Is it possible to read a Q_PROPERTY with a QMetaObject pointer only?

raphasauer

Hello everyone. I'm tinkering a bit with QMetaObjects as of late, and I know you can read their properties using QMetaObjects readOnGadget(const void* gadget). However, let's assume I do not have a pointer to the gadget, I only have a QMetaObject of the gadget. Is there a way to do it? Here's my snippet of code:

Q_GADGET that makes the QMetaObject:

#ifndef EXAMPLE_H
#define EXAMPLE_H
#include <QObject>

struct Player {
    Q_GADGET

public:
    enum class CarType {
        NO_CAR = 0,
        SLOW_CAR,
        FAST_CAR,
        HYPER_CAR
    }; Q_ENUM(CarType)

    Q_PROPERTY(Player player READ getPlayer)
    Q_PROPERTY(int m_speed READ getSpeed WRITE setSpeed)
    Q_PROPERTY(CarType m_carType READ getCarType)

    Player getPlayer() { return *this;}
    int getSpeed() {return this->m_speed;}
    void setSpeed(int speed) {this->m_speed = speed;}
    CarType getCarType() { return m_carType;}


public:
    CarType m_carType;
    int m_speed;

}; Q_DECLARE_METATYPE(Player)

#endif

My main:

#include <QCoreApplication>
#include <iostream>
#include <QMetaObject>
#include "example.h"
#include <QDebug>
#include <QMetaProperty>


void gadgetExplorer(const QMetaObject *mo)
{
    qInfo() << "Q_GADGET: " << mo->className();
    QMetaProperty mp = mo->property(1);
    qInfo() << "Q_PROPERTY: " << mp.name();
    //Read player's speed
    //Is it possible to do it just with the meta object?
}


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    qRegisterMetaType<Player>();
    //Create a player and set the speed
    Player p1;
    p1.setSpeed(30);
    const QMetaObject *mo = &p1.staticMetaObject;
    qInfo() << "Player speed before gadgetExplorer: " << p1.getSpeed();
    gadgetExplorer(mo);
    qInfo() << "Player speed after gadgetExplorer: " << p1.getSpeed();

    return a.exec();
}

Thanks for your time.

kshegunov

Think about it, read a property of what? The data is in the object, the QMetaObject just facilitates the reflection capabilities for the class, so without an object there's nothing to read.

JKSH

@raphasauer said in Is it possible to read a Q_PROPERTY with a QMetaObject pointer only?:

I only have a QMetaObject of the gadget.

To add to @kshegunov's post: All instances of your QGadget share the same QMetaObject. You can check this yourself:

Player p1;
Player p2;

qDebug() << "Are they the same?" << (&p1.staticMetaObject == &p2.staticMetaObject)