Overload of operator— (prefix)

Question

I tried overload operator-- prefix, but I have errors, Someone for help?

#include <iostream>
#include "Circulo.h"

using namespace std;

int main()
{
    //parametrizado
    Circulo c1(10.5, 20.0, 1, "Rojo Intenso"),c2, c3;
/*
    c2 = c1--;    // decrementa, si se puede, el diámetro

    c3 = c2++;    // incrementa, si se puede, el diámetro

    Circulo c4 = c3++;

    c4.CambiarColor("Amarillo patito flúo");


    cout << c1 << c2 << c3 << (c4 = c3) << endl;
*/
    return 0;
}

Circulo.cpp

#include "Circulo.h"
#include <string.h>

using namespace std;

Circulo :: Circulo() //por defecto
{
    x=5.0;
    y=6.0;
    diam=50;
    strcpy(Color,"Rojo");
}

Circulo::Circulo(float x,float y,unsigned diam,char *Color) //parametrizado
{
    this->x=x;
    this->y=y;
    this->diam=diam;
    this->Color= Color;
}

Circulo::Circulo(const Circulo& obj) //por copia
{
    this->x=obj.x;
    this->y=obj.y;
    this->diam=diam;
    this->Color=Color;
}

Circulo :: operator--(const Circulo &obj)const
{
    Circulo aux(this->real+obj.x);
    return aux;
}

Circulo.h

#ifndef CIRCULO_H
#define CIRCULO_H
#include <iostream>

using namespace std;

class Circulo
{
    public:
        Circulo();
        Circulo(float x,float y,unsigned diam,char *Color);
        Circulo(const Circulo& obj);
        Circulo operator --(const Circulo &obj) const;
    private:
        float x,y;
        unsigned diam;
        char *Color;

};

#endif // CIRCULO_H

error: postfix 'Circulo Circulo::operator--(const Circulo&) const' must take 'int' as its argument

C:\Documents and Settings\laboratorios\Mis documentos\Clase Circulo\Circulo.h|13|error: postfix 'Circulo Circulo::operator--(const Circulo&) const' must take 'int' as its argument|
C:\Documents and Settings\laboratorios\Mis documentos\Clase Circulo\Circulo.cpp|30|error: ISO C++ forbids declaration of 'operator--' with no type [-fpermissive]|
C:\Documents and Settings\laboratorios\Mis documentos\Clase Circulo\Circulo.cpp|30|error: postfix 'int Circulo::operator--(const Circulo&) const' must take 'int' as its argument|
||=== Build finished: 3 errors, 0 warnings (0 minutes, 0 seconds) ===|

Answer 1

When you overload an operator that is a member function (and not a friend ), you don't need to specify its parameter.

Circulo operator --(const Circulo &obj) const;

should be

Circulo operator --(); // probably not const, since you modify the instance

The error tells you that the compiler expects a (dummy) int , which is the convention used by the compiler to realize that you wanted to define the postfix operator--() ,

Circulo operator --(int); // dummy int, convention postfix

See Operator overloading (Unary arithmetic operators) for one of the best guides on operator overloading.

You invoke these operators on the current instance, like

Circulo c;
--c; // same as c.operator--();

so there is no need to specify a parameter.

Also, your implementation is incorrect (thanks @Praetorian), you don't need to make a copy of the object, but modify the instance directly, then return a reference to *this (for prefix operator--() ).