How can I cast Gtk::Widget to GtK::ScrolledWindow on gtkmm? - c++

Question

On the code bellow I am setting ScrolledWindows on a grid container and then setting their properties in a loop, because I want to put TextViews on the grid. I do not want to use TreeView because it will be ackward to put long or moderate texts there, which is what I am intending. Unfortunately the function get_child_at(int col,int row), for getting the attached widgets returns Gtk::Widget and for the properties I need I must do a cast for Gtk::ScrolledWindow; which I am not being able to do.
(some code)
descriptors->attach(* (new Gtk::ScrolledWindow ()), 0,0);
descriptors->attach(* (new Gtk::ScrolledWindow ()), 1,0);
descriptors->attach(* (new Gtk::ScrolledWindow ()), 2,0);
(some other code)
for(int i = 0; i < 3; i++)
{
//Glib::RefPtr<Gtk::ScrolledWindow> * disposable_pointer = new Glib::RefPtr<Gtk::ScrolledWindow>();
//disposable_pointer = descriptors->get_child_at (i,0);
//Glib::RefPtr<Gtk::ScrolledWindow> disposable_pointer = Glib::RefPtr<Gtk::Widget>::cast_dynamic(*descriptors->get_child_at (i,0));
//Glib::RefPtr<Gtk::ScrolledWindow>Glib::RefPtr<Gtk::ScrolledWindow>::cast_dynamic
//current_grid_child_scrolledWin = Glib::RefPtr<Gtk::ScrolledWindow>cast_dynamic(descriptors->get_child_at (i,0));
//current_grid_child_scrolledWin = disposable_pointer->get();
Glib::RefPtr<Gtk::Widget> * something = new Glib::RefPtr<Gtk::Widget>(descriptors->get_child_at (i,0));
Glib::RefPtr<Gtk::ScrolledWindow> disposable_pointer = *something;
current_grid_child_scrolledWin->set_vadjustment (Glib::RefPtr<Gtk::Adjustment>());
current_grid_child_scrolledWin->set_hadjustment (Glib::RefPtr<Gtk::Adjustment>());
current_grid_child_scrolledWin->set_policy (Gtk::PolicyType::POLICY_AUTOMATIC, Gtk::PolicyType::POLICY_AUTOMATIC);
current_grid_child_scrolledWin->set_vexpand (true);
current_grid_child_scrolledWin->set_hexpand (true);
current_grid_child_scrolledWin->set_margin_end (10);
current_grid_child_scrolledWin->set_margin_bottom (10);
current_grid_child_scrolledWin->set_visible (true);
}
the last error I got was
/usr/include/glibmm-2.4/glibmm/refptr.h:309:31: error: invalid conversion from 'Gtk::Widget*' to 'Gtk::ScrolledWindow*' [-fpermissive]
309 | pCppObject_(src.operator->())
| ^
| |
| Gtk::Widget*
I considered the gtkmm documentation on using casting with Glib::RefPtr

Answer 1

As we can see from the Gtk::Grid documentation, the return type of get_child_at is a raw pointer to a widget:
Widget* Gtk::Grid::get_child_at(int column,
int row
)
and not a smart pointer (in your case a Glib::Refptr). This is because the caller of get_child_at is not responsible to manage the lifetime of the object pointed to by the returned pointer. You simply get a handle, work on it and then leave it alone. Someone else is responsible to call delete on it. Here is how I would do it in your case:
#include <gtkmm.h>
#include <iostream>
int main(int argc, char *argv[])
{
// Initialize Gtkmm:
auto app = Gtk::Application::create(argc, argv, "so.question.q66162108");
// Create a grid and attach a scrolled window:
Gtk::Grid container;
Gtk::ScrolledWindow scrolledWindow;
// Add the scrolled window to the grid. Make sure the container is responsible
// of deleting it! Otherwise, remeber to call delete on it:
container.attach(*Gtk::manage(new Gtk::ScrolledWindow()), 0, 0, 1, 1);
// Here, you get a raw pointer to a widget:
Gtk::Widget* pWidget = container.get_child_at(0, 0);
// We cast it to its most derived type:
Gtk::ScrolledWindow* pScrolledWindow = dynamic_cast<Gtk::ScrolledWindow*>(pWidget);
// If the cast fail (i.e. not a ScrolledWindow at (0, 0), then the
// casted pointer will be set to nullptr:
if(pScrolledWindow)
{
std::cout << "Casting worked" << std::endl;
// Handle properties here...
pScrolledWindow->set_visible (true);
// Set other properties here...
}
return 0;
} // At this point the container is destroyed and since Gtk::manage
// was used, the scrolled window will be automatically deleted.
Notice that nowhere is this code I am using a smart pointer. This is because when I new my Gtk::ScrolledWindow, I use Gtk::manage*. This function marks the object (here the Gtk::ScrolledWindow) as owned by its parent container widget (Here the Gtk::Grid), so you don't need to delete it manually. I becomes the container's responsibility to call delete on its children when it goes away.
*If you use a newer Gtkmm version, you will have to use make_managed<T> instead. See here.