PagedEntityContainer: add ReuseSlots boolean template parameter

This allows to disable slot reuse in `PagedEntityContainer`.

The feature is temporary and will be used only until
we are confident that we don't have any issues in the
WZ code base regarding object pointers not updated
properly, e.g. when transitioning between the base
and offworld missions.

So, it's to prevent cases where one object would be
overwritten by another and the game code haven't noticed
that. It's better to crash/assert rather than silently
use one object instead of another, which could lead to
unobvious and weird bugs.

Signed-off-by: Pavel Solodovnikov <[email protected]>

lib/framework/paged_entity_container.h

@@ -99,7 +99,8 @@
 /// </summary>
 /// <typeparam name="T">Entity type. Should be a complete type.</typeparam>
 /// <typeparam name="MaxElementsPerPage">The fixed number of elements each page may hold.</typeparam>
-template <typename T, size_t MaxElementsPerPage = 1024>
+/// <typeparam name="ReuseSlots">If `false`, slots are one-shot and set to expire after single use.</typeparam>
+template <typename T, size_t MaxElementsPerPage = 1024, bool ReuseSlots = true>
 class PagedEntityContainer
 {
 	using SlotIndexType = size_t;
@@ -150,6 +151,11 @@ class PagedEntityContainer
 			return _isAlive;
 		}
 
+		void invalidate()
+		{
+			_generation = INVALID_GENERATION;
+		}
+
 		void set_dead()
 		{
 			_isAlive = false;
@@ -399,8 +405,11 @@ class PagedEntityContainer
 		{
 			return;
 		}
-		// Advance slot generation number.
-		slotMetadata.advance_generation();
+
+		// Either advance slot generation number or invalidate the slot right away,
+		// the behavior depends on whether we reuse the slots or not.
+		advance_slot_generation<ReuseSlots>(slotMetadata);
+
 		// Ensure that the element pointed-to by this slot is dead.
 		slotMetadata.set_dead();
 
@@ -580,7 +589,7 @@ class PagedEntityContainer
 
 	const_iterator begin() const
 	{
-		return const_iterator(const_cast<PagedEntityContainer<T>*>(this)->begin());
+		return const_iterator(const_cast<PagedEntityContainer*>(this)->begin());
 	}
 
 	iterator begin()
@@ -657,7 +666,7 @@ class PagedEntityContainer
 
 	const_iterator find(const T& x) const
 	{
-		return const_iterator(const_cast<PagedEntityContainer<T>*>(this)->find(const_cast<T&>(x)));
+		return const_iterator(const_cast<PagedEntityContainer*>(this)->find(const_cast<T&>(x)));
 	}
 
 	void erase(const_iterator it)
@@ -787,17 +796,32 @@ class PagedEntityContainer
 		return (reinterpret_cast<uintptr_t>(addr) % alignof(T)) == 0;
 	}
 
+	template <bool ReuseSlotsAux>
+	void advance_slot_generation(SlotMetadata& meta)
+	{
+		// Advance slot generation number, when `ReuseSlots=true`.
+		meta.advance_generation();
+	}
+
+	// Specialization for the case when `ReuseSlots=false`.
+	template <>
+	void advance_slot_generation<false>(SlotMetadata& meta)
+	{
+		// Invalidate slot right away so that it cannot be reused anymore.
+		meta.invalidate();
+	}
+
 	std::vector<Page> _pages;
 	SlotIndexType _maxIndex = INVALID_SLOT_IDX;
 	size_t _size = 0;
 	size_t _capacity = 0;
 	size_t _expiredSlotsCount = 0;
 };
 
-template <typename T, size_t MaxElementsPerPage>
-constexpr typename PagedEntityContainer<T, MaxElementsPerPage>::SlotIndexType
-	PagedEntityContainer<T, MaxElementsPerPage>::INVALID_SLOT_IDX;
+template <typename T, size_t MaxElementsPerPage, bool ReuseSlots>
+constexpr typename PagedEntityContainer<T, MaxElementsPerPage, ReuseSlots>::SlotIndexType
+	PagedEntityContainer<T, MaxElementsPerPage, ReuseSlots>::INVALID_SLOT_IDX;
 
-template <typename T, size_t MaxElementsPerPage>
-constexpr size_t PagedEntityContainer<T, MaxElementsPerPage>::INVALID_PAGE_IDX;
+template <typename T, size_t MaxElementsPerPage, bool ReuseSlots>
+constexpr size_t PagedEntityContainer<T, MaxElementsPerPage, ReuseSlots>::INVALID_PAGE_IDX;