Compile time "null" using forward declaration? - c++

Question

I'm facing a problem using forward declaration, and I don't know how to fix it. Here's my files:
BubblePlug.h
#ifndef _BUBBLEPLUG_
#define _BUBBLEPLUG_
#include "IPlug_include_in_plug_hdr.h"
#include "resource.h"
#include "IControl.h"
class IPianoRoll;
class IMidiEngine;
class BubblePlug: public IPlug
{
private:
public:
IMidiEngine *pMidiEngine;
IPianoRoll *pPianoRoll;
BubblePlug(IPlugInstanceInfo instanceInfo);
~BubblePlug();
};
#endif // !_BUBBLEPLUG_
BubblePlug.cpp
#include "BubblePlug.h"
#include "IPlug_include_in_plug_src.h"
#include "IPianoRoll.h"
#include "IMidiEngine.h"
BubblePlug::BubblePlug(IPlugInstanceInfo instanceInfo) : IPLUG_CTOR(10, 1, instanceInfo) {
pPianoRoll = new IPianoRoll(this, 8, 8);
pMidiEngine = new IMidiEngine(this);
}
BubblePlug::~BubblePlug() {
delete pPianoRoll;
delete pMidiEngine;
}
IPianoRoll.h
#ifndef _IPIANOROLL_
#define _IPIANOROLL_
#include "IMidiEngine.h"
class IPianoRoll : public IControl
{
private:
BubblePlug *pBubblePlug;
public:
IPianoRoll(BubblePlug *bubbleplug, int x, int y) : IControl(bubbleplug, IRECT(x, y, x + 10, y + 10)), pBubblePlug(bubbleplug) {
}
~IPianoRoll() {
};
bool Draw(IGraphics *pGraphics) {
return true;
}
void Random(bool onlyScore = false) {
pBubblePlug->pMidiEngine->Init();
}
void Start() {
}
};
#endif // !_IPIANOROLL_
IMidiEngine.h
#ifndef _IMIDIENGINE_
#define _IMIDIENGINE_
class IMidiEngine
{
private:
BubblePlug *pBubblePlug;
public:
IMidiEngine(BubblePlug *bubbleplug) : pBubblePlug(bubbleplug) {
}
~IMidiEngine() {
};
void Init(bool randomScore = true) {
pSamplwhk->pPianoRoll->Start();
}
};
#endif // !_IMIDIENGINE_
when I compile, it says around pSamplwhk->pPianoRoll->Start();:
use of undefined type 'IPianoRoll'
left of '->Start' must point to class/struct/union/generic type
VS2015 find each element writing the code (I've no problem), it happens only when I compile (Build).
Why? I pass BubblePlug and I do forward of both IPianoRoll and IMidiEngine, including them in order (on BubblePlug.cpp).
IMidiEngine should know everythings about IPianoRoll (which it is included first).
At least, I should have problem at "runtime", why at compile?
Can you help me to understand the problem and how to fix it? Thanks.

Answer 1

IPianoRoll.h includes IMidiEngine.h, so no matter in which order you include the two files, the definition of IPianoRoll will always come after the init function where it is being used.
One way to avoid this is to move the body of the init function into a separate .cpp file:
In IMidiEngine.h:
void Init(bool randomScore=true);
In IMidiEngine.cpp:
void IMidiEngine::Init(bool randomScore) {
pSamplwhk->pPianoRoll->Start();
}