How and when to use Q_DECLARE_METATYPE

Question

I need to cast a QSqlRecord to QVariant and back throughout my project. In order to do so I added

Q_DECLARE_METATYPE(QSqlRecord);

in the .h files of the classes which require the casting. I also have a base class from which several children inherit, in this case I assume it is sufficient to include the Q_DECLARE_METATYPE only once in the base class. I therefore have for example:

• widgetBaseClass: declares metatype
• widgetChildClass1: inherits widgetBaseClass does not declare metatype
• widgetChildClass2: inherits widgetBaseClass does not declare metatype
• myTableModel: declares metatype

When I try to run the program like this I get

Redefinition of 'QMetaTypeId<QSqlRecord>

from widgetBaseClass, pointing to the previous declaration in myTableModel. If on the other hand I remove the declaration I get:

static_assert failed "Type is not registered, please use the Q_DECLARE_METATYPE macro to make it known to Qt's meta-object system");

From how I understand the workings of Q_DECLARE_METATYPE this means that if i declare it, it result in an error because it was already declared somewhere else, but if I don't declare it I cannot cast from QVariant, because it does not recognise the object as a valid QVariant, what am I missing?

Answer 1

You should place Q_DECLARE_METATYPE(QSqlRecord) in only one header and then just include it everewhere it needed. Q_DECLARE_METATYPE(QSqlRecord) have to be outside any classes and namespaces. From Qt documentation:

Ideally, this macro should be placed below the declaration of the class or struct. If that is not possible, it can be put in a private header file which has to be included every time that type is used in a QVariant

Answer 2

You use Q_DECLARE_METATYPE on classes with public constructor, destructor, copy-constructor, so QSqlRecord fits. Just make sure that this macro is used only once per class declaration. Possibly you missed some #pragma once?

Use it on your own classes, after declaration of the class, outside namespace braces:

//mystruct.h
namespace MyNamespace
{
struct MyStruct
{
    int i;
    ...
};
}

Q_DECLARE_METATYPE(MyNamespace::MyStruct)

See documentation here: http://doc.qt.io/qt-5/qmetatype.html#Q_DECLARE_METATYPE