Use C++11 decltype where possible

There are various places glib uses __typeof__ for type safety, but
that's a GNUC extension. C++11 has standard decltype() that does a
similar job, at least for cases we care about.

This avoids C++ code to always have to cast return value of
g_object_ref() which was causing type kind of error:

error: invalid conversion from ‘gpointer’ {aka ‘void*’} to
‘GstElementFactory*’ {aka ‘_GstElementFactory*’} [-fpermissive]

glib/gmacros.h

@@ -236,6 +236,10 @@
      ((defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8))) || \
       defined(__clang__))
 #define glib_typeof(t) __typeof__ (t)
+#elif defined(__cplusplus) && __cplusplus >= 201103L
+/* C++11 decltype() is close enough for our usage */
+#include <type_traits>
+#define glib_typeof(t) typename std::remove_reference<decltype (t)>::type
 #endif
 
 /*

glib/tests/cxx.cpp

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2020 Xavier Claessens
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <glib.h>
+
+typedef struct
+{
+  int dummy;
+} MyObject;
+
+static void
+test_typeof (void)
+{
+#if __cplusplus >= 201103L
+  // Test that with C++11 we don't get those kind of errors:
+  // error: invalid conversion from ‘gpointer’ {aka ‘void*’} to ‘MyObject*’ [-fpermissive]
+  MyObject *obj = g_rc_box_new0 (MyObject);
+  MyObject *obj2 = g_rc_box_acquire (obj);
+  g_assert_true (obj2 == obj);
+
+  MyObject *obj3 = g_atomic_pointer_get (&obj2);
+  g_assert_true (obj3 == obj);
+
+  MyObject *obj4 = nullptr;
+  g_atomic_pointer_set (&obj4, obj3);
+  g_assert_true (obj4 == obj);
+
+  MyObject *obj5 = nullptr;
+  g_atomic_pointer_compare_and_exchange (&obj5, nullptr, obj4);
+  g_assert_true (obj5 == obj);
+
+  MyObject *obj6 = g_steal_pointer (&obj5);
+  g_assert_true (obj6 == obj);
+
+  g_clear_pointer (&obj6, g_rc_box_release);
+  g_rc_box_release (obj);
+#else
+  g_test_skip ("This test requires C++11 compiler");
+#endif
+}
+
+int
+main (int argc, char *argv[])
+{
+  g_test_init (&argc, &argv, NULL);
+
+  g_test_add_func ("/C++/typeof", test_typeof);
+
+  return g_test_run ();
+}

glib/tests/meson.build

@@ -14,6 +14,9 @@ glib_tests = {
   'collate' : {},
   'cond' : {},
   'convert' : {},
+  'cxx' : {
+    'source' : ['cxx.cpp'],
+  },
   'dataset' : {},
   'date' : {},
   'dir' : {},