Write function not working for Q_PROPERTY

raphasauer

Hello everyone. I'm trying to modify a property of a Q_GADGET using meta-objects. So far I can read the property's name and type, however when I try to read its value or set a value it does not work. Here's my code:

Q_GADGET:

#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

Here is my main:

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


template <class T>
void gadgetExplorer(T *ptr)
{
    const QMetaObject *mo = &ptr->staticMetaObject;
    qInfo() << "Q_GADGET: " << mo->className();
    QMetaProperty mp = mo->property(1);
    qInfo() << "Q_PROPERTY: " << mp.name();
    //Read player's speed
    QVariant i = mp.readOnGadget(&mp);
    qInfo() << "Q_PROPERTY_VALUE: " << i;
    i = 40;
    //Set a new value = 40
    bool operationSuccess = mp.writeOnGadget((void*)&mp, i);
    qInfo() << "WAS_WRITE_SUCCESSFUL? " << operationSuccess;

}


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

    return a.exec();
}

Here's what I get from the console:

Player speed before gadgetExplorer:  30
Q_GADGET:  Player
Q_PROPERTY:  m_speed
Q_PROPERTY_VALUE:  QVariant(int, 21940)
WAS_WRITE_SUCESSFUL?  true
Player speed after gadgetExplorer:  30

I was expecting to get a Q_PROPERTY_VALUE of 30, and, after the function is executed, to get a Player speed = 40. However, the results are wrong. What am I doing wrong?

Thanks for your time.

VRonin

The argument to readOnGadget/writeOnGadget should be the gadget, not the QMetaProperty.

QVariant i = mp.readOnGadget(ptr);
bool operationSuccess = mp.writeOnGadget(ptr, i);

KroMignon

@raphasauer said in Write function not working for Q_PROPERTY:

I was expecting to get a Q_PROPERTY_VALUE of 30, and, after the function is executed, to get a Player speed = 40. However, the results are wrong. What am I doing wrong?

Your QMetaProperty usage is wrong!

QVariant i = mp.readOnGadget(&mp); should be QVariant i = mp.readOnGadget(ptr);
bool operationSuccess = mp.writeOnGadget((void*)&mp, i); should be bool operationSuccess = mp.writeOnGadget((void*)ptr, i);

raphasauer

Thanks @VRonin and @KroMignon for your insights!