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...