Codechange: Don't use cpp_offsetof in the TTO/TTD savegame loader.

Many of the member variables that are used in the oldloader are inside types
that are not standard layout types. Using pointer arithmetics to determine
addresses of members inside types that are not standard layout is generally
undefined behaviour. If we'd use C++17, it is conditionally supported, which means
each compiler may or may not support it. And even then using it for individual
array elements is syntactically not supported the the standard offsetof function.

2 files changed, 14 insertions, 15 deletions

diff --git a/src/saveload/oldloader.cpp b/src/saveload/oldloader.cpp
index 42970c9a2..6bc07ad27 100644
--- a/src/saveload/oldloader.cpp
+++ b/src/saveload/oldloader.cpp
@@ -108,8 +108,6 @@ byte ReadByte(LoadgameState *ls)
  */
 bool LoadChunk(LoadgameState *ls, void *base, const OldChunks *chunks)
 {
-	byte *base_ptr = (byte*)base;
-
 	for (const OldChunks *chunk = chunks; chunk->type != OC_END; chunk++) {
 		if (((chunk->type & OC_TTD) && _savegame_type == SGT_TTO) ||
 				((chunk->type & OC_TTO) && _savegame_type != SGT_TTO)) {
@@ -134,8 +132,8 @@ bool LoadChunk(LoadgameState *ls, void *base, const OldChunks *chunks)
 						break;
 
 					case OC_ASSERT:
-						DEBUG(oldloader, 4, "Assert point: 0x%X / 0x%X", ls->total_read, chunk->offset + _bump_assert_value);
-						if (ls->total_read != chunk->offset + _bump_assert_value) throw std::exception();
+						DEBUG(oldloader, 4, "Assert point: 0x%X / 0x%X", ls->total_read, (uint)(size_t)chunk->ptr + _bump_assert_value);
+						if (ls->total_read != (size_t)chunk->ptr + _bump_assert_value) throw std::exception();
 					default: break;
 				}
 			} else {
@@ -153,10 +151,10 @@ bool LoadChunk(LoadgameState *ls, void *base, const OldChunks *chunks)
 				}
 
 				/* When both pointers are nullptr, we are just skipping data */
-				if (base_ptr == nullptr && chunk->ptr == nullptr) continue;
+				if (base == nullptr && chunk->ptr == nullptr) continue;
 
-				/* Writing to the var: bits 8 to 15 have the VAR type */
-				if (chunk->ptr == nullptr) ptr = base_ptr + chunk->offset;
+				/* Chunk refers to a struct member, get address in base. */
+				if (chunk->ptr == nullptr) ptr = (byte *)chunk->offset(base);
 
 				/* Write the data */
 				switch (GetOldChunkVarType(chunk->type)) {
diff --git a/src/saveload/oldloader.h b/src/saveload/oldloader.h
index 1600a9ac2..6879192ec 100644
--- a/src/saveload/oldloader.h
+++ b/src/saveload/oldloader.h
@@ -82,13 +82,14 @@ enum OldChunkType {
 DECLARE_ENUM_AS_BIT_SET(OldChunkType)
 
 typedef bool OldChunkProc(LoadgameState *ls, int num);
+typedef void *OffsetProc(void *base);
 
 struct OldChunks {
 	OldChunkType type;   ///< Type of field
 	uint32 amount;       ///< Amount of fields
 
-	void *ptr;           ///< Pointer where to save the data (may only be set if offset is 0)
-	uint offset;         ///< Offset from basepointer (may only be set if ptr is nullptr)
+	void *ptr;           ///< Pointer where to save the data (takes precedence over #offset)
+	OffsetProc *offset;  ///< Pointer to function that returns the actual memory address of a member (ignored if #ptr is not nullptr)
 	OldChunkProc *proc;  ///< Pointer to function that is called with OC_CHUNK
 };
 
@@ -123,12 +124,12 @@ static inline uint32 ReadUint32(LoadgameState *ls)
  *  - OCL_CHUNK: load another proc to load a part of the savegame, 'amount' times
  *  - OCL_ASSERT: to check if we are really at the place we expect to be.. because old savegames are too binary to be sure ;)
  */
-#define OCL_SVAR(type, base, offset)         { type,                 1, nullptr, (uint)cpp_offsetof(base, offset), nullptr }
-#define OCL_VAR(type, amount, pointer)       { type,            amount, pointer,    0,                             nullptr }
-#define OCL_END()                            { OC_END,               0, nullptr,    0,                             nullptr }
-#define OCL_CNULL(type, amount)              { OC_NULL | type,  amount, nullptr,    0,                             nullptr }
-#define OCL_CCHUNK(type, amount, proc)       { OC_CHUNK | type, amount, nullptr,    0,                             proc }
-#define OCL_ASSERT(type, size)               { OC_ASSERT | type,     1, nullptr, size,                             nullptr }
+#define OCL_SVAR(type, base, offset)         { type,                 1, nullptr, [] (void *b) -> void * { return std::addressof(static_cast<base *>(b)->offset); }, nullptr }
+#define OCL_VAR(type, amount, pointer)       { type,            amount, pointer, nullptr, nullptr }
+#define OCL_END()                            { OC_END,               0, nullptr, nullptr, nullptr }
+#define OCL_CNULL(type, amount)              { OC_NULL | type,  amount, nullptr, nullptr, nullptr }
+#define OCL_CCHUNK(type, amount, proc)       { OC_CHUNK | type, amount, nullptr, nullptr, proc }
+#define OCL_ASSERT(type, size)               { OC_ASSERT | type,     1, (void *)(size_t)size, nullptr, nullptr }
 #define OCL_NULL(amount)        OCL_CNULL((OldChunkType)0, amount)
 #define OCL_CHUNK(amount, proc) OCL_CCHUNK((OldChunkType)0, amount, proc)