Q_Property and reference data type

MasterQ

Hi,

I try to start with Qt's property mechanism.

The following is a first attemp

#ifndef DBCONTEXT_H
#define DBCONTEXT_H

#include "dbset.h"
#include "dokumententity.h"
#include "qsqldatabase.h"
#include "quoteentity.h"

#include <QObject>

class DBContext : QObject {
  Q_OBJECT
public:
  DBContext();
  ~DBContext();
  Q_PROPERTY(DBSet<DocumentEntity> Documents READ Documents CONSTANT)

  DBSet<DocumentEntity> Documents() const;

private:
  DBSet<DocumentEntity> m_Documents;
};
#endif // DBCONTEXT_H

and

DBSet<DocumentEntity> DBContext::Documents() const { return m_Documents; }

this seems to work, but...

... should it be a better way to give reference type to avoid unnecessary copying?

DBSet<DocumentEntity> &DBContext::Documents() const { return &m_Documents; }

And this fails with

dbcontext.cpp:29:62: Non-const lvalue reference to type 'DBSet<DocumentEntity>' cannot bind to a temporary of type 'const DBSet<DocumentEntity> *'

I am confused since the lvalue is not taken as const as in the declaration.

C++ const behaviour is still some miracle for me.

Any hints, how to deal with it? And how should Q_PROPERTY look like to deal with reference types?

greets

Joachim

JoeCFD

@MasterQ C++ does not allow overloading a function with different return types.
in your case, the two funcs are same, but return types are different.

MasterQ

@JoeCFD

sry, maybe my post was misleading

class DBContext : QObject {
  Q_OBJECT
public:
  DBContext();
  ~DBContext();

  Q_PROPERTY(DBSet<DocumentEntity> Documents READ Documents CONSTANT)
  Q_PROPERTY(DBSet<BillEntity> Bills READ Bills CONSTANT)

  DBSet<DocumentEntity> Documents() const { return m_Documents; }
  
  DBSet<BillEntity> &Bills() const { return &m_Bills; };

  private:
  const DBSet<DocumentEntity> m_Documents;
  const DBSet<BillEntity> m_Bills;

};

The first version of the getter (Documents) returns a full copy of the DBSet. This is, to my opinion, not a good solution. The second getter (Bills) returns a reference. While syntax of version 1 is OK, the error given above is thrown for version 2 with reference type.

Question:
Why is the lvalue not recognized as const in version2? How to do it right?

Another question: How to write the line with Q_PROPERTY to achieve a getter with returning reference type?

JoeCFD

@MasterQ said in Q_Property and reference data type:

DBSet<BillEntity> &Bills() const { return &m_Bills; };

const DBSet<BillEntity> &Bills() const { return &m_Bills; };

jsulm

@MasterQ said in Q_Property and reference data type:

DBSet<BillEntity> &Bills() const { return &m_Bills; };

This is wrong: you're trying to return a pointer, not reference!
Should be:

const DBSet<BillEntity> &Bills() const { return m_Bills; };

MasterQ

@jsulm

dbcontext.cpp:29:62: Binding reference of type 'DBSet<...>' to value of type 'const DBSet<...>' drops 'const' qualifier

I had tried this also before. But the main issue remains. Why is "const" dropped?

This is not simply a warning, it is an error!

--EDIT--

OK, I got you poltergeist!

I just deleted all and rewrote the whole thing new. Now it is working and no errors about dropping const appears anymore.

Just wasted a whole day by struggling with this bullsh*t

jsulm

@MasterQ said in Q_Property and reference data type:

Why is "const" dropped?

Because you were trying to return a non-const reference to a const object.