Qt：如何继承QQuickPath类

Question

I would like to create new class MyPath which inherits QQuickPath . As far as I found out QQuickPath is private class. Is there any "clean" way of doing it?

My intent is creating custom QML type MyPath which has some extended functionality using QPainterPath atribute of QQuickPath (according to this link ).

Thanks for help.

Edit:

Qt Creator is now able to suggest QQuickPath but compilation fails.

I followed instructions posted by @michał-w-urbańczyk and have created this class header

#ifndef C_H
#define C_H

#include <QtQuick/private/qquickpath_p.h>
#include <QObject>

class C : public QQuickPath
{
    Q_OBJECT
public:
    C(QObject *parent = 0);
    ~C();

signals:

public slots:
};

#endif // C_H

and source file

 #include "c.h"

 C::C(QObject *parent)
  : QQuickPath(parent)
 {
 }

 C::~C()
 {
 }

and I got these errors:

c.o: In function `C::C(QObject*)':
../app/gui/c.cpp:4: undefined reference to `QQuickPath::QQuickPath(QObject*)'
c.o: In function `C::~C()':
../app/gui/c.cpp:8: undefined reference to `QQuickPath::~QQuickPath()'
moc_c.o: In function `C::qt_metacast(char const*)':
../app/moc_c.cpp:79: undefined reference to `QQuickPath::qt_metacast(char const*)'
moc_c.o: In function `C::qt_metacall(QMetaObject::Call, int, void**)':
../app/moc_c.cpp:84: undefined reference to `QQuickPath::qt_metacall(QMetaObject::Call, int, void**)'
moc_c.o:(.data.rel.ro+0x0): undefined reference to `QQuickPath::staticMetaObject'
moc_c.o:(.data.rel.ro._ZTV1C[_ZTV1C]+0x70): undefined reference to `QQuickPath::componentComplete()'
moc_c.o:(.data.rel.ro._ZTV1C[_ZTV1C]+0x78): undefined reference to `QQuickPath::classBegin()'
moc_c.o:(.data.rel.ro._ZTV1C[_ZTV1C]+0xa0): undefined reference to `non-virtual thunk to QQuickPath::classBegin()'
moc_c.o:(.data.rel.ro._ZTV1C[_ZTV1C]+0xa8): undefined reference to `non-virtual thunk to QQuickPath::componentComplete()'
moc_c.o:(.data.rel.ro._ZTI1C[_ZTI1C]+0x10): undefined reference to `typeinfo for QQuickPath'

Thanks

Answer 1

Short answer: such scenario is not supported by Qt so there is no "clean" way to do that. If you are really determined, it is possible to achieve when using the developer build of Qt.

Long answer: Qt does provide a support for accessing its private headers (where are the private classes like `QQuickPath1 defined) but it is not enough. Apparently it is not possible to inherit a private Qt class when using the official Qt binary distribution, as it does not seem to have necessary QQuickPath-related methods available.

I was however able to get this working by building the Qt from the official source code package. Depending on the configure options, the necessary methods might become accessible. I have built Qt with the -developer-build option passed to configure — it apparently defines the QT_BUILD_INTERNAL macro that exports symbols necessary for the non-Qt-code to be able to inherit Qt private classes.

The following comment from qglobal.h seems relevant:

/*
   No, this is not an evil backdoor. QT_BUILD_INTERNAL just exports more symbols
   for Qt's internal unit tests. If you want slower loading times and more
   symbols that can vanish from version to version, feel free to define QT_BUILD_INTERNAL.
*/

The whole thing is possible but obviously not recommended, as your application will require custom Qt binaries build. If you're ready to accept such drawback (along the other drawbacks of using Qt private headers like loosing binary compatibility with future Qt 5 releases), build the Qt on your own with the said flag defined and follow the instructions below.

First you need to add appropriate info to the .pro file, so the qmake will add private include directories to the includes path. Along with quick build option you need to provide quick-private . Because of the QQuickPath include dependencies, you'll need to add qml-private and gui-private as well. In the end, you'll end with something like this in your .pro file:

QT += qml quick quick-private qml-private gui-private

If you are not using qmake to build your project, then you'll need to add the path to the folder with priovate includes manually. The private includes are in the locations like: <QtDir>/include/<ModuleName>/<QtVersion>/<ModuleName>/private

Then simply add the include with QQuickPath to one of your files and define the derived type:

#include <QtQuick/private/qquickpath_p.h>

class MyPath : public QQuickPath
{
    Q_OBJECT
public:
    // any additional code
};