Qt. How to understand why undefined reference to a class?

Question

Qt

Greetings!
I don’t even know how to describe it, but I’ll try:
There are several classes that are large enough in one project,
these classes are in a separate folder (for convenience, I distribute the classes into folders)
As usual, I include productItem.h in the
header

#ifndef PRODUCTITEM_H
#define PRODUCTITEM_H
.....
#include "productitemrelatives.h"
#include "productitemmetavaluesmodel.h"

class ProductItem : public WBaseItem
.......

Qt generates
productitem.cpp:233 on build: error: undefined reference to `ProductItemRelatives::saveRelatives()
' for any use of ProductItemRelatives or ProductItemMetaValuesModel
The project was rebuilt, qmake was launched, moc files were deleted, pro.user was deleted, still the same error.
At the same time, the compilation in line passes the norms, in Windows there are such delights.
Qt 5.5, mingw
Tell me where to look?

Answer 1

in ProductItemRelativesexactly there статическийsaveRelatives?

Answer 2

Please provide a declaration of the ProductItemRelatives class and show how you call its methods from other code.