Destructing Glib::RefPtr causes failed assertions in the GTK 3 core

问题

The guys from Gtkmm are comparing Glib::RefPtr with std::auto_ptr<>:

Glib::RefPtr is a smartpointer. Specifically, it is a reference-counting smartpointer. You might be familiar with std::auto_ptr<>, which is also a smartpointer, but Glib::RefPtr<> is much simpler, and more useful.

But for some strange reason, I can't get my work done with the RefPtr. The same code is just fine with a auto_ptr.

In the following code, SmartPtr is just a placeholder for one of these two smartpointers.

#include <gtkmm.h>
#include <iostream>
#include <tr1/memory>

struct WindowHolder {
  SmartPtr<Gtk::Window> ptr;

  WindowHolder()
    : ptr(new Gtk::Window)
  {
    ptr->signal_delete_event().connect(sigc::mem_fun(*this, &WindowHolder::reset));
    ptr->show_all();
  }

  bool reset(GdkEventAny* event)
  {
    Gtk::Main::quit();
  }
};

int main(int argc, char *argv[])
{
  Gtk::Main kit(argc, argv);
  WindowHolder w;
  kit.run();
}

When compiling, I first define SmartPtr as Glib::RefPtr and then as std::auto_ptr.

$ g++ '-DSmartPtr=Glib::RefPtr' `pkg-config --cflags --libs gtkmm-3.0` main.cc && ./a.out 
(main:22093): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed
$ g++ '-DSmartPtr=std::auto_ptr' `pkg-config --cflags --libs gtkmm-3.0` main.cc && ./a.out 
$

The problem is this GLib-GObject-CRITICAL. In my real application, this isn't just a single line but a whole bunch of them. In the second version with std::auto_ptr everything gets destructed well.

Strange enough the code it is just fine in GTK 2:

$ g++ '-DSmartPtr=Glib::RefPtr' `pkg-config --cflags --libs gtkmm-2.4` main.cc && ./a.out 
$

I don't want to depend on std::auto_ptr because it is deprecated and I also don't want to work with a raw pointer because then the destructors have to manually delete the pointers which adds extra complexity ...

My questions are:

Why causes Glib::RefPtr this "critical warning" (probably a double free)?
Why does it work with gtkmm 2.4 but not in 3.0?
Can I fix the code with Glib::RefPtr and gtkmm 3.0?
How should I handle such situations in general?

回答1:

The reference count is too low, and you can fix it by adding a ptr->reference() after ptr->show_all(). I have an explanation, but take it with a grain of salt:

Glib::RefPtr doesn't increment the reference count of its object initially.
The GtkWindow will have a reference count initially of 1.
When your window is closed, the library decrements the reference count of its GtkWindow once.
Since the GtkWindow's count is zero, it is destroyed.
kit.run() seeing there is no more windows, returns.
w goes out of scope and the RefPtr's object's count is decremented causing the error.

I can't really answer #2 or #4 unfortunately, as this area of gtk/gtkmm is still a little mysterious (to me).

Reference: http://www.gtkforums.com/viewtopic.php?t=2412

回答2:

Glib::RefPtr is not meant to be for general use. You should use it when the API forces you to, but not otherwise. GtkWindow (or Gtk::Window) has its own odd memory management which is not really compatible with RefPtr.

If you want a general purpose smartpointer, try std::shared_ptr or std::unique_ptr. Or you could find something in boost.

来源：https://stackoverflow.com/questions/10156565/destructing-glibrefptr-causes-failed-assertions-in-the-gtk-3-core

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