GLib Object failed to create derivable type object, getting "GLib-GObject-CRITICAL: invalid unclassed type"

Question

I try to create a simple derivable type but i seem to miss something. When i try to call the virtual method, "MY_ITEM_CLASS(result)" throws the critical error.

Compile with

gcc `pkg-config glib-2.0 gtk4 --cflags` test_gtk.cpp -o test_gtk `pkg-config glib-2.0 gtk4 --libs` -lstdc++

The error message is

GLib-GObject-CRITICAL **: 16:01:24.795: invalid unclassed type '(null)' in class cast to 'MyItem'

#include <glib-object.h>

#include <iostream>
 
typedef struct _MyItem MyItem;
#define MY_TYPE_ITEM (my_item_get_type())
G_DECLARE_DERIVABLE_TYPE(MyItem, my_item, MY, ITEM, GObject)

struct _MyItemClass {
    GObjectClass parent;
    void (*print)(MyItem* self);
};
 
typedef struct { 
    const char* name;    
} MyItemPrivate;

auto static my_item_init (MyItem *object) -> void;
auto static my_item_class_init (MyItemClass* klass) -> void;

G_DEFINE_TYPE_WITH_PRIVATE (MyItem, my_item, G_TYPE_OBJECT)


auto static item__print(MyItem* self) -> void {
    auto priv = (MyItemPrivate*)my_item_get_instance_private(self);
    std::cout << (priv->name?priv->name:"unnamed") << std::endl;
}

auto static my_item_init (MyItem *object) -> void {
    auto priv = (MyItemPrivate*)my_item_get_instance_private(object);
    priv->name = "This is my name";
}

auto static my_item_class_init (MyItemClass* klass) -> void {
    GObjectClass *object_class = G_OBJECT_CLASS(klass);
    klass->print = item__print;
}



auto main() -> int {
    auto result = (MyItem*)g_object_new(MY_TYPE_ITEM, nullptr);
    MY_ITEM_CLASS(result)->print(result);
    g_object_unref(result);    
    return 0;
}

This seems more like c than c++. Like typedef struct _MyItem MyItem; etc — Richard
– Richard, Commented Apr 6, 2024 at 15:35
@user12002570 It is mostly C, largely because the code uses Glib (C library) instead of glibmm (C++ wrapper). On a related note, since it uses the g_object_* functions instead of Glib::Object, the "glib-object" tag looks inaccurate. I'll change it to a more accurate tag. — JaMiT
– JaMiT, Commented Apr 6, 2024 at 16:13
Looking at the history, this question was originally tagged for C instead of C++. Given that the question is about using a C library, the C tag would be good, if only the code did not use C++ features. If you replace <iostream> usage with C-style output and replace auto with explicit types, you could try again with the C tag. (Did I miss any other C++-isms?) — JaMiT
– JaMiT, Commented Apr 6, 2024 at 16:20

JaMiT · Accepted Answer · 2024-04-06 22:39:32Z

1

It does not work because instead of

MY_ITEM_CLASS(result)->print(result);

it must be

MY_ITEM_GET_CLASS(result)->print(result);

edited Apr 6, 2024 at 22:39

JaMiT

18.6k5 gold badges20 silver badges44 bronze badges

answered Apr 6, 2024 at 22:01

Lothar

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