Struggling to bind a pointer to a member function to a T member of a class template then called in a slot

Question

I've been trying to create a connection(QObject::connect) at run time.

I wanted to pass a pointer to a member function to another class template. It complied successfully, but link errors raised at run time.

DynamicSlotCreator.h:

template <typename OBJ, typename FUNC>
class DynamicSlotCreator : public QObject
{
    Q_OBJECT;

private:
    OBJ obj;
    FUNC func;

public:
    DynamicSlotCreator(OBJ o, FUNC f) :
        obj(o), func(f)
    {}

public slots:

    void dynamicSlot()
    {
        (obj.*f)();
    }

};

main.cpp:

class Function
{
public:
    void greet()
    {
        qDebug() << "hello world";
    }
};


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QPushButton* pb = new QPushButton("Btn");
    pb->show();

    Function x;
    DynamicSlotCreator<Function, decltype(&Function::greet)> dsc(x, &Function::greet);

    QObject::connect(pb, SIGNAL(clicked()), &dsc, SLOT(dynamicSlot()));

    return a.exec();

}

ERRORS:

1>main.obj : error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl DynamicSlotCreator<class Function,void (__cdecl Function::*)(void)>::metaObject(void)const " (?metaObject@?$DynamicSlotCreator@VFunction@@P81@EAAXXZ@@UEBAPEBUQMetaObject@@XZ)
1>main.obj : error LNK2001: unresolved external symbol "public: virtual void * __cdecl DynamicSlotCreator<class Function,void (__cdecl Function::*)(void)>::qt_metacast(char const *)" (?qt_metacast@?$DynamicSlotCreator@VFunction@@P81@EAAXXZ@@UEAAPEAXPEBD@Z)
1>main.obj : error LNK2001: unresolved external symbol "public: virtual int __cdecl DynamicSlotCreator<class Function,void (__cdecl Function::*)(void)>::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@?$DynamicSlotCreator@VFunction@@P81@EAAXXZ@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z)

The problem is most likely [Why can templates only be implemented in the header file?](https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file?rq=1) Because of how Qt works, some of the implementation will be in auto-generated source files. — Some programmer dude, Nov 11 '19 at 09:51
I doubt you can use templates in this way. Signal/slot connection code generated by Qt moc tool **before** the compilation, while class template instantiated at compile time. — vahancho, Nov 11 '19 at 09:53
dont confuse runtime errors with linker errors. Linker errors happen long before you can run it — 463035818_is_not_an_ai, Nov 11 '19 at 09:56
`Q_OBJECT` is a macro that adds some functions, and these functions will be implemented in an automatically generated source file by the `moc` tool. — Some programmer dude, Nov 11 '19 at 09:58
@Someprogrammerdude alright Is there a possible way to do the same thing? — SpellTheif, Nov 11 '19 at 10:05
It is not possible to mix template and Q_OBJECT, see [answer](https://stackoverflow.com/questions/4397478/qt-templated-q-object-class) — Paolo, Nov 11 '19 at 10:14
Assuming you're using Qt5 why not just [pass a lambda to `QObject::connect`](https://doc.qt.io/qt-5/qobject.html#connect-4)? — G.M., Nov 11 '19 at 10:39

score 0 · Answer 1 · answered Nov 11 '19 at 10:25

Instead of a pointer to a function, you need to pass a pointer to QAction Something like this:

auto action = new QAction(myClass1);
connect(action, &QAction::triggered, myClass1, &MyClass1::myFunction);
... --> action
auto button = new QPushButton(myClass2);
connect(button, &QPushButton::clicked, action, &QAction::trigger);

Struggling to bind a pointer to a member function to a T member of a class template then called in a slot

1 Answers1