sys/cdefs.h: add __const_castlf/dev/const-cast

1 files changed, 40 insertions, 0 deletions

diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index e79cf2972f3f..b3c055301f61 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -832,4 +832,44 @@
 #define __align_down(x, y) __builtin_align_down(x, y)
 #define __is_aligned(x, y) __builtin_is_aligned(x, y)
 
+/*
+ * __const_cast: a const-safe cast.
+ *
+ * Use it like this:
+ *
+ * 	__const_cast(type, pointer, return-type, expression)
+ *
+ * 'pointer' should be a pointer to an object of type 'type'.  If 'pointer'
+ * points to a const 'type', then the return value will be cast to
+ * 'const return-type', otherwise the return value will be cast to
+ * 'return-type'.  In either case, the return value is 'expression'.
+ *
+ * For example, a const-safe macro to cast a pointer to an array of char to a
+ * pointer to an array of uint8_t:
+ *
+ * #define GETBYTES(s) __const_cast(char, (s), uint8_t, (s))
+ */
+
+#ifdef __cplusplus
+template<typename... _Ts>
+struct __const_cast_visitor : _Ts... { using _Ts::operator()...; };
+template<typename... _Ts>
+__const_cast_visitor(_Ts...) -> __const_cast_visitor<_Ts...>;
+#define __const_cast(T, p, U, expr)					\
+	(__const_cast_visitor{						\
+		[&](const T *) -> const U {				\
+			return reinterpret_cast<const U>(expr);		\
+		},							\
+		[&](T *) -> U {						\
+			return reinterpret_cast<U>(expr);		\
+		}							\
+	})(p)
+#else
+#define __const_cast(T, p, U, expr)		\
+	_Generic((p),				\
+		const T *: (const U)expr,	\
+		T *: (U)expr			\
+	)
+#endif
+
 #endif /* !_SYS_CDEFS_H_ */