[libcalamares] add a convenience optional-setter for use with pointers

src/libcalamares/utils/RAII.h

@@ -14,6 +14,7 @@
 #include <QObject>
 #include <QSignalBlocker>
 
+#include <optional>
 #include <type_traits>
 
 /** @brief Convenience to zero out and deleteLater of any QObject-derived-class
@@ -58,4 +59,20 @@ struct cBoolSetter
 /// @brief Blocks signals on a QObject until destruction
 using cSignalBlocker = QSignalBlocker;
 
+/** @brief Writes a value on destruction to a pointed-to location.
+ *
+ * If the pointer is non-null, write the last-given-value if there
+ * is one to the pointed-to object.
+ */
+template < typename T >
+struct cPointerSetter
+{
+    std::optional< T > m_value;
+    T* m_pointer;
+
+    cPointerSetter( T* p ) : m_pointer(p) {}
+    ~cPointerSetter() { if ( m_pointer && m_value.has_value() ) { *m_pointer = m_value.value(); } }
+
+    const T& operator=(const T& v) { m_value = v; return v; }
+};
 #endif

src/libcalamares/utils/Tests.cpp

@@ -56,6 +56,7 @@ private Q_SLOTS:
 
     /** @brief Tests the RAII bits. */
     void testBoolSetter();
+    void testPointerSetter();
 
     /** @brief Tests the Traits bits. */
     void testTraits();
@@ -360,6 +361,50 @@ LibCalamaresTests::testBoolSetter()
     QVERIFY( b );
 }
 
+void
+LibCalamaresTests::testPointerSetter()
+{
+    int special = 17;
+
+    QCOMPARE( special, 17 );
+    {
+        cPointerSetter p( &special );
+    }
+    QCOMPARE( special, 17 );
+    {
+        cPointerSetter p( &special );
+        p = 18;
+    }
+    QCOMPARE( special, 18 );
+    {
+        cPointerSetter p( &special );
+        p = 20;
+        p = 3;
+    }
+    QCOMPARE( special, 3 );
+    {
+        cPointerSetter<int> p( nullptr );
+    }
+    QCOMPARE( special, 3 );
+    {
+        // "don't do this" .. order of destructors is important
+        cPointerSetter p( &special );
+        cPointerSetter q( &special );
+        p = 17;
+    }
+    QCOMPARE( special, 17 );
+    {
+        // "don't do this" .. order of destructors is important
+        cPointerSetter p( &special );
+        cPointerSetter q( &special );
+        p = 34;
+        q = 2;
+        // q destroyed first, then p
+    }
+    QCOMPARE( special, 34 );
+}
+
+
 /* Demonstration of Traits support for has-a-method or not.
  *
  * We have two classes, c1 and c2; one has a method do_the_thing() and the