Using template on a QWidget constructor

[raphasauer]

Hello everyone. I have created a QWidget class that should take a template pointer as an input. Doing so with a method works fine, however when I try to use it on the QWidget's constructor I get an error. Here's the code:

#ifndef GADGET_H
#define GADGET_H

#include <QObject>


struct P2D {
    Q_GADGET
    Q_PROPERTY(P2D p2d READ getP2D )
    Q_PROPERTY(float m_x READ getX WRITE setX)
    Q_PROPERTY(float m_y READ getY WRITE setY)

    P2D getP2D() {return *this;}
    float getX() {return this->m_x;}
    void setX(float x) {this->m_x = x;}
    float getY() {return this->m_y;}
    void setY(float y) {this->m_y = y;}

public:
    float m_x;
    float m_y;
}; Q_DECLARE_METATYPE(P2D)

#endif

Here's my main:

#include "mainwindow.h"
#include "gadget.h"

#include <QApplication>
#include <QDebug>
#include <QMetaObject>
#include <QMetaProperty>
#include <QVariant>
#include <QWidget>
#include "objectintrospector.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    qRegisterMetaType<P2D>();
    P2D point;
    point.m_x = 10;
    point.m_y = 25;

    objectIntrospector *oi = new objectIntrospector(nullptr, "P2D", &point);
    oi->show();
    return a.exec();
}

And here is my QWidget:

#ifndef OBJECTINTROSPECTOR_H
#define OBJECTINTROSPECTOR_H

#include <QWidget>

class objectIntrospector : public QWidget
{
    Q_OBJECT
public:
    template <class T>objectIntrospector(QWidget *parent = nullptr, const char* typeName =  nullptr, T *ptr = nullptr);
    ~objectIntrospector();

private:
    Ui::objectIntrospector *ui;
};

I get the following error:

undefined reference to `objectIntrospector::objectIntrospector<P2D>(QWidget*, char const*, P2D*)'

Is there a way to use a template on QWidget's constructor?

Thanks for your time.

[Christian Ehrlicher]

@raphasauer said in Using template on a QWidget constructor:

Is there a way to use a template on QWidget's constructor?

You would also get this function when you don't derive from QWidget since you nowhere define and instantiate objectIntrospector<P2D >::objectIntrospector(QWidget* const char* T*).

[raphasauer]

@Christian-Ehrlicher is it a problem with instantiating? Because I can get functions like this to run just fine:

template <class T>
const QMetaObject* metaConverter(T* ptr)
{
    const QMetaObject *mo = &ptr->staticMetaObject;
    return mo;
}

I was thinking that I could use the template on the constructor in a similar way.

[Christian Ehrlicher]

@raphasauer said in Using template on a QWidget constructor:

I was thinking that I could use the template on the constructor in a similar way.

You can but you do not in your example.

[raphasauer]

@Christian-Ehrlicher What would be the proper way to do it? I am resorting to templates because the constructor should receive a generic pointer to an object that contains a Q_GADGET. Or is there a better approach?

[Christian Ehrlicher]

As for any other template - make the implementation visible to the compiler when it instantiates the template as you did in your second example.

[raphasauer]

@Christian-Ehrlicher by implementation you are referring to objectIntrospector::objectIntrospector<P2D>(QWidget*, char const*, P2D*) ?
I wasn't using an explicit implementation because "P2D" could be any element that has a Q_GADGET on it.

[Christian Ehrlicher]

I never said you should do this. I said the compiler must see the code when it instantiates the template. Basic c++ stuff.

[raphasauer]

@Christian-Ehrlicher thanks! My C++ is a bit rusty, but I understand what you are saying now.

Basically I should declare the logic with the header, this way the compiler sees that there is an implementation.

[Christian Ehrlicher]

Correct.