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  1. Qt
  2. QTBUG-33091

Q_PROPERTY READ returning pointers to the property's type are confusing

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Details

    • Bug
    • Resolution: Invalid
    • Not Evaluated
    • None
    • 5.0.2, 5.1.0
    • None
    • Windows 7, 8

    Description

      Hello, so there is a curious issue concerning Q_PROPERTY definitions with READ methods that return a reference to the read object rather than a copy of it. Here's what the docs claim concerning READ methods...

      "A READ accessor function is required. It is for reading the property value. Ideally, a const function is used for this purpose, and it must return either the property's type or a pointer or reference to that type"

      And yet Thiago's comment in the following bug seems completely contradictory to the part of that statement concerning the returning of references:

      https://bugreports.qt-project.org/browse/QTBUG-7407

      I ran across this issue by chance when exporting custom C++ classes to qml with the following error:

      "QMetaProperty::read: Unable to handle unregistered datatype..."

      The following are examples of the multiple ways I attempted to circumvent this error. When I found that the pointer solution worked while a reference version would not, I could tell something was wrong under the hood.

      Class1 {
      ...
      Q_PROPERTY(Class2 class2 READ getClass2)
      ...
      Class2& getClass2() {return m_class2;}
      ...
      private:
      Class2 m_class2;
      }
      

      When accessing class2 property in qml returns QMetaProperty::read error

      Class1 {
      ...
      Q_PROPERTY(Class2& class2 READ getClass2)
      ...
      Class2& getClass2() {return m_class2;}
      ...
      private:
      Class2 m_class2;
      }
      

      Produces moc error

      Class1 {
      ...
      Q_PROPERTY(Class2* class2 READ getClass2)
      ...
      Class2* getClass2() {return &m_class2;}
      ...
      private:
      Class2 m_class2;
      }
      

      Works as expected.

      Either the docs should be updated noting the deficiency in Q_PROPERTY's ability to handle READ methods which return references to avoid confusing users, or the backend should be appropriately updated to fix the root issue.

      Thank you,
      David

      Attachments

        1. readPropertyTest.rar
          6 kB
          David Chamberlain
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            ogoffart Olivier Goffart (Woboq GmbH)
            dchamberlain David Chamberlain
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            Dates

              Created:
              Updated:
              Resolved:

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