util/Cast: new utility library

Makefile.am

@@ -249,6 +249,7 @@ endif
 
 libutil_a_SOURCES = \
 	src/util/Macros.hxx \
+	src/util/Cast.hxx \
 	src/util/Clamp.hxx \
 	src/util/Error.cxx src/util/Error.hxx \
 	src/util/Domain.hxx \

src/output/HttpdInternal.hxx

@@ -30,6 +30,7 @@
 #include "thread/Mutex.hxx"
 #include "event/ServerSocket.hxx"
 #include "event/DeferredMonitor.hxx"
+#include "util/Cast.hxx"
 
 #ifdef _LIBCPP_VERSION
 /* can't use incomplete template arguments with libc++ */
@@ -157,7 +158,7 @@ public:
 #endif
 
 	static constexpr HttpdOutput *Cast(audio_output *ao) {
-		return (HttpdOutput *)((char *)ao - offsetof(HttpdOutput, base));
+		return ContainerCast(ao, HttpdOutput, base);
 	}
 
 #if GCC_CHECK_VERSION(4,6) || defined(__clang__)

src/tag/TagPool.cxx

@@ -20,6 +20,7 @@
 #include "config.h"
 #include "TagPool.hxx"
 #include "TagItem.hxx"
+#include "util/Cast.hxx"
 
 #include <glib.h>
 
@@ -67,7 +68,7 @@ calc_hash(TagType type, const char *p)
 static inline struct slot *
 tag_item_to_slot(TagItem *item)
 {
-	return (struct slot*)(((char*)item) - offsetof(struct slot, item));
+	return ContainerCast(item, slot, item);
 }
 
 static struct slot *slot_alloc(struct slot *next,

src/util/Cast.hxx

@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2013 Max Kellermann <max@duempel.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
+ * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef CAST_HXX
+#define CAST_HXX
+
+#include <stddef.h>
+
+/**
+ * Offset the given pointer by the specified number of bytes.
+ */
+static constexpr void *
+OffsetPointer(void *p, ptrdiff_t offset)
+{
+    return (char *)p + offset;
+}
+
+template<typename T, typename U>
+static constexpr T *
+OffsetCast(U *p, ptrdiff_t offset)
+{
+    return reinterpret_cast<T *>(OffsetPointer(p, offset));
+}
+
+/**
+ * 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)))
+
+#endif