Best way to access a cpp structure in QML

[pra7]

@Wieland Thanks!!! for your suggestion after lot of reading and searching I got the answer ,Actually we can use structs or any object which is not derived from QObject by using Q_GADGET :

struct MyStruct {
    Q_GADGET
    int m_val;
    QString m_name1;
    QString m_name2;
    QString m_name3;
    QString m_name4;
    Q_PROPERTY(int val MEMBER m_val)
    Q_PROPERTY(QString name1 MEMBER m_name1)
    Q_PROPERTY(QString name2 MEMBER m_name2)
    Q_PROPERTY(QString name3 MEMBER m_name3)
    Q_PROPERTY(QString name4 MEMBER m_name4)
};

Then in my class i just replaced QVariant as below :

class MyClass:public QObject
{
    Q_OBJECT
    Q_PROPERTY(MyStruct mystr READ getMyStruct
                WRITE setMyStruct NOTIFY myStructChanged)

public:
    explicit MyClass(QObject *parent = nullptr);
    MyStruct strObj;

     // Edit: changed get function
     MyStruct getMyStruct() const
     {
         return strObj;
     }

// Edit: Added set function
     void setMyStruct(myStruct val)
        {
            mystr = val;
            emit myStructChanged();
        }
signals:
void myStructChanged();

}

Now in QML file i can just use classObj.mystr.name1 to access the members and i can just use classObj.mystr.name1 = "abc" to set the values.

[A Former User]

@pra7 Yes, that's possible, too. Just note that with that solution, MyStruct won't send notifications when a member changes. This is how basic QML types (e.g. color) are implemented.

[A Former User]

One more thing, you did this...

struct MyStruct {
    Q_GADGET 
    int m_val;
    QString m_name1;
    QString m_name2;
    QString m_name3;
    QString m_name4;
    Q_PROPERTY(int val MEMBER m_val)
    Q_PROPERTY(QString name1 MEMBER m_name1)
    Q_PROPERTY(QString name2 MEMBER m_name2)
    Q_PROPERTY(QString name3 MEMBER m_name3)
    Q_PROPERTY(QString name4 MEMBER m_name4)
};

... where you put Q_GADGET in the public section. This has the side effect that all the members that come after Q_GADGET are private now. That's because Q_GADGET is a macro that expands to...

#define Q_GADGET \
public: \
    static const QMetaObject staticMetaObject; \
    void qt_check_for_QGADGET_macro(); \
    typedef void QtGadgetHelper; \
private: \
    QT_WARNING_PUSH \
    Q_OBJECT_NO_ATTRIBUTES_WARNING \
    Q_DECL_HIDDEN_STATIC_METACALL static void qt_static_metacall(QObject *, QMetaObject::Call, int, void **); \
    QT_WARNING_POP \
    QT_ANNOTATE_CLASS(qt_qgadget, "") \
    /*end*/

... , see qobjectdefs.h.

So maybe better follow the usual convention to avoid surprises:

class MyStruct {
    Q_GADGET 
    Q_PROPERTY(int val MEMBER m_val)
    Q_PROPERTY(QString name1 MEMBER m_name1)
    Q_PROPERTY(QString name2 MEMBER m_name2)
    Q_PROPERTY(QString name3 MEMBER m_name3)
    Q_PROPERTY(QString name4 MEMBER m_name4)
public:
    int m_val;
    QString m_name1;
    QString m_name2;
    QString m_name3;
    QString m_name4;
};

[pra7]

@Wieland Thanks !! for pointing that .

[pra7]

@Wieland Is there any possibility that I can access structure inside a structure?

struct MyStruct {
Q_GADGET
int m_val;
QString m_name1;
QString m_name2;
QString m_name3;
QString m_name4;
MyNewStruct m_newStr; //** new Struct includes Q_GADGET macro and member definations. 

Q_PROPERTY(int val MEMBER m_val)
Q_PROPERTY(QString name1 MEMBER m_name1)
Q_PROPERTY(QString name2 MEMBER m_name2)
Q_PROPERTY(QString name3 MEMBER m_name3)
Q_PROPERTY(QString name4 MEMBER m_name4)

Q_PROPERTY(MyNewStruct newStr MEMBER m_newStr) //**Currently getting error 
};

[A Former User]

@pra7 said in Best way to access a cpp structure in QML:

//**Currently getting error

What error?

[Praveen_2017]

This post is deleted!

[pra7]

@Wieland

error: no match for 'operator!=' (operand types are 'myStruct1' and 'myStruct1') if (_t->mynewstr != *reinterpret_cast< myStruct1*>(_v)) {

where Mystruct1 is myNewStruct and the error is in MOC ...

[A Former User]

The MOC generates some code for you to implement all the Gadget / Property stuff. Looks like the generated code uses the != operator for MyStruct. So you need to implement that:

public:
    bool operator==(MyStruct const &other) const;
    bool operator!=(MyStruct const &other) const;

bool MyStruct::operator==(const MyStruct &other) const
{
    // compare members 
    return true;
}

bool MyStruct::operator!=(MyStruct const &other) const
{
    return !(*this == other);
}

[pra7]

@Wieland That worked !!! Thanks, How to know that which all operators should be overloaded?

[A Former User]

@pra7 said in Best way to access a cpp structure in QML:

How to know that which all operators should be overloaded?

You can't really know. But when the compiler complains about a missing operator, just implement it. In this case we didn't really need the == operator, but I'd say it's common practice to implement the != operator using the == operator.

[pra7]

@Wieland Thanks for all your suggestions.

[Mammamia]

@pra7, @Wieland how can I access a list of Structure inside another Structure.
Eg: From the above example, I need to use

QList<MyNewStructure> m_StructList;

How is that possible? I have tried,
QList<MyNewStructure> m_StructList;

Q_PROPERTY(QList<MyNewStruct> newStr MEMBER m_newStr) but didnt help.

[seyed]

@pra7 but it does not sufficient! I had to use Q_DECLARE_METATYPE() and qRegisterMetaType<>() to avoid unknown type error. Did I missed something?