Qt Quick: QML/C++ expression cannot be used as function? - c++

Question

could you take a look and explain, what am I doing wrong in code below?
#ifndef BACKEND_H
#define BACKEND_H
#include <QObject>
#include <QDebug>
#include <QtQml>
class BackEnd : public QObject
{
Q_OBJECT
Q_PROPERTY(int player READ player WRITE player_change)
QML_ELEMENT
int player;
public:
explicit BackEnd(QObject *parent = nullptr);
signals:
int player_changed(int player);
public slots:
int player_change(int player);
};
#endif // BACKEND_H
cpp:
#include "backend.h"
BackEnd::BackEnd(QObject *parent)
: QObject{parent}
{
}
int BackEnd::player_change(int player)
{
return (player == 1) ? 2 : 1;
}
Error:
Expression cannot be used as a function
Error appears in auto-edited moc file...
Removing Q_PROPERTY makes it fine...

Answer 1

The error says that the Q_PROPERTY macro can't use the member player as a function.
A READ accessor function is required if no MEMBER variable was
specified. It is for reading the property value. Ideally, a const
function is used for this purpose, and it must return either the
property's type or a const reference to that type. e.g.,
QWidget::focus is a read-only property with READ function,
QWidget::hasFocus().
You need to create a getter and use it as the READ function. Or you use MEMBER.
A MEMBER variable association is required if no READ accessor function
is specified. This makes the given member variable readable and
writable without the need of creating READ and WRITE accessor
functions. It's still possible to use READ or WRITE accessor functions
in addition to MEMBER variable association (but not both), if you need
to control the variable access.
Have a look here.
#ifndef BACKEND_H
#define BACKEND_H
#include <QDebug>
#include <QObject>
#include <QtQml>
class BackEnd : public QObject
{
Q_OBJECT
Q_PROPERTY(int foo READ foo WRITE setFoo NOTIFY fooChanged)
Q_PROPERTY(int bar MEMBER bar NOTIFY barChanged)
QML_ELEMENT
public:
explicit BackEnd(QObject *parent = nullptr) : QObject{parent} {}
int foo() const { return m_foo; }
void setFoo(int val) { m_foo = val; emit fooChanged(); }
signals:
void fooChanged();
void barChanged();
private:
int m_foo;
int bar;
};
#endif // BACKEND_H