[英]Solving this specific C++ diamond problem for Qt classes
I'm using QT's QQuickFramebufferObject
class which inherits from QQuickItem
in Qt library. 我正在使用QT的QQuickFramebufferObject
类,该类继承自Qt库中的QQuickItem
。
I have the following user-defined class: 我有以下用户定义的类:
class OpenGlBufferItem: public QQuickFramebufferObject
And I need my OpenGlBufferItem
class to also derive from ReactItem
. 我需要我的OpenGlBufferItem
类也可以从ReactItem
派生。 The problem is that ReactItem
ultimately derives from QQuickItem
too: 问题是ReactItem
最终也从QQuickItem
派生:
class ReactItem : public QQuickPaintedItem
because QQuickPaintedItem
inherits from QQuickItem
因为QQuickPaintedItem
继承自QQuickItem
So we have the following problem: 所以我们有以下问题:
QQuickItem
/ \
/ \
QQuickPaintedItem QQuickFramebufferObject
/ \
ReactItem OpenGlBufferItem
What I need is 我需要的是
QQuickItem
/ \
/ \
QQuickPaintedItem QQuickFramebufferObject
/ \
ReactItem /
\ /
\ /
OpenGlBufferItem
Normally, to solve diamond problems we'd simply declare some classes as virtually inheriting from others. 通常,为了解决钻石问题,我们只需声明某些类实际上是从其他类继承而来。 However, I cannot declare the classes RectItem
, QQuickPaintedItem
, QQuickFrameBufferObject
, because they are already given. 但是,我无法声明类RectItem
, QQuickPaintedItem
, QQuickFrameBufferObject
,因为它们已经给出了。
How should I proceed? 我应该如何进行?
UPDATE: 更新:
If I simply tryo to do 如果我只是尝试做
class OpenGlBufferItem: public QQuickFramebufferObject, public ReactItem
I get these kind of errors: 我得到这些错误:
Command failed: ./build.sh -e "modules/mediaplayer/desktop"
In file included from /home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/EWIEGA46WW/moc_OpenGlBufferQtQuick.cpp:9:0,
from /home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/mocs_compilation.cpp:2:
/home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/EWIEGA46WW/../../../../../../../../OpenGlBufferQtQuick.h: In constructor ‘OpenGlBufferItem::OpenGlBufferItem(QQuickItem*)’:
/home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/EWIEGA46WW/../../../../../../../../OpenGlBufferQtQuick.h:90:13: error: reference to ‘connect’ is ambiguous
connect(parent, SIGNAL(widthChanged()), this, SLOT(parentWidthChanged()));
^~~~~~~
In file included from /home/lz/Qt5.11.2/5.11.2/gcc_64/include/QtCore/QObject:1:0,
from /home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/EWIEGA46WW/../../../../../../../../OpenGlBufferQtQuick.h:3,
from /home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/EWIEGA46WW/moc_OpenGlBufferQtQuick.cpp:9,
from /home/lz/orwell/orwellJS/desktop/modules/mediaplayer/desktop/orwell_subdir/orwell_autogen/mocs_compilation.cpp:2:
/home/lz/Qt5.11.2/5.11.2/gcc_64/include/QtCore/qobject.h:308:13: note: candidates are: template<class Func1, class Func2> static typename std::enable_if<(QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1), QMetaObject::Connection>::type QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const QObject*, Func2, Qt::ConnectionType)
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
^~~~~~~
/home/lz/Qt5.11.2/5.11.2/gcc_64/include/QtCore/qobject.h:300:13: note: template<class Func1, class Func2> static typename std::enable_if<(QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1), QMetaObject::Connection>::type QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, Func2)
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
^~~~~~~
/home/lz/Qt5.11.2/5.11.2/gcc_64/include/QtCore/qobject.h:269:13: note: template<class Func1, class Func2> static typename std::enable_if<(((int)(QtPrivate::FunctionPointer<Func2>::ArgumentCount) >= 0) && (! QtPrivate::FunctionPointer<Func2>::IsPointerToMemberFunction)), QMetaObject::Connection>::type QObject::connect(const typename QtPrivate::FunctionPointer<Func>::Object*, Func1, const QObject*, Func2, Qt::ConnectionType)
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
^~~~~~~
/
and a lot more 还有更多
This is not possible. 这是不可能的。
I was thinking about using the CRTP (Curiously Recurring Template Pattern), which could theorhetically be employed if you can modify one of the classes and can handle its restrictions. 我正在考虑使用CRTP(好奇地重复模板模式),如果您可以修改其中一个类并可以处理其限制,则可以从理论上使用它。 But it will not work in your case as ReactItem is not deriving directly from QQuickItem, but from QQuickPaintedItem. 但这不适用于您的情况,因为ReactItem不是直接来自QQuickItem,而是直接来自QQuickPaintedItem。
Let's assume that was not the case: 假设情况并非如此:
ReactItem would need to be changed to look like this: ReactItem需要更改为如下所示:
template <class T> class ReactItem : public T {/*...*/};
The class hiearchy would look like this: 类hiearchy将如下所示:
QQuickItem
/ \
/ QQuickFramebufferObject
/ \
ReactItem<QQuickItem> \
\
ReactItem<QQuickFramebufferObject>
/
OpenGlBufferItem
The restrictions of this approach is, that the two ReactItem instantiations are unrelated types with regards to the type hierarchy. 这种方法的限制是,就类型层次结构而言,两个ReactItem实例是不相关的类型。 This means a lot of the code previously referring to ReactItem
might needs to be changed. 这意味着以前引用ReactItem
的许多代码可能需要更改。
This is quite easy to do for functions: 对于函数,这很容易做到:
void f(ReactItem* item) { item->setVisible(true); }
// becomes
template<class T> void f(ReactItem<T>* item) { item->setVisible(true); }
It is a lot trickier for templated data members like std::vector<ReactItem*>
. 对于像std::vector<ReactItem*>
这样的模板数据成员来说,这要麻烦std::vector<ReactItem*>
。
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