple(iWC, fTLoW)
    {
        fWeightInKilos = fWiK;
    }

    friend cBigApple operator -- (cBigApple tmp) 
    {
        return cBigApple(--tmp.iWormCount, --tmp.fTotalLengthOfWorms, 0.0);
    }
};

class cBigRedApple:public cBigApple
{
private:
public:
    cBigRedApple() : cBigApple()
    {
    }

    cBigRedApple(int iWC, float fTLoW, float fWiK) : cBigApple(iWC, fTLoW, fWiK)
    {
    }

    cBigRedApple operator -- (int)
    {
        return cBigRedApple(iWormCount--, fTotalLengthOfWorms--, 0.0);
    }
};

int main(){
    cApple oMyApple;
    ++oMyApple;
    cout << "oMyApple:" << endl;
    oMyApple.put();

    cBigApple oMyBigApple;
    ++oMyBigApple;
    ++oMyBigApple;
    ++oMyBigApple;
    cout << "\noMyBigApple:" << endl;
    oMyBigApple.put();

    cBigRedApple oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    ++oMyBigRedApple;
    --oMyBigRedApple;
    oMyBigRedApple--;
    cout << "\noMyBigRedApple:" << endl;
    oMyBigRedApple.put();

    getch();
    return 0;
}

mikhailberis · #8 (**permalink**) February 7th, 2006, 05:20 PM

Quote:

quote:Originally posted by Jonax
Thanks Vector - the following seems to work:

Code:

...
    friend cBigApple operator -- (cBigApple tmp) 
    {
        return cBigApple(--tmp.iWormCount, --tmp.fTotalLengthOfWorms, 0.0);
    }
};
...

You might want to make the argument to operator-- a reference to the original object, and return a reference to the object itself. It might look like:

Code:

...
const cBigApple & operator -- (cBigApple & tmp) {
    iWormCount -= 1;
    fTotalLenthOfWorms -= 1;
    return *this;
}
...

This will make sure that the returned object is not a new instance, but rather the same instance with which you called the operator-- with.

This should apply to the other unary operators too.

Hope this helps.

Dean Michael C. Berris
Orange and Bronze Technologies, Inc.
Mobile +639287291459

vector · #9 (**permalink**) February 27th, 2006, 08:50 AM

Yup...agreed

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