util/Cast: reimplement as template without macro

src/output/plugins/httpd/HttpdInternal.hxx

@@ -152,19 +152,13 @@ public:
 	HttpdOutput(EventLoop &_loop);
 	~HttpdOutput();
 
-#if GCC_CHECK_VERSION(4,6) || defined(__clang__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Winvalid-offsetof"
+#if defined(__clang__) || GCC_CHECK_VERSION(4,7)
+	constexpr
 #endif
-
-	static constexpr HttpdOutput *Cast(AudioOutput *ao) {
-		return ContainerCast(ao, HttpdOutput, base);
+	static HttpdOutput *Cast(AudioOutput *ao) {
+		return &ContainerCast(*ao, &HttpdOutput::base);
 	}
 
-#if GCC_CHECK_VERSION(4,6) || defined(__clang__)
-#pragma GCC diagnostic pop
-#endif
-
 	using DeferredMonitor::GetEventLoop;
 
 	bool Init(const config_param &param, Error &error);

src/tag/TagPool.cxx

@@ -87,10 +87,13 @@ calc_hash(TagType type, const char *p)
 	return hash ^ type;
 }
 
-static inline constexpr TagPoolSlot *
+#if defined(__clang__) || GCC_CHECK_VERSION(4,7)
+	constexpr
+#endif
+static inline TagPoolSlot *
 tag_item_to_slot(TagItem *item)
 {
-	return ContainerCast(item, TagPoolSlot, item);
+	return &ContainerCast(*item, &TagPoolSlot::item);
 }
 
 static inline TagPoolSlot **

src/util/Cast.hxx

 /*
- * Copyright (C) 2013 Max Kellermann <max@duempel.org>
+ * Copyright (C) 2013-2014 Max Kellermann <max@duempel.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -64,11 +64,31 @@ OffsetCast(const U *p, ptrdiff_t offset)
 	return reinterpret_cast<const T *>(OffsetPointer(p, offset));
 }
 
+template<class C, class A>
+static constexpr inline ptrdiff_t
+ContainerAttributeOffset(const C *null_c, const A C::*p)
+{
+    return ptrdiff_t((const char *)null_c - (const char *)&(null_c->*p));
+}
+
+template<class C, class A>
+static constexpr inline ptrdiff_t
+ContainerAttributeOffset(const A C::*p)
+{
+    return ContainerAttributeOffset<C, A>(nullptr, p);
+}
+
 /**
  * Cast the given pointer to a struct member to its parent structure.
  */
-#define ContainerCast(p, container, attribute) \
-	OffsetCast<container, decltype(((container*)nullptr)->attribute)> \
-	((p), -ptrdiff_t(offsetof(container, attribute)))
+template<class C, class A>
+#if defined(__clang__) || GCC_CHECK_VERSION(4,7)
+constexpr
+#endif
+static inline C &
+ContainerCast(A &a, A C::*member)
+{
+    return *OffsetCast<C, A>(&a, ContainerAttributeOffset<C, A>(member));
+}
 
 #endif