Introduced the BL_UCAST() macro as an alternative to explicit casts

- The benefit of using BL_UCAST() instead of an explicit cast is that it
  produces a compiler error if the argument is not a block_list.
- The benefit of using BL_UCAST() instead of BL_CAST() is that the
  block_list argument is guaranteed to be evaluated only once. As such
  it can be used in places where the argument is a function.
- No block_list type checking is done. As such, if a different
  block_list type is passed, behavior is undefined. The user should do
  the checking beforehand.

Signed-off-by: Haru <haru@dotalux.com>

1 files changed, 40 insertions, 1 deletions

diff --git a/src/map/map.h b/src/map/map.h
index 7047feab6..f785ddc83 100644
--- a/src/map/map.h
+++ b/src/map/map.h
@@ -828,8 +828,47 @@ typedef struct homun_data       TBL_HOM;
 typedef struct mercenary_data   TBL_MER;
 typedef struct elemental_data   TBL_ELEM;
 
+/**
+ * Casts a block list to a specific type.
+ *
+ * @remark
+ *   The `bl` argument may be evaluated more than once.
+ *
+ * @param type_ The block list type (using symbols from enum bl_type).
+ * @param bl    The source block list to cast.
+ * @return The block list, cast to the correct type.
+ * @retval NULL if bl is the wrong type or NULL.
+ */
 #define BL_CAST(type_, bl) \
-	( ((bl) == (struct block_list*)NULL || (bl)->type != (type_)) ? (T ## type_ *)NULL : (T ## type_ *)(bl) )
+	( ((bl) == (struct block_list *)NULL || (bl)->type != (type_)) ? (T ## type_ *)NULL : (T ## type_ *)(bl) )
+
+/**
+ * Helper function for `BL_UCAST`.
+ *
+ * @warning
+ *   This function shouldn't be called on it own.
+ *
+ * The purpose of this function is to produce a compile-timer error if a non-bl
+ * object is passed to BL_UCAST. It's declared as static inline to let the
+ * compiler optimize out the function call overhead.
+ */
+static inline struct block_list *BL_UCAST_(struct block_list *bl)
+{
+	return bl;
+}
+
+/**
+ * Casts a block list to a specific type, without performing any type checks.
+ *
+ * @remark
+ *   The `bl` argument is guaranteed to be evaluated once and only once.
+ *
+ * @param type_ The block list type (using symbols from enum bl_type).
+ * @param bl    The source block list to cast.
+ * @return The block list, cast to the correct type.
+ */
+#define BL_UCAST(type_, bl) \
+	((T ## type_ *)BL_UCAST_(bl))
 
 struct charid_request {
 	struct charid_request* next;