与泛型混淆

Question

I have

class Foo : public QFrame {...}

and in the module when this class is used, I have

QWidget* screen = this->parentWidget();
Foo* foo = (Foo*) screen->findChild<QFrame*>("foo1"); // foo1 is the name of the control from .ui file

and this works. If I change it to

QWidget* screen = this->parentWidget();
Foo* foo = screen->findChild<Foo*>("foo1"); // foo1 is the name of the control from .ui file

I get this linker error

Error 4 error LNK2001: unresolved external symbol "public: static struct QMetaObject const Foo::staticMetaObject" (?staticMetaObject@Foo@@2UQMetaObject@@B) Foo.obj

What's the difference between the two snippets and why does the first one work and the second one doesn't?

Answer 1

This is probably because Q_DECLARE_METATYPE is missing on Foo class

From Qt Documenation

Adding a Q_DECLARE_METATYPE() makes the type 
known to all template based functions

Something like

class Foo : public QFrame
{
 //everything
}

Q_DECLARE_METATYPE(Foo)

Answer 2

Your class Foo is probably missing the Q_OBJECT macro. Add it and, if you use QMake, add the header to the HEADERS list and rerun qmake.