Welcome!

The error message:

 "gtk-notebook-append-page: assertion 'GTK-IS-WIDGET (child)' failed" indicates that a GtkWidget cannot be cast into a GtkWidget.

Therefore, please check whether all GtkWidgets have been created as a GtkWidget.

GtkWidget *child = gtk_....._new();

GtkWidget *label = gtk_label_new();  ect

The widgets must be cast accordingly if they are to be used in a function.

Example:

gtk_label_set_label(GTK_LABEL(label),"my Text");

More code would be necessary for a more detailed troubleshooting.

Have fun programming.

Before I come to my reply, I would like to point out once again that consistent cast seems to be a problem of the given programme!!
Please check again thoroughly.

The other problem is that the function

gtk-notebook-remove-page (GTK-NOTEBOOK(notebook), page);

Also deletes the connected GtkWidget “child” or decrement the reference pointer by 1.

I solved the sample as follows:

/* move a page from notebook_1 to notebook_2 */
void move_page (GtkButton *button)
{
    gint page;
    GtkWidget *child;
    GtkWidget *label;
    GtkWidget *scroll_win;


    page = gtk_notebook_get_current_page(GTK_NOTEBOOK(notebook_1));
    g_print("Current_page ist %in",page);
    child = gtk_notebook_get_nth_page(GTK_NOTEBOOK(notebook_1), page);

    if (GTK_IS_WIDGET(child))
                g_print("child is GtkWidget %in",page);
        else
                g_print("child is not GtkWidget %in",page);

    g_object_ref(G_OBJECT(child));

    label = gtk_notebook_get_tab_label(GTK_NOTEBOOK(notebook_1), child);
    gtk_notebook_remove_page (GTK_NOTEBOOK(notebook_1), page);

    if (GTK_IS_WIDGET(child))
                g_print("child is GtkWidget %in",page);
        else
                g_print("child is not GtkWidget %in",page);



    gtk_notebook_append_page(GTK_NOTEBOOK (notebook_2), child, label);

    /* If I use this instead of child, no errors are thrown in the console and the
       label info is transferred to notebook_2
    scroll_win = gtk_scrolled_window_new();
    gtk_notebook_append_page(notebook_2, scroll_win, label);
    */
    gtk_widget_queue_draw(GTK_WIDGET(notebook_1));

}

That’s how it worked for me.

You can comment out g_object_ref(G_OBJECT(child)); times, you can do so, you also see how I came to it.

Have fun programming.

So I figured out a way to make this work, for the most part. If I connect to the “page_closed” signal and move the page in the callback, it works.

Unfortunately I seem to lose the label information, but no warnings/errors in the console and the page information is moved. Updated solution in C below:

#include <gtk/gtk.h>

GtkWidget *notebook_1;
GtkWidget *notebook_2;



void remove_page (GtkNotebook *nbook, GtkWidget *child, gint page)
{

    GtkWidget *label;
    GtkWidget *scroll_win;
    
    label = gtk_notebook_get_tab_label(nbook, child);

    gtk_notebook_append_page(GTK_NOTEBOOK (notebook_2), GTK_WIDGET(child), GTK_LABEL(label));
    

}    



/* move a page from notebook_1 to notebook_2 */
void move_page (GtkButton *button)
{
    gint page;
    GtkWidget *child;
    GtkWidget *label;
    GtkWidget *scroll_win;
    
    
    page = gtk_notebook_get_current_page(notebook_1);
    child = gtk_notebook_get_nth_page(notebook_1, page);
    label = gtk_notebook_get_tab_label(notebook_1, child);
    gtk_notebook_remove_page (notebook_1, page);
    
    
}


static void
activate (GtkApplication* app,
          gpointer        user_data)
{
    GtkWidget *window;
    GtkWidget *button;
    GtkWidget *table;

    GtkWidget *frame;
    GtkWidget *label;
    GtkWidget *checkbutton;
    int i;
    char bufferf[32];
    char bufferl[32];
    char istr[32];
    
    
    
    window = gtk_application_window_new (app);
    
    
    gtk_window_set_default_size (GTK_WINDOW (window), 400,400);
    table = gtk_grid_new();
    gtk_window_set_child (window, table);
    
    /* Create a new notebook, place the position of the tabs */
    notebook_1 = gtk_notebook_new ();
    gtk_notebook_set_tab_pos (GTK_NOTEBOOK (notebook_1), GTK_POS_TOP);
    gtk_grid_attach(GTK_GRID (table), notebook_1, 0,0,6,1);
    gtk_widget_show(notebook_1);
    
    /* lets append a bunch of pages to the notebook */
    for (i=0; i < 5; i++) {
        sprintf(bufferf, "Append Frame %d", i+1);
        sprintf(bufferl, "Page %d", i+1);
        
        frame = gtk_frame_new (bufferf);
        gtk_widget_set_margin_start(frame, 10);
        gtk_widget_set_margin_end(frame, 10);
        gtk_widget_set_margin_top(frame, 10);
        gtk_widget_set_margin_bottom(frame, 10);
        gtk_widget_set_size_request (frame, 100, 75);
        gtk_widget_show (frame);
        
        label = gtk_label_new (bufferf);

        label = gtk_label_new (bufferl);
        gtk_notebook_append_page (GTK_NOTEBOOK (notebook_1), frame, label);
    }
    
    
    /* now lets add a page to a specific spot */
    checkbutton = gtk_check_button_new_with_label ("Check me please!");
    gtk_widget_set_size_request(checkbutton, 100, 75);
    gtk_widget_show (checkbutton);
    
    label = gtk_label_new ("Add spot");
    label = gtk_label_new ("Add page");
    gtk_notebook_insert_page (GTK_NOTEBOOK (notebook_1), checkbutton, label, 2);
    
    /* Now finally lets prepend pages to the notebook */
    for (i=0; i < 5; i++) {
        sprintf(bufferf, "Prepend Frame %d", i+1);
        sprintf(bufferl, "PPage %d", i+1);
        
        frame = gtk_frame_new (bufferf);
        gtk_widget_set_margin_start(frame, 10);
        gtk_widget_set_margin_end(frame, 10);
        gtk_widget_set_margin_top(frame, 10);
        gtk_widget_set_margin_bottom(frame, 10);
        gtk_widget_set_size_request (frame, 100, 75);
        gtk_widget_show (frame);
        
        label = gtk_label_new (bufferf);

        
        label = gtk_label_new (bufferl);
        gtk_notebook_prepend_page (GTK_NOTEBOOK(notebook_1), frame, label);
    }
    
    /* Set what page to start at (page 4) */
    gtk_notebook_set_current_page (GTK_NOTEBOOK(notebook_1), 3);
    
    
    button = gtk_button_new_with_label ("move page");
    g_signal_connect (button, "clicked",
                               G_CALLBACK (move_page),
                               NULL);
    gtk_grid_attach(GTK_GRID(table), button, 6,1,1,1);
    gtk_widget_show(button);

    
    notebook_2 = gtk_notebook_new();
    gtk_notebook_set_tab_pos (GTK_NOTEBOOK (notebook_2), GTK_POS_TOP);
    gtk_grid_attach(GTK_GRID (table), notebook_2, 0,2,6,1);
    gtk_widget_show(notebook_2);
    

    gtk_notebook_set_group_name(notebook_2,"group");


    for (i=0; i < 5; i++) {
      sprintf(istr, "N2 %d", i);
      

        frame = gtk_frame_new (istr);
        gtk_widget_set_margin_start(frame, 10);
        gtk_widget_set_margin_end(frame, 10);
        gtk_widget_set_margin_top(frame, 10);
        gtk_widget_set_margin_bottom(frame, 10);
        gtk_widget_set_size_request (frame, 100, 75);
        gtk_widget_show (frame);
        
        label = gtk_label_new (istr);
        gtk_notebook_prepend_page (GTK_NOTEBOOK(notebook_2), frame, label);
    }

    gtk_notebook_popup_enable(notebook_2);

    g_signal_connect (notebook_1, "page_removed",
                               G_CALLBACK (remove_page),
                               NULL);


    gtk_widget_show(table);
    gtk_widget_show(window);


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

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

  return status;
}