Q_DECLARE_METATYPE doesn't work at all

Question

I'm trying to use custom classes in QAbstractListModel, and the Q_DECLARE_METATYPE doesn't work at all!

To test where the problem is, I've simplified the code as the following:

#include <QMetaType>
#include <QVariant>
#include <QDebug>

typedef int x;
Q_DECLARE_METATYPE(x)

void main() {
    QVariant v;
    qDebug() << v.canConvert<x>();
}

and the output is still false!

btw, the code I want to implement is like:

namespace ns{
    class a {
        public:
            a();    //default constructor
            a(const a&);    //copy constructor
            ~a();
    }
}
Q_DECALRE_METATYPE(ns::a);

and when I try to reimplement QAbstractListModel::data like this:

QList<ns::s> list;    //this is actually a member field of the custom model.
QVariant MyListModel::data(const QModelIndex& index, int role) const {      
    Q_UNUSED(role)
    return list.at(index.row());        
}

the compiler will report and error like:

cannot convert const ns::a to QVariant::Type

Answer 1

Your example is overly simplified as the docs quite clearly state that the class/struct being passed to Q_DECLARE_METATYPE must have a default constructor, a copy constructor and a public destructor: http://qt-project.org/doc/qt-5.0/qtcore/qmetatype.html#Q_DECLARE_METATYPE

That being said, here's a pretty simple example that shows Q_DECLARE_METATYPE working:

#include <QMetaType>
#include <QVariant>
#include <QDebug>

namespace MyNS {
    class MyClass {
    public:
        MyClass() : value(0) { }
        MyClass(int value) : value(value) { }
        MyClass(const MyClass &other) { value = other.value; }
        ~MyClass() { }
        int getValue() const { return value; }
    private:
        int value;
    };
};

Q_DECLARE_METATYPE(MyNS::MyClass);

int main(int argc, char *argv[])
{
    MyNS::MyClass m(15);
    QVariant v = QVariant::fromValue(m);
    qDebug() << v.canConvert<MyNS::MyClass>();
    qDebug() << v.value<MyNS::MyClass>().getValue();
}