Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.

Dariusz

Hey

I'm trying to learn how Qt is build so it has a private&public classes/headers that are able to inherit private-from-private / public-from-public and "work".

I've gone over https://community.kde.org/Policies/Library_Code_Policy and https://techbase.kde.org/Policies/Library_Code_Policy/Shared_D-Pointer_Example

But as far as I can tell... that is a little "outdated" and I'm not sure what is the current way of doing it.

What I understand is that I have to declare Q_DECLAER_PRIVATE(myClassName) in public header and in private header declare Q_DECLARE_PUBLIC(myClassName)

So that "appear" to work, but then when I try to compile I get > error C2065: 'q_ptr': undeclared identifier in my private header.

I can also see Q_DECLARE_PRIVATE_D but I've no idea what this one do o.o.
Is that meant to be used by inherited classes? So say if I do :

class myGraphicsItem : public QGraphicsItem {
Q_DECLARE_PRIVATE_D(QGraphicsItem ::d_ptr.data(), myGraphicsItem )
}

In which case, if I inherit from more than 1 class say >

class myGraphicsItem : public QObject, public QGraphicsItem{
Q_DECLARE_PRIVATE_D(QObject::d_ptr.data(), myGraphicsItem )
Q_DECLARE_PRIVATE_D(QGraphicsItem::d_ptr.data(), myGraphicsItem )
}

is this correct ?

How can I learn about this black magic O_O ?

TIA
Regards
Dariusz

jsulm

@Dariusz Check this: https://wiki.qt.io/D-Pointer

Dariusz

@jsulm thats awesome thank you!
But still stuck :- D
I made an example project > https://github.com/Dariusz1989/dPointerLearn
This currently gives me >

error C2143: syntax error: missing ';' before '*'
error C7524: 'inline' specifier cannot appear on a block-scope declaration or non-static data member
error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
error C2334: unexpected token(s) preceding '{'; skipping apparent function body
error C2327: 'extra::controllers::inheritedPrivate::inherited': is not a type name, static, or enumerator

I'm lost :- (
What do I do ?

JonB

@Dariusz
You should state which file and which line number the first error occurs on.

Dariusz

@JonB said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

@Dariusz
You should state which file and which line number the first error occurs on.

ops, my bad >

inherited_p.h(18): error C2143: syntax error: missing ';' before '*'
inherited_p.h(18): error C7524: 'inline' specifier cannot appear on a block-scope declaration or non-static data member
inherited_p.h(18): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
inherited_p.h(18): error C2334: unexpected token(s) preceding '{'; skipping apparent function body
inherited_p.h(20): error C2327: 'extra::controllers::inheritedPrivate::inherited': is not a type name, static, or enumerator

All of it point to

            Q_DECLARE_PUBLIC(inherited);

and

            inheritedPrivate(inherited *item);

TIA!

jsulm

@Dariusz said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

Q_DECLARE_PUBLIC(inherited);

Remove ;

Dariusz

@jsulm said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

@Dariusz said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

Q_DECLARE_PUBLIC(inherited);

Remove ;

Hmmm, now why? As far as I can tell QWidget_p.h has it declared?

also, it appears to "work" I've pushed git for the public.

Ok I got the Q_DECLARE_PUBLIC working as well, looks like I had circular include there.

JonB

@jsulm said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

Remove ;

OOI, why?! https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html#_M/Q_DECLARE_PUBLIC

#define Q_DECLARE_PUBLIC(Class)                                    \
    inline Class* q_func() { return static_cast<Class *>(q_ptr); } \
    inline const Class* q_func() const { return static_cast<const Class *>(q_ptr); } \
    friend class Class;

So it gets an extra ; at the end, so what? :)

Dariusz

@JonB said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

@jsulm said in Learning & Undertanding Q_DECLARE_PRIVATE/PUBLIC macros/sdk.:

Remove ;

OOI, why?! https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html#_M/Q_DECLARE_PUBLIC
#define Q_DECLARE_PUBLIC(Class)                                    \
    inline Class* q_func() { return static_cast<Class *>(q_ptr); } \
    inline const Class* q_func() const { return static_cast<const Class *>(q_ptr); } \
    friend class Class;
So it gets an extra ; at the end, so what? :)

oh you said to remove ; uhh ok, my bad :D

Ok so ! Multiple inheritance!

As far as I can tell when it comes to Q_DECLARE_PRIVATE_D I have to specify which base -private class I want to inherit. As it appears that I can only grab one.

        Q_DECLARE_PRIVATE_D(QObject::d_ptr.data(), inherited);
        //Q_DECLARE_PRIVATE_D(core::base::baseClass::d_ptr, inherited); // I can only inherit 1 base class?

With the code above, I can access QObject members, but core::base::baseClass are invalid. Where as if I swap the // around, and use baseClass as target, then I can access baseClass members, but QObject are invalid then.

I take in this case... I would have to make a "new" macro, something like Q_DECLARE_PRIVATE2_D & Q_DECLARE_PRIVATE3_D &Q_DECLARE_PRIVATE4_D etc etc & have multi base ptr storage as well like

            baseClassPrivate *d_ptr;
            baseClassPrivate *d_ptr2;
            baseClassPrivate *d_ptr3;
            baseClassPrivate *d_ptr4;

At which point I should look in to map logic and more complex macro. As well as my inherited(inheritedPrivate &d);would need to be more like inherited(QMap<QString,inheritedPrivate >&d); or something like that.

So that when I do Q_D / Q_Q I should be able to specify which baseClass source I want to use ?

I take Qt is fairly "strict" when it comes to sdk/d-pointers and thus there are no multi-inheritances issues. However if I want to inherit form both Qt & my own base class then I need to solve it ?

TIA

Dariusz

Welp I hit bu bu.
When I do

Q_D(const myCLass)
d->doStuff();

I get

Exception: Exception 0xc0000005 encountered at address 0x7ff62fe17f70: User-mode data execution prevention (DEP) violation at location 0x7ff62fe17f70

It happens on d->doStuff();

but if I do

auto d = static_cast<myClassPrivate*>(d_ptr);
d->doStuff();

then it all works :D SHISH!

sierdzio

If doStuff() is not const, then don't declare d pointer as const:

Q_D(myClass) // NO "const" here!
d->doStuff();