Codechange: ability to store structs and list of structs in savegames

The commits following this will use this new functionality.

Currently, a few places do this manually. This has as drawback that
the Save() and Load() code need to be in sync, and that any change
can result in (old) savegames no longer loading. In general, it is
annoying code to maintain.

By putting everything in a description table, and use that for
both Save() and Load(), it becomes easier to see what is going on,
and hopefully less likely for people to make mistakes.

3 files changed, 157 insertions, 19 deletions

diff --git a/src/saveload/saveload.cpp b/src/saveload/saveload.cpp
index f9abac3ba..6be86a7ac 100644
--- a/src/saveload/saveload.cpp
+++ b/src/saveload/saveload.cpp
@@ -1450,6 +1450,27 @@ size_t SlCalcObjMemberLength(const void *object, const SaveLoad &sld)
 		case SL_WRITEBYTE: return 1; // a byte is logically of size 1
 		case SL_VEH_INCLUDE: return SlCalcObjLength(object, GetVehicleDescription(VEH_END));
 		case SL_ST_INCLUDE: return SlCalcObjLength(object, GetBaseStationDescription());
+		case SL_STRUCT:
+		case SL_STRUCTLIST: {
+			if (!SlIsObjectValidInSavegame(sld)) break;
+
+			NeedLength old_need_length = _sl.need_length;
+			size_t old_obj_len = _sl.obj_len;
+
+			_sl.need_length = NL_CALCLENGTH;
+			_sl.obj_len = 0;
+
+			/* Pretend that we are saving to collect the object size. Other
+			 * means are difficult, as we don't know the length of the list we
+			 * are about to store. */
+			sld.handler->Save(const_cast<void *>(object));
+			size_t length = _sl.obj_len;
+
+			_sl.obj_len = old_obj_len;
+			_sl.need_length = old_need_length;
+
+			return length;
+		}
 		default: NOT_REACHED();
 	}
 	return 0;
@@ -1505,8 +1526,6 @@ size_t SlCalcObjMemberLength(const void *object, const SaveLoad &sld)
 
 static bool SlObjectMember(void *object, const SaveLoad &sld)
 {
-	void *ptr = GetVariableAddress(object, sld);
-
 	assert(IsVariableSizeRight(sld));
 
 	VarType conv = GB(sld.conv, 0, 8);
@@ -1517,10 +1536,12 @@ static bool SlObjectMember(void *object, const SaveLoad &sld)
 		case SL_STR:
 		case SL_REFLIST:
 		case SL_DEQUE:
-		case SL_STDSTR:
+		case SL_STDSTR: {
 			/* CONDITIONAL saveload types depend on the savegame version */
 			if (!SlIsObjectValidInSavegame(sld)) return false;
 
+			void *ptr = GetVariableAddress(object, sld);
+
 			switch (sld.cmd) {
 				case SL_VAR: SlSaveLoadConv(ptr, conv); break;
 				case SL_REF: SlSaveLoadRef(ptr, conv); break;
@@ -1532,11 +1553,14 @@ static bool SlObjectMember(void *object, const SaveLoad &sld)
 				default: NOT_REACHED();
 			}
 			break;
+		}
 
 		/* SL_WRITEBYTE writes a value to the savegame to identify the type of an object.
 		 * When loading, the value is read explicitly with SlReadByte() to determine which
 		 * object description to use. */
-		case SL_WRITEBYTE:
+		case SL_WRITEBYTE: {
+			void *ptr = GetVariableAddress(object, sld);
+
 			switch (_sl.action) {
 				case SLA_SAVE: SlWriteByte(*(uint8 *)ptr); break;
 				case SLA_LOAD_CHECK:
@@ -1546,15 +1570,34 @@ static bool SlObjectMember(void *object, const SaveLoad &sld)
 				default: NOT_REACHED();
 			}
 			break;
+		}
 
 		/* SL_VEH_INCLUDE loads common code for vehicles */
-		case SL_VEH_INCLUDE:
+		case SL_VEH_INCLUDE: {
+			void *ptr = GetVariableAddress(object, sld);
 			SlObject(ptr, GetVehicleDescription(VEH_END));
 			break;
+		}
 
-		case SL_ST_INCLUDE:
+		case SL_ST_INCLUDE: {
+			void *ptr = GetVariableAddress(object, sld);
 			SlObject(ptr, GetBaseStationDescription());
 			break;
+		}
+
+		case SL_STRUCT:
+		case SL_STRUCTLIST:
+			if (!SlIsObjectValidInSavegame(sld)) return false;
+
+			switch (_sl.action) {
+				case SLA_SAVE: sld.handler->Save(object); break;
+				case SLA_LOAD_CHECK: sld.handler->LoadCheck(object); break;
+				case SLA_LOAD: sld.handler->Load(object); break;
+				case SLA_PTRS: sld.handler->FixPointers(object); break;
+				case SLA_NULL: break;
+				default: NOT_REACHED();
+			}
+			break;
 
 		default: NOT_REACHED();
 	}
diff --git a/src/saveload/saveload.h b/src/saveload/saveload.h
index 28af1fd37..ff0982569 100644
--- a/src/saveload/saveload.h
+++ b/src/saveload/saveload.h
@@ -400,6 +400,73 @@ struct ChunkHandler {
 /** A table of ChunkHandler entries. */
 using ChunkHandlerTable = span<const ChunkHandler>;
 
+/** A table of SaveLoad entries. */
+using SaveLoadTable = span<const struct SaveLoad>;
+
+/** Handler for saving/loading an object to/from disk. */
+class SaveLoadHandler {
+public:
+	virtual ~SaveLoadHandler() {}
+
+	/**
+	 * Save the object to disk.
+	 * @param object The object to store.
+	 */
+	virtual void Save(void *object) const {}
+
+	/**
+	 * Load the object from disk.
+	 * @param object The object to load.
+	 */
+	virtual void Load(void *object) const {}
+
+	/**
+	 * Similar to load, but used only to validate savegames.
+	 * @param object The object to load.
+	 */
+	virtual void LoadCheck(void *object) const {}
+
+	/**
+	 * A post-load callback to fix #SL_REF integers into pointers.
+	 * @param object The object to fix.
+	 */
+	virtual void FixPointers(void *object) const {}
+
+	/**
+	 * Get the description of the fields in the savegame.
+	 */
+	virtual SaveLoadTable GetDescription() const = 0;
+};
+
+/**
+ * Default handler for saving/loading an object to/from disk.
+ *
+ * This handles a few common things for handlers, meaning the actual handler
+ * needs less code.
+ *
+ * Usage: class SlMine : public DefaultSaveLoadHandler<SlMine, MyObject> {}
+ *
+ * @tparam TImpl The class initializing this template.
+ * @tparam TObject The class of the object using this SaveLoadHandler.
+ */
+template <class TImpl, class TObject>
+class DefaultSaveLoadHandler : public SaveLoadHandler {
+public:
+	SaveLoadTable GetDescription() const override { return static_cast<const TImpl *>(this)->description; }
+
+	virtual void Save(TObject *object) const {}
+	void Save(void *object) const override { this->Save(static_cast<TObject *>(object)); }
+
+	virtual void Load(TObject *object) const {}
+	void Load(void *object) const override { this->Load(static_cast<TObject *>(object)); }
+
+	virtual void LoadCheck(TObject *object) const {}
+	void LoadCheck(void *object) const override { this->LoadCheck(static_cast<TObject *>(object)); }
+
+	virtual void FixPointers(TObject *object) const {}
+	void FixPointers(void *object) const override { this->FixPointers(static_cast<TObject *>(object)); }
+};
+
 /** Type of reference (#SLE_REF, #SLE_CONDREF). */
 enum SLRefType {
 	REF_ORDER          =  0, ///< Load/save a reference to an order.
@@ -501,10 +568,12 @@ enum SaveLoadType : byte {
 	SL_REFLIST     =  4, ///< Save/load a list of #SL_REF elements.
 	SL_DEQUE       =  5, ///< Save/load a deque of #SL_VAR elements.
 	SL_STDSTR      =  6, ///< Save/load a \c std::string.
+	SL_STRUCT      =  7, ///< Save/load a struct.
+	SL_STRUCTLIST  =  8, ///< Save/load a list of structs.
 	/* non-normal save-load types */
-	SL_WRITEBYTE   =  8,
-	SL_VEH_INCLUDE =  9,
-	SL_ST_INCLUDE  = 10,
+	SL_WRITEBYTE   =  9,
+	SL_VEH_INCLUDE = 10,
+	SL_ST_INCLUDE  = 11,
 };
 
 typedef void *SaveLoadAddrProc(void *base, size_t extra);
@@ -519,11 +588,9 @@ struct SaveLoad {
 	size_t size;                    ///< the sizeof size.
 	SaveLoadAddrProc *address_proc; ///< callback proc the get the actual variable address in memory
 	size_t extra_data;              ///< extra data for the callback proc
+	SaveLoadHandler *handler;       ///< Custom handler for Save/Load procs.
 };
 
-/** A table of SaveLoad entries. */
-using SaveLoadTable = span<const SaveLoad>;
-
 /**
  * Storage of simple variables, references (pointers), and arrays.
  * @param cmd      Load/save type. @see SaveLoadType
@@ -535,7 +602,7 @@ using SaveLoadTable = span<const SaveLoad>;
  * @param extra    Extra data to pass to the address callback function.
  * @note In general, it is better to use one of the SLE_* macros below.
  */
-#define SLE_GENERAL(cmd, base, variable, type, length, from, to, extra) {cmd, type, length, from, to, cpp_sizeof(base, variable), [] (void *b, size_t) -> void * { assert(b != nullptr); return const_cast<void *>(static_cast<const void *>(std::addressof(static_cast<base *>(b)->variable))); }, extra}
+#define SLE_GENERAL(cmd, base, variable, type, length, from, to, extra) {cmd, type, length, from, to, cpp_sizeof(base, variable), [] (void *b, size_t) -> void * { assert(b != nullptr); return const_cast<void *>(static_cast<const void *>(std::addressof(static_cast<base *>(b)->variable))); }, extra, nullptr}
 
 /**
  * Storage of a variable in some savegame versions.
@@ -671,13 +738,13 @@ using SaveLoadTable = span<const SaveLoad>;
  * @param from   First savegame version that has the empty space.
  * @param to     Last savegame version that has the empty space.
  */
-#define SLE_CONDNULL(length, from, to) {SL_ARR, SLE_FILE_U8 | SLE_VAR_NULL, length, from, to, 0, nullptr, 0}
+#define SLE_CONDNULL(length, from, to) {SL_ARR, SLE_FILE_U8 | SLE_VAR_NULL, length, from, to, 0, nullptr, 0, nullptr}
 
 /** Translate values ingame to different values in the savegame and vv. */
 #define SLE_WRITEBYTE(base, variable) SLE_GENERAL(SL_WRITEBYTE, base, variable, 0, 0, SL_MIN_VERSION, SL_MAX_VERSION, 0)
 
-#define SLE_VEH_INCLUDE() {SL_VEH_INCLUDE, 0, 0, SL_MIN_VERSION, SL_MAX_VERSION, 0, [] (void *b, size_t) { return b; }, 0}
-#define SLE_ST_INCLUDE() {SL_ST_INCLUDE, 0, 0, SL_MIN_VERSION, SL_MAX_VERSION, 0, [] (void *b, size_t) { return b; }, 0}
+#define SLE_VEH_INCLUDE() {SL_VEH_INCLUDE, 0, 0, SL_MIN_VERSION, SL_MAX_VERSION, 0, [] (void *b, size_t) { return b; }, 0, nullptr}
+#define SLE_ST_INCLUDE() {SL_ST_INCLUDE, 0, 0, SL_MIN_VERSION, SL_MAX_VERSION, 0, [] (void *b, size_t) { return b; }, 0, nullptr}
 
 /**
  * Storage of global simple variables, references (pointers), and arrays.
@@ -689,7 +756,7 @@ using SaveLoadTable = span<const SaveLoad>;
  * @param extra    Extra data to pass to the address callback function.
  * @note In general, it is better to use one of the SLEG_* macros below.
  */
-#define SLEG_GENERAL(cmd, variable, type, length, from, to, extra) {cmd, type, length, from, to, sizeof(variable), [] (void *, size_t) -> void * { return static_cast<void *>(std::addressof(variable)); }, extra}
+#define SLEG_GENERAL(cmd, variable, type, length, from, to, extra) {cmd, type, length, from, to, sizeof(variable), [] (void *, size_t) -> void * { return static_cast<void *>(std::addressof(variable)); }, extra, nullptr}
 
 /**
  * Storage of a global variable in some savegame versions.
@@ -739,6 +806,14 @@ using SaveLoadTable = span<const SaveLoad>;
 #define SLEG_CONDSSTR(variable, type, from, to) SLEG_GENERAL(SL_STDSTR, variable, type, 0, from, to, 0)
 
 /**
+ * Storage of a structs in some savegame versions.
+ * @param handler  SaveLoadHandler for the structs.
+ * @param from     First savegame version that has the struct.
+ * @param to       Last savegame version that has the struct.
+ */
+#define SLEG_CONDSTRUCT(handler, from, to) {SL_STRUCT, 0, 0, from, to, 0, nullptr, 0, new handler()}
+
+/**
  * Storage of a global reference list in some savegame versions.
  * @param variable Name of the global variable.
  * @param type     Storage of the data in memory and in the savegame.
@@ -748,6 +823,14 @@ using SaveLoadTable = span<const SaveLoad>;
 #define SLEG_CONDREFLIST(variable, type, from, to) SLEG_GENERAL(SL_REFLIST, variable, type, 0, from, to, 0)
 
 /**
+ * Storage of a list of structs in some savegame versions.
+ * @param handler  SaveLoadHandler for the list of structs.
+ * @param from     First savegame version that has the list.
+ * @param to       Last savegame version that has the list.
+ */
+#define SLEG_CONDSTRUCTLIST(handler, from, to) {SL_STRUCTLIST, 0, 0, from, to, 0, nullptr, 0, new handler()}
+
+/**
  * Storage of a global variable in every savegame version.
  * @param variable Name of the global variable.
  * @param type     Storage of the data in memory and in the savegame.
@@ -783,6 +866,12 @@ using SaveLoadTable = span<const SaveLoad>;
 #define SLEG_SSTR(variable, type) SLEG_CONDSSTR(variable, type, SL_MIN_VERSION, SL_MAX_VERSION)
 
 /**
+ * Storage of a structs in every savegame version.
+ * @param handler SaveLoadHandler for the structs.
+ */
+#define SLEG_STRUCT(handler) SLEG_CONDSTRUCT(handler, SL_MIN_VERSION, SL_MAX_VERSION)
+
+/**
  * Storage of a global reference list in every savegame version.
  * @param variable Name of the global variable.
  * @param type     Storage of the data in memory and in the savegame.
@@ -790,12 +879,18 @@ using SaveLoadTable = span<const SaveLoad>;
 #define SLEG_REFLIST(variable, type) SLEG_CONDREFLIST(variable, type, SL_MIN_VERSION, SL_MAX_VERSION)
 
 /**
+ * Storage of a list of structs in every savegame version.
+ * @param handler SaveLoadHandler for the list of structs.
+ */
+#define SLEG_STRUCTLIST(handler) SLEG_CONDSTRUCTLIST(handler, SL_MIN_VERSION, SL_MAX_VERSION)
+
+/**
  * Empty global space in some savegame versions.
  * @param length Length of the empty space.
  * @param from   First savegame version that has the empty space.
  * @param to     Last savegame version that has the empty space.
  */
-#define SLEG_CONDNULL(length, from, to) {SL_ARR, SLE_FILE_U8 | SLE_VAR_NULL, length, from, to, 0, nullptr, 0}
+#define SLEG_CONDNULL(length, from, to) {SL_ARR, SLE_FILE_U8 | SLE_VAR_NULL, length, from, to, 0, nullptr, 0, nullptr}
 
 /**
  * Checks whether the savegame is below \a major.\a minor.
diff --git a/src/settings.cpp b/src/settings.cpp
index 2b222ad78..23f43d24f 100644
--- a/src/settings.cpp
+++ b/src/settings.cpp
@@ -2024,7 +2024,7 @@ static std::vector<SaveLoad> GetSettingsDesc(const SettingTable &settings, bool
 
 		if (is_loading && (sd->flags & SF_NO_NETWORK_SYNC) && _networking && !_network_server) {
 			/* We don't want to read this setting, so we do need to skip over it. */
-			saveloads.push_back({sd->save.cmd, GetVarFileType(sd->save.conv) | SLE_VAR_NULL, sd->save.length, sd->save.version_from, sd->save.version_to, 0, nullptr, 0});
+			saveloads.push_back({sd->save.cmd, GetVarFileType(sd->save.conv) | SLE_VAR_NULL, sd->save.length, sd->save.version_from, sd->save.version_to, 0, nullptr, 0, nullptr});
 			continue;
 		}