Private (hidden) QSharedData

Question

I have a class that has some data members that I want to be hidden from the caller (because including the headers for their types significantly increases the compile time, and it would require every project using this class to add an additional path to their include paths).

This class uses QSharedDataPointer to store this data. This way it can be copied by using the default copy constructor.

The basic structure of this class is:

class MyClass {
private:
  QSharedDataPointer<MySharedClassData> m_data;
};

Is there any fancy trick to do this without defining MySharedClassData (which inherits from QSharedData) in the same header file? Or is there any other good way of hiding data fields?

I've already tried a forward declaration of MySharedClassData but this didn't work (despite the fact that m_data is private).

The only solution I can currently thing of is to declare m_data as QSharedDataPointer<QSharedData> but then I need to cast the data member every time I want to access it. Is there a better solution?

Is [Pimpl](http://en.wikipedia.org/wiki/Opaque_pointer) idiom a thing I can think of in this case? — maverik, Feb 07 '12 at 12:27
@maverik: Yes. "The `QSharedDataPointer` class represents a `p`ointer to an `impl`icitly shared object." — MSalters, Feb 07 '12 at 12:35
Yes, this is exactly what I'm trying to do. Unfortunately this doesn't seem to work with `QSharedDataPointer`. Or at least I can't figure out, how to do this. — Sebastian Krysmanski, Feb 07 '12 at 12:35

score 6 · Accepted Answer · answered Feb 07 '12 at 12:35

6

The forward declaration should be working as long as you constructor and destructor are not defined in the header. The following class compiles on my computer:

#ifndef MAIN_WINDOW_HXX
#define MAIN_WINDOW_HXX

#include <QMainWindow>
#include <ui_MainWindow.h>

#include <QSharedDataPointer>

class MySharedClassData;

class MainWindow : public QMainWindow, private Ui_MainWindow {
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = 0, Qt::WindowFlags flags = 0);
    virtual ~MainWindow();

    QSharedDataPointer<MySharedClassData> m_data;

};

#endif

If you try to inline your constructor/destructor, then you might receive a: C2027: use of undefined type 'type' under VS.

answered Feb 07 '12 at 12:35

tibur

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This is the right hint. But in my case I also needed to manually implement the copy constructor and the assignment operator. – Sebastian Krysmanski Feb 07 '12 at 12:49
I can confirm that operator=() appears to be required for MSVC and possibly other compilers. Clang and GCC work without it. – Jake Petroules Oct 17 '12 at 23:16
I havn't known that i should declare the destructor (even empty implementation in the cpp file)! thanks!! – Brent81 Oct 23 '13 at 16:32

score 0 · Answer 2 · answered Feb 07 '12 at 12:38

0

Yes. There's no really fancy trick required. However, all methods that do need MySharedClassData must be defined after the definition of MySharedClassData. If you move the class definition to a .cpp file, the methods have to be moved there too.

answered Feb 07 '12 at 12:38

MSalters

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score 0 · Answer 3 · answered Feb 07 '12 at 12:40

In general, if you want to use the pimpl idiom with a smart pointer to a forward-declared impl (rather than managing the impl object manually), you need a smart pointer with an out-of-line deleter, like boost::shared_ptr (you should be able to use std::unique_ptr with a custom deleter, but I haven't tried).

The requirement is that you can instantiate the smart pointer template without seeing the impl class destructor: this rules out std::auto_ptr for example.

I don't know whether QSharedDataPointer meets the requirement or not, but tibur seems to say it does.

Private (hidden) QSharedData

3 Answers3