Qt-interest Archive, September 2007
reinterpret_cast for d_ptr
Message 1 in thread
I was looking for the definition of Q_D macro used frequently in qt. And
Q_D() macro is defined in qglobal.h header as:
#define Q_D(Class) Class##Private * const d = d_func()
And d_func() is:
#define Q_DECLARE_PRIVATE(Class) \
inline Class##Private* d_func() { return reinterpret_cast(d_ptr); } \
inline const Class##Private* d_func() const { return reinterpret_cast(d_ptr); } \
friend class Class##Private;
if one look at the definition d_func(), could see the d_ptr variable. In QObject
source code d_ptr is simply a QObjectData type variable declared as a protected
member of QObject class(qobject.h). Now when the above Q_D() macro is used:
Q_D(SpinBox);
would be:
QSpinBoxPrivate *const d = d_func
And d_func() becomes:
inline QSpinBoxPrivate* d_func() { return reinterpret_cast(d_ptr); }
Now my question is why use reinterpret_cast inside the definition of d_func? I
mean the safer static_cast could be used. Then why use something that is unsafe
and not necessary? Is there any hidden reason that i'm not aware of?
Thanks.
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Message 2 in thread
On 26.09.07 14:13:38, A D wrote:
>
> I was looking for the definition of Q_D macro used frequently in qt. And
> Q_D() macro is defined in qglobal.h header as:
>
> #define Q_D(Class) Class##Private * const d = d_func()
>
> And d_func() is:
>
> #define Q_DECLARE_PRIVATE(Class) \
> inline Class##Private* d_func() { return reinterpret_cast(d_ptr); } \
> inline const Class##Private* d_func() const { return reinterpret_cast(d_ptr); } \
> friend class Class##Private;
>
>
> if one look at the definition d_func(), could see the d_ptr variable. In QObject
> source code d_ptr is simply a QObjectData type variable declared as a protected
> member of QObject class(qobject.h). Now when the above Q_D() macro is used:
>
> Q_D(SpinBox);
>
> would be:
>
> QSpinBoxPrivate *const d = d_func
>
> And d_func() becomes:
>
> inline QSpinBoxPrivate* d_func() { return reinterpret_cast(d_ptr); }
>
> Now my question is why use reinterpret_cast inside the definition of d_func? I
> mean the safer static_cast could be used. Then why use something that is unsafe
> and not necessary? Is there any hidden reason that i'm not aware of?
Look into the archives, this exact same question was asked a few days
ago.
Andreas
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Message 3 in thread
>A D wrote:
>
> I was looking for the definition of Q_D macro used frequently in qt. And
> Q_D() macro is defined in qglobal.h header as:
>
> #define Q_D(Class) Class##Private * const d = d_func()
>
> And d_func() is:
>
> #define Q_DECLARE_PRIVATE(Class) \
> inline Class##Private* d_func() { return reinterpret_cast(d_ptr); } \
> inline const Class##Private* d_func() const { return reinterpret_cast(d_ptr); } \
> friend class Class##Private;
>
>
> if one look at the definition d_func(), could see the d_ptr variable. In QObject
> source code d_ptr is simply a QObjectData type variable declared as a protected
> member of QObject class(qobject.h). Now when the above Q_D() macro is used:
>
> Q_D(SpinBox);
>
> would be:
>
> QSpinBoxPrivate *const d = d_func
>
> And d_func() becomes:
>
> inline QSpinBoxPrivate* d_func() { return reinterpret_cast(d_ptr); }
>
>
>Now my question is why use reinterpret_cast inside the definition of d_func? I
>mean the safer static_cast could be used. Then why use something that is unsafe
>and not necessary? Is there any hidden reason that i'm not aware of?
>>
>>Andreas wrote:
>>Look into the archives, this exact same question was asked a few days
>>ago.
Thanks Andreas. The problem is now resolved.
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