Error with dynamic_cast - c++

Question

I'm having problems with my code with dynamic_cast. I have spent many hours trying to find a solution for this, but I still don't find the answer. I read that the problem could be because I didn't write forward declarations but I have already done that and still with the same problem.
Abstract class
#include "CRoute.h"
class CScreen
{
protected:
CRoute* m_pRoute;
public:
virtual ~CScreen();
virtual void connecToRoute(CRoute* route) = 0;
virtual void drawRoute() = 0;
};
Derived class
#include "CScreen.h"
class CGUIScreen : public CScreen
{
public:
void drawRoute();
void connecToRoute(CRoute* route);
};
Derived class
#include "CScreen.h"
class CCRTScreen : public CScreen
{
public:
void drawRoute();
void connecToRoute(CRoute* route);
};
Base Class
#include <string>
#include <iostream>
using namespace std;
class CScreen;
class CCRTScreen;
class CGUIScreen;
class CWaypoint
{
public:
CWaypoint();
void print(int format, CScreen* screenType);
};
Derived class
#include <iostream>
#include <string>
#include "CWaypoint.h"
using namespace std;
class CScreen;
class CCRTScreen;
class CGUIScreen;
class CPOI : public CWaypoint
{
public:
void print(int format, CScreen* screenType);
};
Method of CPOI
void CPOI::print(int format, CScreen* screenType)
{
if(dynamic_cast<CGUIScreen*>(screenType)) ---> Here is the error <<----
{
cout << "printing POI GUI " << endl;
}
else if(dynamic_cast<CCRTScreen*>(screenType)) ---> Here is the error <<----
{
cout << "printing POI CRT " << endl;
}
}
And the error I'm getting is the next one
..\myCode\CWaypoint.cpp:184:41: error: cannot dynamic_cast 'screenType' (of type 'struct CScreen*') to type 'struct CGUIScreen*' (target is not pointer or reference to complete type)
..\myCode\CWaypoint.cpp:184:44: error: expected unqualified-id before ')' token
..\myCode\CWaypoint.cpp:188:46: error: cannot dynamic_cast 'screenType' (of type 'struct CScreen*') to type 'struct CCRTScreen*' (target is not pointer or reference to complete type)

Answer 1

I read that the problem could be because I didn't write forward
declarations but I have already done that and still with the same
problem.
Quite the contrary; your forward declarations are what causes the errors.
A forward declaration, such as your class CScreen; line, simply tells the compiler: "There is a class called 'CScreen'. I'll give you more details later, but for now just keep in mind that this is a valid class name, OK?"
The compiler can then do very basic things with that class name; for example, it will accept pointer or reference declarations with it. That's why your print(int format, CScreen* screenType) line works. You don't need to know anything about CScreen other than its name to declare a pointer to it.
But how is the compiler supposed to accept a dynamic_cast with the class name? It does not really know anything about the class. In particular, it does not know that CGUIScreen or CCRTScreen are derived from CScreen. That's why at the point where you use dynamic_cast, the full class definitions are needed.
The header files for CWaypoint and CPOI (possible called waypoint.h and point.h?), can thus safely use forward declarations. As you correctly did:
waypoint.h:
class CScreen;
class CCRTScreen;
class CGUIScreen;
class CWaypoint
{
public:
CWaypoint();
void print(int format, CScreen* screenType);
};
point.h:
class CScreen;
class CCRTScreen; // not necessary but not invalid
class CGUIScreen; // not necessary but not invalid
class CPOI : public CWaypoint
{
public:
void print(int format, CScreen* screenType);
};
The implementation files, however, (possible called waypoint.cpp and point.cpp?), require the full definitions when you use a dynamic_cast:
point.cpp:
#include "point.h"
#include "screen.h"
#include "gui_screen.h"
#include "crt_screen.h"
#include <iostream>
using std::cout;
using std::endl;
void CPOI::print(int format, CScreen* screenType)
{
if(dynamic_cast<CGUIScreen*>(screenType))
{
cout << "printing POI GUI " << endl;
}
else if(dynamic_cast<CCRTScreen*>(screenType))
{
cout << "printing POI CRT " << endl;
}
}
By the way, it seems that CWaypoint should actually be an abstract base class, and that it possibly doesn't need an implementation file at all:
point.h:
class CScreen;
class CWaypoint
{
public:
virtual ~CWaypoint() {}
virtual void print(int format, CScreen* screenType) = 0;
};
P.S: If I may say so, I think your class names are confusing. A "Point" is definitely something more general than a "Waypoint", yet the inheritance relationship is exactly vice versa. Also, consider getting rid of Hungarian Notation. Just call your classes Screen instead of CScreen etc.

Answer 2

What the error message is telling you is that it does not know what the definition of CScreen or any of the derived classes are because you have forward declared them but not included their definitions.
Instead of
class CScreen;
class CCRTScreen;
class CGUIScreen;
Use
#include "CCRTScreen.h"
#include "CGUIScreen.h"