invalid use of incomplete type struct...

[dragneel]

I have this error.
"Invalid use of incomplete type struct Operand forward declaration of struct Operand". i got the same error some days ago, so i asked somewhere but the code was correct and i resolved creating again those files. But now i have again this error and i can't get rid of it :( I have this error in these 2 files.

barrel.h

#include "operand.h"
#include <string>
using std::string;
 
//class Operand;  Error in both cases.
class Barrel: public Operand
{
        ...
        virtual void empty();
        virtual void fill();
        virtual Barrel* mix(Operand* op2);
 
    protected:
        double capacity, content, temperature;
};

c.h

    #include <string>
    #include <QtGui>
    #include "barrel.h"
    #include "calculatorwindow.h"
    #include "operand.h"
 
 
    using std::string;
 
    class CalculatorWindow;
    class Barrel;
    class Operand;
 
    class Calculator
    {
    public:
        ...
        Operand* operand1;
        Operand* operand2;
        double intoperand;
        string operation;
        string print;
        bool continue;
        ...
        void Fill(CalculatorWindow * calc);
        void Empty(CalculatorWindow * calc);
        Operand* Mix(CalculatorWindow * calc);
    };
 

Oh, i forgot it.
operand.h

#include <string>
using std::string;
 
class Operand
{
public:
    virtual string print();
    virtual Operand* Add(double a);
    virtual void Subtract(double a);
    virtual void empty();
    virtual void fill();
    virtual Operand* mix(Operand* op2);
};

[weaknessforcats]

This code:

//class Operand;  Error in both cases.
 class Barrel: public Operand
 {
          ...
          virtual void empty();
          virtual void fill();
etc...

requies that class Operand has an empty() and fill() method to override. The compiler can't tell if tis is true from a forward reference. You need to supply the full class definition of the Operand class.

A forward reference is only allowable when the compiler is asked to permit a pointer to a class object.

[dragneel]

I have even tried to use operand as an abstract class, but nothing. So you say that i must completely define every method of operand?

[weaknessforcats]

When you use a virtual function you are supposed to use a base class pointer with a derived object. Like this:

Operand* ptr = new Barrel;
 
ptr->fill();
 

The compiler cannot compile this code unless Operand has a fill() method. Thats's because ptr is an Operand*. There is no access to Barrel::fill() using this pointer.

There's no way to tell if there is a fill() method for Operand withut the full class declaration.

On the other hand, if you are doing this:

Barrel obj;
       obj.fill();
 

Then you are misusing polymotphism.

Those virtual functions in Barrel tell the compiler that if there is an Operand* variable being used for fill()and there is a choice between using Operand::fill() and Barrel::fill() that
Barrel::fill() is preferred.

Have a look at how a virtual function table (VTBL) works.

[dragneel]

Well, i already have the SAME virtual methods in both operand and barrel (except for virtual barrel* mix() in barrel.h instead of virtual operand* mix() )but i have the same error.

[weaknessforcats]

One problem is here:

include <string>
 2.     #include <QtGui>
 3.     #include "barrel.h"
 4.     #include "calculatorwindow.h"
 5.     #include "operand.h"
 

You barrel.h is before operand.h therefore when the compiler tries barrel.h it does not know about Operand yet.

Include operand.h before barrel.h.