Question

Why do I get “error C2504: ‘CEntity’ : base class undefined” errors, when all the relevant headers are included?

I have a CMap which does most of the heavy lifting:

// CMap.h
#ifndef _CMAP_H_
#define _CMAP_H_
#include "CEntity.h"
class CMap {
    public:
        CMap(void);
        void OnLoad();
        void OnRender();
        std::vector<CTile*> TileList;
};
#endif

One of the things in the CMap is a list of Tiles:

// CTile.h
#ifndef _CTILE_H_
#define _CTILE_H_
class CEntity; // forward declaration

class CTile {
    public:
        CTile(void);
        std::vector<CEntity*> EntityList;
        char Label[0];
};
#endif

Each Tile contains a list of Entities:

// CEntity.h
#ifndef _CENTITY_H_
#define _CENTITY_H_
class CEntity {
    public:
        CEntity(void);
        char Label[0];
};
#endif

There are also various children of CEntity:

// CEntity_Buggy.h
#ifndef _CENTITY_BUGGY_H_
#define _CENTITY_BUGGY_H_
#include "CEntity.h"
class CEntity_Buggy : public CEntity {
    public:
        CEntity_Buggy(void);    
};
#endif

Now, my main map loading routine works fine, as does the rendering routing, which happens to need to peek into the Tiles for some info:

// CMap.cpp
#include "CMap.h"

void CMap::OnLoad() {
    ...
}

void CMap::OnRender() {
    /* here would be some rendering code ... */

    std::vector<CTile*>::iterator i;
    for (i=this->TileList.begin(); i!=this->TileList.end(); ++i) {
        CTile* tile = *i;

        for(unsigned int i = 0; i < tile->EntityList.size(); i++) {
            label[0] = tile->EntityList[i]->Label[0];
        }
    }
}

That works fine, and in the full app so far, it draws the cells, and adds the labels from the ‘resident’ entities.
The problem comes when I want to put some specific entity subclasses into the system:

// CMap.h
#include "CEntity_Buggy.h" // add this to the header

// CMap.cpp
#include "CMap.h"

void CMap::OnLoad() {
    CEntity_Buggy buggy;
    buggy.OnLoad();
}

And now I get lots of \centity_buggy.h(18): error C2504: 'CEntity' : base class undefined errors, but I’m not sure why. My CEntity_Buggy.h includes CEntity.h.

The full current (and this is a first-C++ project, so it’s quite flawed) is available on GitHub, if that helps.

Answer 1

The specific cause here is, I suspect, this chain of includes:

• CEntity.h includes CCamera.h
• CCamera.h includes CMap.h
• CMap.h includes CEntity_Buggy.h

So CEntity.h is causing CEntity_Buggy.h to be included before CEntity has been defined. In this case it does not look like CMap.h really needs CEntity_Buggy.h – you could probably just include it in the cpp.

In general avoid including in .h files wherever humanly possible. Forward declaration is your friend 🙂