util/WritableBuffer: add cast methods

src/util/WritableBuffer.hxx

@@ -32,7 +32,11 @@
 
 #include "Compiler.h"
 
-#include <stddef.h>
+#include <cstddef>
+
+#ifndef NDEBUG
+#include <assert.h>
+#endif
 
 /**
  * A reference to a memory area that is writable.
@@ -52,6 +56,8 @@ struct WritableBuffer {
 
 	WritableBuffer() = default;
 
+	constexpr WritableBuffer(std::nullptr_t):data(nullptr), size(0) {}
+
 	constexpr WritableBuffer(pointer_type _data, size_type _size)
 		:data(_data), size(_size) {}
 
@@ -59,6 +65,29 @@ struct WritableBuffer {
 		return { nullptr, 0 };
 	}
 
+	/**
+	 * Cast a WritableBuffer<void> to a WritableBuffer<T>.  A "void"
+	 * buffer records its size in bytes, and when casting to "T",
+	 * the assertion below ensures that the size is a multiple of
+	 * sizeof(T).
+	 */
+#ifdef NDEBUG
+	constexpr
+#endif
+	static WritableBuffer<T> FromVoid(WritableBuffer<void> other) {
+		static_assert(sizeof(T) > 0, "Empty base type");
+#ifndef NDEBUG
+		assert(other.size % sizeof(T) == 0);
+#endif
+		return WritableBuffer<T>(pointer_type(other.data),
+				      other.size / sizeof(T));
+	}
+
+	constexpr WritableBuffer<void> ToVoid() const {
+		static_assert(sizeof(T) > 0, "Empty base type");
+		return WritableBuffer<void>(data, size * sizeof(T));
+	}
+
 	constexpr bool IsNull() const {
 		return data == nullptr;
 	}