Trouble with Q_PROPERTY()

[RobM]

I have a method from a header file that I want to access from QML: foo() and here is how that method is defined:

extern bool foo();

in the header: TheHeader.h

So I have a "shell" code built by someone else and I want to add a new Q_PROPERTY() that will allow the front end (QML) to see if foo() is returning true or false, here is the "shell" code:

#pragma once
#include "UiControls/Presentation/Presenter.h"
namespace TheSpace
{
  namespace  TheCore
  {
    class SettingsInterface;
  }
  namespace Ui
  {
    class GuiSettings : public UiControls::Presenter
    {
        Q_OBJECT
        Q_PROPERTY(TheSpace::TheCore::SettingsInterface* model READ model CONSTANT)
        TheSpace::TheCore::SettingsInterface* m_model;
      public:
        GuiSettings(TheCore::SettingsInterface* _model);
        TheSpace::TheCore::SettingsInterface* model() const
        {
          return m_model;
        }
    };
  }
}

So I included the header and added the following:

#pragma once
#include "UiControls/Presentation/Presenter.h"
#include <TheHeader.h>

namespace TheSpace
{
  namespace  TheCore
  {
    class SettingsInterface;
  }
  namespace Ui
  {
    class GuiSettings : public UiControls::Presenter
    {
        Q_OBJECT
        //added
        Q_PROPERTY(bool fooBool READ fooBool)
        
        Q_PROPERTY(TheSpace::TheCore::SettingsInterface* model READ model CONSTANT)

        TheSpace::TheCore::SettingsInterface* m_model;

      public:
        GuiSettings(TheCore::SettingsInterface* _model);
        //added
        bool fooBool() const;

        TheSpace::TheCore::SettingsInterface* model() const
        {
          return m_model;
        }
      //added
      private:
        bool m_fooBool = false;
    };
  }
}

Then in the .cpp file:

#include "GuiSettings.h"
#include "TheCore/Interfaces/SettingsInterface.h"
#include <QtCore/QSettings>

namespace TheSpace
{
  namespace Ui
  {
    GuiSettings::GuiSettings(TheCore::SettingsInterface* _model)
      : m_model(_model)
    {
    }
  }
}

I added:

#include "GuiSettings.h"
#include "Core/Interfaces/SettingsInterface.h"
#include <QtCore/QSettings>

namespace TheSpace
{
  namespace Ui
  {
    GuiSettings::GuiSettings(TheCore::SettingsInterface* _model)
      : m_model(_model)
     //added
      ,m_fooBool(false)
    {
    }
    //added
    bool GuiSettings::fooBool() const
    {
      if(TheHeader::foo() == true)
      {
        return !m_fooBool;
      }
      else
      {
        return m_fooBool;
      }
    }
  }
}

the problem is that when I compile I get an error:

undefined reference to `TheHeader::foo()`

I am rusty with C++ and new with connect QML too C++ so if someone could tell me what I am doing wrong here?

[SGaist]

Hi,

It's a extern method named foo. Your call means "execute the foo method from class TheHeader".

[JonB]

@RobM said in Trouble with Q_PROPERTY():

extern bool foo();

If it's just written like that then TheHeader isn't relevant, you just want
if(foo() == true)

Or am I misunderstanding something? Is this a header file from a C library?

[RobM]

@SGaist I am not sure what to do. Are you saying I need to redefine the foo() method inside of the header file?

[SGaist]

Well, the first thing is: why is it declared as external ?

[RobM]

@SGaist I think it means I need to make a declaration of the function if I want to use it then I don't have to import the header?

"the extern keyword extends the the function’s visibility to the whole program, the function can be used (called) anywhere in any of the files of the whole program, provided those files contain a declaration of the function" -- GeeksForGeeks

but I tried that:

TheHeader::foo() {}

and I got:

GuiSettings.cpp:16:20: error: cannot define or redeclare 'foo' here because namespace 'Ui' does not enclose namespace 'TheHeader'

[SGaist]

I meant why are you using it like that ?
Where does that method come from originally ?

[RobM]

@SGaist Well it's declared in TheHeader.cpp

bool foo()
{
  return true
}

I am just trying to learn how to use it...

[JonB]

@RobM
Did you try just calling it via foo() like I said?

I assume this not inside any class. If it is, then you need to call on an instance. But I don't know why you'd have extern bool foo() inside a class in a header.

[RobM]

@JonB Yes that's the first thing I tried :(

use of undeclared identifier `foo`

[JonB]

@RobM
Perhaps you ought show more of TheHeader.h because I can't figure what's going on from your extracts.

[RobM]

I figured it out. I needed to include a reference in the .pro file