Can I use g_autoptr in a class declaration?

While following Add unique_ptr with custom deleter as class member I found that smart pointers were already implemented for GLib as g_autoptr - see GLib - Automatic Cleanup for details. Instead of:

#include <glib.h>
#include <memory>

GError* error = nullptr;
std::unique_ptr<GDBusConnection, decltype(&g_object_unref)> conn(g_bus_get_sync(G_BUS_TYPE_SYSTEM, nullptr, &error), &g_object_unref);
if (!conn) {
    print"Couldn't get bus: %s", error ? error->message : "unknown error");
    g_clear_error(&error);
}

I can just do:

#include <glib.h>

g_autoptr(GError) error = nullptr;
g_autoptr(GDBusConnection) conn = g_bus_get_sync(G_BUS_TYPE_SYSTEM, nullptr, &error);
if (!conn) {
    print"Couldn't get bus: %s", error ? error->message : "unknown error");
}

But if I try to do this as part of a class:

#include <glib.h>

class YourClass {
    YourClass();
    ~YourClass();

    ...
private:
    g_autoptr(GDBusConnection) _conn;
};

YourClass::YourClass() {
    g_autoptr(GError) error = nullptr;
    _conn = g_bus_get_sync(G_BUS_TYPE_SYSTEM, nullptr, &error);
    if (!_conn) {
        print"Couldn't get bus: %s", error ? error->message : "unknown error");
    }
}

I get an error from gcc:

error: 'cleanup' attribute ignored [-Werror=attributes]
|     g_autoptr(GDBusConnection) _conn;

Can g_autoptr not be used in a class declaration?

g_autoptr can't be used to declare class member variables. It's intended to use only in local scopes.

g_autoptr is not a smart pointer in terms of RAII. This is a macro that uses compiler specific extensions for registering cleanup handlers in function local scopes. This limits g_autoptr to use only with a single assignment. Reassignment doesn't free a previously assigned pointer.