[SOLVED] How and when to use Q_DECLARE

How and when to use Q_DECLARE_METATYPE

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?

Solution

You should place Q_DECLARE_METATYPE(QSqlRecord) in only one header and then just include it everywhere it is needed.

Q_DECLARE_METATYPE(QSqlRecord) has 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.