[libcalamares] Introduce generic delete-later class

Don't bother with QScopeGuard just right now, since I can't
find a use-case in the Calamares codebase.

FIXES #1358

src/libcalamares/utils/RAII.h

@@ -0,0 +1,43 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   Copyright 2020, Adriaan de Groot <groot@kde.org>
+ *
+ *   Calamares is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   Calamares 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 General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with Calamares. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef UTILS_RAII_H
+#define UTILS_RAII_H
+
+#include <QObject>
+
+#include <type_traits>
+
+/// @brief Convenience to zero out and deleteLater of any QObject-derived-class
+template< typename T >
+struct cqDeleter
+{
+    T*& p;
+
+    ~cqDeleter()
+    {
+        static_assert( std::is_base_of<QObject, T>::value, "Not a QObject-class" );
+        if ( p )
+        {
+            p->deleteLater();
+        }
+        p = nullptr;
+    }
+};
+
+#endif

src/modules/netinstall/Config.cpp

@@ -23,6 +23,7 @@
 
 #include "network/Manager.h"
 #include "utils/Logger.h"
+#include "utils/RAII.h"
 #include "utils/Yaml.h"
 
 #include <QNetworkReply>
@@ -96,21 +97,6 @@ Config::loadGroupList( const QUrl& url )
     }
 }
 
-/// @brief Convenience to zero out and deleteLater on the reply, used in dataIsHere
-struct ReplyDeleter
-{
-    QNetworkReply*& p;
-
-    ~ReplyDeleter()
-    {
-        if ( p )
-        {
-            p->deleteLater();
-        }
-        p = nullptr;
-    }
-};
-
 void
 Config::receivedGroupData()
 {
@@ -123,7 +109,7 @@ Config::receivedGroupData()
 
     cDebug() << "NetInstall group data received" << m_reply->size() << "bytes from" << m_reply->url();
 
-    ReplyDeleter d { m_reply };
+    cqDeleter< QNetworkReply > d{ m_reply };
 
     // If m_required is *false* then we still say we're ready
     // even if the reply is corrupt or missing.