GTK+: Cannot initialize GtkWidget * data member

Question

I am trying to assign a GtkWidget pointer in my class within the constructor, however I am encountering run-time errors.

#include <gtk/gtk.h>

class MainWindowController
{
private:
  GtkWidget * appWindow;
  const gchar * windowTitle = "Window title";
public:
  MainWindowController(GtkApplication * app);
  ~MainWindowController();
  void show();
};

MainWindowController::MainWindowController(GtkApplication* app)
{
  //this works
  GtkWidget* window = gtk_application_window_new(app);

  //not this 
  //appWindow = gtk_application_window_new(app);

  /*adding it into the init list does not work either... I have tried
   MainWindowController::MainWindowController(GtkApplication* app)
      :appWindow(gtk_application_window_new(app)) {}
  */

  gtk_window_set_title(GTK_WINDOW(appWindow), windowTitle);
  gtk_window_set_default_size(GTK_WINDOW(appWindow), 500, 500);
}

The following errors are being reported at the terminal:

(main:156): GLib-GObject-WARNING **: instance with invalid (NULL) class pointer
(main:156): GLib-GObject-CRITICAL **: g_signal_handlers_destroy: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed
(main:156): GLib-GObject-WARNING **: instance with invalid (NULL) class pointer
(main:156): GLib-GObject-CRITICAL **: g_signal_handlers_destroy: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

Why doesn't this cause an error

GtkWidget* window = gtk_application_window_new(app);

but this does?

appWindow = gtk_application_window_new(app);

Update: Here is my main.cpp

#include <gtk/gtk.h>
#include "mainWindowController.hpp"

static void
activate (GtkApplication* app,
          gpointer        user_data)
{
  MainWindowController mainController(app);
}

int
main (int    argc,
      char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), 0, 0);
  g_object_unref (app);

  return status;
}

I am also compiling with: g++ pkg-config --cflags gtk+-3.0 -o main main.cpp pkg-config --libs gtk+-3.0 -std=c++0x

We'll need to see more code. Nothing seems wrong with what you posted. — andlabs
– andlabs, Commented Dec 27, 2016 at 20:05

rodrigo · Accepted Answer · 2016-12-27 23:55:22Z

2

You are not showing the full code, but I think that the problem is in this line:

MainWindowController mainController(app);

Here you create a local variable mainController of type MainWindowController and immediately after that, the variable is destroyed, as it goes out of scope.

Now, the details of what happens next depend on what the destructor of this class does. I'm guessing that you are doing something to appWindow that makes the object invalid (gtk_widget_destroy() maybe?). Or maybe you are trying to use the appWindow in a signal after the destructor is called, and that is undefined behavior.

answered Dec 27, 2016 at 23:55

rodrigo

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1 Comment

RedShirt Over a year ago

Spot on! Was my destructor, I was calling g_object_unref(appWindow)

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