problem with forward declaration

[MarkoSan]

Hi to all!

I have some .h file with:

class ForwardDeclarationClass;    // forward declaration of class

class ForwardDeclarationClass;    // forward declaration of class

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In same header file there is:

class MyClass : public QGlWidget
{
    ....
    private:
        ForwardDeclarationClass* d;
}

class MyClass : public QGlWidget
{
    ....
    private:
        ForwardDeclarationClass* d;
}

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In the .cpp file I have:

...
...
...
class ForwardDeclarationClass
{
     ....
     ....
}

...
...
...
class ForwardDeclarationClass
{
     ....
     ....
}

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and after that there are defined all corresponding methods (in same .cpp file).

I try to compile this Qt project and I get following error:

forward declaration of `struct ForwardDeclarationClass'

forward declaration of `struct ForwardDeclarationClass'

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What is wrong?

[jacek]

Could you post the whole error message?

[MarkoSan]

Here is it:

CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'

CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'

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Namely:
- CMerchandizeBrowser is MyClass
- CMerchandizeBrowserPrivate is ForwardDeclarationClass

[jpn]

I suspect you're using a non-pointer or non-reference in the header. In this case a forward declaration is insufficient. Could this be the case? What does CMerchandizeBrowser.h:34 contain?

[MarkoSan]

Here is line:

class CMerchandizeBrowserPrivate;    // forward declaration

class CMerchandizeBrowserPrivate;    // forward declaration

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And later in the same file in the class CMerchandizeBrowser I have:

private:
    CMerchandizeBrowserPrivate* d;

private:
    CMerchandizeBrowserPrivate* d;

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[jpn]

But is that line 34? As you can see from the error message, it comes from line 34.

[MarkoSan]

Like I said, the line 34, where error occurs, is:

class CMerchandizeBrowserPrivate;    // forward declaration

class CMerchandizeBrowserPrivate;    // forward declaration

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[jacek]

Is that the only error message you get? What happens if you comment out that line #34?

[MarkoSan]

I get several errors after that:

CMerchandizeBrowser.cpp: In member function `int CMerchandizeBrowser::slideCount() const':
CMerchandizeBrowser.cpp:372: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setSlideCount(int)':
CMerchandizeBrowser.cpp:377: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `QSize CMerchandizeBrowser::slideSize() const':
CMerchandizeBrowser.cpp:382: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setSlideSize(QSize)':
CMerchandizeBrowser.cpp:387: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `int CMerchandizeBrowser::zoomFactor() const':
CMerchandizeBrowser.cpp:392: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setZoomFactor(int)':
CMerchandizeBrowser.cpp:397: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual QImage CMerchandizeBrowser::slide(int) const':
CMerchandizeBrowser.cpp:402: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::setSlide(int, const QImage&)':
CMerchandizeBrowser.cpp:407: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::setSlide(int, const QPixmap&)':
CMerchandizeBrowser.cpp:412: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `int CMerchandizeBrowser::currentSlide() const':
CMerchandizeBrowser.cpp:417: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setCurrentSlide(int)':
CMerchandizeBrowser.cpp:422: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::clear()':
CMerchandizeBrowser.cpp:427: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::render()':
CMerchandizeBrowser.cpp:432: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::showPrevious()':
CMerchandizeBrowser.cpp:438: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::showNext()':
CMerchandizeBrowser.cpp:443: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::showSlide(int)':
CMerchandizeBrowser.cpp:448: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::paintEvent(QPaintEvent*)':
CMerchandizeBrowser.cpp:481: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::resizeEvent(QResizeEvent*)':
CMerchandizeBrowser.cpp:486: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::timerEvent(QTimerEvent*)':
CMerchandizeBrowser.cpp:492: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp:493: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
mingw32-make[1]: *** [debug/CMerchandizeBrowser.o] Error 1
mingw32-make[1]: Leaving directory `C:/Documents and Settings/markofr/workspace/eROSystem'
mingw32-make: *** [debug] Error 2

CMerchandizeBrowser.cpp: In member function `int CMerchandizeBrowser::slideCount() const':
CMerchandizeBrowser.cpp:372: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setSlideCount(int)':
CMerchandizeBrowser.cpp:377: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `QSize CMerchandizeBrowser::slideSize() const':
CMerchandizeBrowser.cpp:382: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setSlideSize(QSize)':
CMerchandizeBrowser.cpp:387: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `int CMerchandizeBrowser::zoomFactor() const':
CMerchandizeBrowser.cpp:392: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setZoomFactor(int)':
CMerchandizeBrowser.cpp:397: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual QImage CMerchandizeBrowser::slide(int) const':
CMerchandizeBrowser.cpp:402: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::setSlide(int, const QImage&)':
CMerchandizeBrowser.cpp:407: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::setSlide(int, const QPixmap&)':
CMerchandizeBrowser.cpp:412: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `int CMerchandizeBrowser::currentSlide() const':
CMerchandizeBrowser.cpp:417: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::setCurrentSlide(int)':
CMerchandizeBrowser.cpp:422: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::clear()':
CMerchandizeBrowser.cpp:427: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::render()':
CMerchandizeBrowser.cpp:432: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::showPrevious()':
CMerchandizeBrowser.cpp:438: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::showNext()':
CMerchandizeBrowser.cpp:443: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `void CMerchandizeBrowser::showSlide(int)':
CMerchandizeBrowser.cpp:448: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::paintEvent(QPaintEvent*)':
CMerchandizeBrowser.cpp:481: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::resizeEvent(QResizeEvent*)':
CMerchandizeBrowser.cpp:486: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp: In member function `virtual void CMerchandizeBrowser::timerEvent(QTimerEvent*)':
CMerchandizeBrowser.cpp:492: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.cpp:493: error: invalid use of undefined type `struct CMerchandizeBrowserPrivate'
CMerchandizeBrowser.h:34: error: forward declaration of `struct CMerchandizeBrowserPrivate'
mingw32-make[1]: *** [debug/CMerchandizeBrowser.o] Error 1
mingw32-make[1]: Leaving directory `C:/Documents and Settings/markofr/workspace/eROSystem'
mingw32-make: *** [debug] Error 2

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but I assumed these errors are invoked because forward class declaration reports error.

[jacek]

It looks like you are missing #include directive in CMerchandizeBrowser.cpp. Where is CMerchandizeBrowserPrivate defined?

[MarkoSan]

CMerchandizeBrowserPrivate is defined in .cpp file. I think I will get some replies that defining class in .cpp file is not ok...

[Thomas]

Correct, you cannot define a class in a .cpp file.

[jacek]

CMerchandizeBrowserPrivate is defined in .cpp file.

Is the definition placed before the lines mentioned in error messages?

I think I will get some replies that defining class in .cpp file is not ok...

No, there's nothing wrong with it, but you won't be able to use that class outside that .cpp file.

[Thomas]

No, there's nothing wrong with it, but you won't be able to use that class outside that .cpp file.

I stand corrected. Yes you can. I was thinking of the accessability of those classes.

[MarkoSan]

Well, I've moved the class definition in .cpp file to the very beginning of the file, it compiles now!!!