#include problems

[ibrewster]

I have three header files (and associated cpp files as well, of course) like the following:

calculator.h:

#ifndef CALCULATOR_H
#define CALCULATOR_H

#include "ui_calculator.h"

class calculator
{
   //various standard class stuff here
};

#endif // CALCULATOR_H 

ui_calculator.h:

#ifndef UI_CALCULATOR_H
#define UI_CALCULATOR_H

#include "calcentrywidget.h"

class Ui_calculator{
 //yada yada yada
};
{

and, finally, calcentrywidget.h:

#ifndef CALCENTRYWIDGET_H
#define CALCENTRYWIDGET_H

class calcEntryWidget
{
    //various class members
};

#endif // CALCENTRYWIDGET_H 

This all works well, compiles, runs, everyone is happy. Now I want to add a pointer to a calculator object to my calcEntryWidget class - basically a pointer back to its parent object. So, obviously, I have to add a #include "calculator.h" to the calcentrywidget.h file. And everything falls apart - the instant I add that #include, having made no other changes, compilation fails with a

ISO C++ forbids declaration of 'calcEntryWidget' with no type

error, followed by a slew of other errors stemming, I believe, from that one. What am I missing here? How can I add a calculator pointer to the calcEntryWidget class? Thanks.

[jsmith]

Forward declare calculator in calcentrywidget.h. You don't need to include the entire header for it.

[ibrewster]

Well, that seems to work, except that I still need to include calculator.h in the calentrywidget.cpp file for it to compile. So I needed the forward declaration in the header, and the include in the cpp. Is that right? Seems a bit redundant, but maybe that's just the way it is supposed to work. Thanks!

[jsmith]

Yes, that is the way it is supposed to work.

In general, if all you do is declare a pointer-to-type or reference-to-type in your header file, then you can and should forward declare type rather than include its header file. This not only helps eliminate circular inclusion problems (which in a large system can be much harder to find) but also helps reduce compile times (assuming dependencies are used).