Pimpl implemenation - compilation error on static_cast base* to Derieved*

[sunil.thaha]

Hi,

I was trying out the pimpl idiom and its implementation in Qt.
I was wondering why the following code gives me an error in the data_pointer_test function. the error happens in both the static and the dynamic cast.

AFAIK, static cast should be able perform a downcast. as the following code does not give me any error

PrivateData *d = new QstObjectPrivate(0);     
    QstObjectPrivate *back = static_cast<QstObjectPrivate *>( d);

PrivateData *d = new QstObjectPrivate(0);     
    QstObjectPrivate *back = static_cast<QstObjectPrivate *>( d);

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Am I getting the basics wrong ?
Here is the Pimpl implementation code ...

p, li { white-space: pre-wrap; }   // --- Private Data ---
 class PublicInterface;
 class PrivateData {
     friend class PublicInterface;
 public:
     PrivateData( PublicInterface *iface )
         :_interface(iface){}
     virtual ~PrivateData(){
     }
 // ---- data ----    
     PublicInterface *_interface;
 };
   // --- Public Interface --- 
 class PublicInterface{
     friend class PrivateData;
 protected:
     inline PublicInterface( PrivateData *d ):_data( d ){
     }
 
     inline ~PublicInterface(){
         delete _data;
     }    
 // ---- data ----    
     PrivateData *_data;
 };
    // --- My Private Test Harness ---
 class QstObjectPrivate;
 class QstObject : public PublicInterface{
 public:
     QstObject();
 private:
     QstObjectPrivate* data_pointer_test() {
         return static_cast<QstObjectPrivate*>( _data );
     }
 };
  class QstObjectPrivate : public PrivateData{
public:
     QstObjectPrivate( PublicInterface *iface ):PrivateData( iface ){
     }
 };
   QstObject::QstObject()
     :PublicInterface( new QstObjectPrivate(this) ){
 }
    int main(){ 
     QstObject test;
 }

p, li { white-space: pre-wrap; }   // --- Private Data ---
 class PublicInterface;
 class PrivateData {
     friend class PublicInterface;
 public:
     PrivateData( PublicInterface *iface )
         :_interface(iface){}
     virtual ~PrivateData(){
     }
 // ---- data ----    
     PublicInterface *_interface;
 };
   // --- Public Interface --- 
 class PublicInterface{
     friend class PrivateData;
 protected:
     inline PublicInterface( PrivateData *d ):_data( d ){
     }
     
     inline ~PublicInterface(){
         delete _data;
     }    
 // ---- data ----    
     PrivateData *_data;
 };
    // --- My Private Test Harness ---
 class QstObjectPrivate;
 class QstObject : public PublicInterface{
 public:
     QstObject();
 private:
     QstObjectPrivate* data_pointer_test() {
         return static_cast<QstObjectPrivate*>( _data );
     }
 };
  class QstObjectPrivate : public PrivateData{
public:
     QstObjectPrivate( PublicInterface *iface ):PrivateData( iface ){
     }
 };
   QstObject::QstObject()
     :PublicInterface( new QstObjectPrivate(this) ){
 }
    int main(){ 
     QstObject test;
 }

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[sunil.thaha]

Is this very simple that no one want to answer ? or

[spud]

You can't define the function there, since at that point the compiler doesn't know that QstObjectPrivate inherits PrivateData you have to implement it at the end of the file:

...
class QstObject : public PublicInterface
{
public:
	QstObject();
private:
	inline QstObjectPrivate* data_pointer_test();
};
 
class QstObjectPrivate : public PrivateData
{
public:
	QstObjectPrivate( PublicInterface *iface )
		: PrivateData( iface )
	{
	}
};
 
QstObject::QstObject()
	: PublicInterface( new QstObjectPrivate(this) )
{
}
 
QstObjectPrivate* QstObject::data_pointer_test()
{
	return static_cast<QstObjectPrivate*>( _data );
}
...

...
class QstObject : public PublicInterface
{
public:
	QstObject();
private:
	inline QstObjectPrivate* data_pointer_test();
};

class QstObjectPrivate : public PrivateData
{
public:
	QstObjectPrivate( PublicInterface *iface )
		: PrivateData( iface )
	{
	}
};

QstObject::QstObject()
	: PublicInterface( new QstObjectPrivate(this) )
{
}

QstObjectPrivate* QstObject::data_pointer_test()
{
	return static_cast<QstObjectPrivate*>( _data );
}
...

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[sunil.thaha]

Thanks a lot !!
and that helped me make sense of why Qt's pimpl implementation uses reinterpret cast