Make VariantSlot a union. Include next slot id in VariantData

src/ArduinoJson/Array/ArrayImpl.hpp

@@ -20,11 +20,11 @@ inline ArrayData::iterator ArrayData::at(
 }
 
 inline VariantData* ArrayData::addElement(ResourceManager* resources) {
-  auto slot = resources->allocSlot();
+  auto slot = resources->allocVariant();
   if (!slot)
     return nullptr;
   CollectionData::appendOne(slot, resources);
-  return slot->data();
+  return slot.data();
 }
 
 inline VariantData* ArrayData::getOrAddElement(size_t index,
@@ -58,12 +58,12 @@ inline void ArrayData::removeElement(size_t index, ResourceManager* resources) {
 template <typename T>
 inline bool ArrayData::addValue(T&& value, ResourceManager* resources) {
   ARDUINOJSON_ASSERT(resources != nullptr);
-  auto slot = resources->allocSlot();
+  auto slot = resources->allocVariant();
   if (!slot)
     return false;
-  JsonVariant variant(slot->data(), resources);
+  JsonVariant variant(slot.data(), resources);
   if (!variant.set(detail::forward<T>(value))) {
-    resources->freeSlot(slot);
+    resources->freeVariant(slot);
     return false;
   }
   CollectionData::appendOne(slot, resources);

src/ArduinoJson/Collection/CollectionData.hpp

@@ -12,7 +12,7 @@
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
 class VariantData;
-class VariantSlot;
+union VariantSlot;
 
 class CollectionIterator {
   friend class CollectionData;
@@ -58,9 +58,9 @@ class CollectionIterator {
   }
 
  private:
-  CollectionIterator(VariantSlot* slot, SlotId slotId);
+  CollectionIterator(VariantData* slot, SlotId slotId);
 
-  VariantSlot* slot_;
+  VariantData* slot_;
   SlotId currentId_, nextId_;
 };
 
@@ -79,7 +79,7 @@ class CollectionData {
   using iterator = CollectionIterator;
 
   iterator createIterator(const ResourceManager* resources) const {
-    return iterator(resources->getSlot(head_), head_);
+    return iterator(resources->getVariant(head_), head_);
   }
 
   size_t size(const ResourceManager*) const;
@@ -98,15 +98,15 @@ class CollectionData {
   }
 
  protected:
-  void appendOne(SlotWithId slot, const ResourceManager* resources);
-  void appendPair(SlotWithId key, SlotWithId value,
+  void appendOne(VariantWithId slot, const ResourceManager* resources);
+  void appendPair(VariantWithId key, VariantWithId value,
                   const ResourceManager* resources);
 
   void removeOne(iterator it, ResourceManager* resources);
   void removePair(iterator it, ResourceManager* resources);
 
  private:
-  SlotWithId getPreviousSlot(VariantSlot*, const ResourceManager*) const;
+  VariantWithId getPreviousSlot(VariantData*, const ResourceManager*) const;
 };
 
 inline const VariantData* collectionToVariant(

src/ArduinoJson/Collection/CollectionImpl.hpp

@@ -12,23 +12,23 @@
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-inline CollectionIterator::CollectionIterator(VariantSlot* slot, SlotId slotId)
+inline CollectionIterator::CollectionIterator(VariantData* slot, SlotId slotId)
     : slot_(slot), currentId_(slotId) {
   nextId_ = slot_ ? slot_->next() : NULL_SLOT;
 }
 
 inline void CollectionIterator::next(const ResourceManager* resources) {
   ARDUINOJSON_ASSERT(currentId_ != NULL_SLOT);
-  slot_ = resources->getSlot(nextId_);
+  slot_ = resources->getVariant(nextId_);
   currentId_ = nextId_;
   if (slot_)
     nextId_ = slot_->next();
 }
 
-inline void CollectionData::appendOne(SlotWithId slot,
+inline void CollectionData::appendOne(VariantWithId slot,
                                       const ResourceManager* resources) {
   if (tail_ != NULL_SLOT) {
-    auto tail = resources->getSlot(tail_);
+    auto tail = resources->getVariant(tail_);
     tail->setNext(slot.id());
     tail_ = slot.id();
   } else {
@@ -37,12 +37,12 @@ inline void CollectionData::appendOne(SlotWithId slot,
   }
 }
 
-inline void CollectionData::appendPair(SlotWithId key, SlotWithId value,
+inline void CollectionData::appendPair(VariantWithId key, VariantWithId value,
                                        const ResourceManager* resources) {
   key->setNext(value.id());
 
   if (tail_ != NULL_SLOT) {
-    auto tail = resources->getSlot(tail_);
+    auto tail = resources->getVariant(tail_);
     tail->setNext(key.id());
     tail_ = value.id();
   } else {
@@ -55,24 +55,24 @@ inline void CollectionData::clear(ResourceManager* resources) {
   auto next = head_;
   while (next != NULL_SLOT) {
     auto currId = next;
-    auto slot = resources->getSlot(next);
+    auto slot = resources->getVariant(next);
     next = slot->next();
-    resources->freeSlot(SlotWithId(slot, currId));
+    resources->freeVariant(VariantWithId(slot, currId));
   }
 
   head_ = NULL_SLOT;
   tail_ = NULL_SLOT;
 }
 
-inline SlotWithId CollectionData::getPreviousSlot(
-    VariantSlot* target, const ResourceManager* resources) const {
-  auto prev = SlotWithId();
+inline VariantWithId CollectionData::getPreviousSlot(
+    VariantData* target, const ResourceManager* resources) const {
+  auto prev = VariantWithId();
   auto currentId = head_;
   while (currentId != NULL_SLOT) {
-    auto currentSlot = resources->getSlot(currentId);
+    auto currentSlot = resources->getVariant(currentId);
     if (currentSlot == target)
       break;
-    prev = SlotWithId(currentSlot, currentId);
+    prev = VariantWithId(currentSlot, currentId);
     currentId = currentSlot->next();
   }
   return prev;
@@ -90,7 +90,7 @@ inline void CollectionData::removeOne(iterator it, ResourceManager* resources) {
     head_ = next;
   if (next == NULL_SLOT)
     tail_ = prev.id();
-  resources->freeSlot({it.slot_, it.currentId_});
+  resources->freeVariant({it.slot_, it.currentId_});
 }
 
 inline void CollectionData::removePair(ObjectData::iterator it,
@@ -101,11 +101,11 @@ inline void CollectionData::removePair(ObjectData::iterator it,
   auto keySlot = it.slot_;
 
   auto valueId = it.nextId_;
-  auto valueSlot = resources->getSlot(valueId);
+  auto valueSlot = resources->getVariant(valueId);
 
   // remove value slot
   keySlot->setNext(valueSlot->next());
-  resources->freeSlot({valueSlot, valueId});
+  resources->freeVariant({valueSlot, valueId});
 
   // remove key slot
   removeOne(it, resources);

src/ArduinoJson/Json/JsonSerializer.hpp

@@ -25,9 +25,9 @@ class JsonSerializer : public VariantDataVisitor<size_t> {
     auto slotId = array.head();
 
     while (slotId != NULL_SLOT) {
-      auto slot = resources_->getSlot(slotId);
+      auto slot = resources_->getVariant(slotId);
 
-      slot->data()->accept(*this);
+      slot->accept(*this);
 
       slotId = slot->next();
 
@@ -47,8 +47,8 @@ class JsonSerializer : public VariantDataVisitor<size_t> {
     bool isKey = true;
 
     while (slotId != NULL_SLOT) {
-      auto slot = resources_->getSlot(slotId);
-      slot->data()->accept(*this);
+      auto slot = resources_->getVariant(slotId);
+      slot->accept(*this);
 
       slotId = slot->next();
 

src/ArduinoJson/Memory/ResourceManager.hpp

@@ -13,8 +13,10 @@
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-class VariantSlot;
+union VariantSlot;
 class VariantPool;
+class VariantData;
+class VariantWithId;
 
 class ResourceManager {
  public:
@@ -49,18 +51,11 @@ class ResourceManager {
     return overflowed_;
   }
 
-  SlotWithId allocSlot() {
-    auto p = variantPools_.allocSlot(allocator_);
-    if (!p)
-      overflowed_ = true;
-    return p;
-  }
+  VariantWithId allocVariant();
 
-  void freeSlot(SlotWithId slot);
+  void freeVariant(VariantWithId slot);
 
-  VariantSlot* getSlot(SlotId id) const {
-    return variantPools_.getSlot(id);
-  }
+  VariantData* getVariant(SlotId id) const;
 
   template <typename TAdaptedString>
   StringNode* saveString(TAdaptedString str) {

src/ArduinoJson/Memory/ResourceManagerImpl.hpp

@@ -7,12 +7,26 @@
 #include <ArduinoJson/Collection/CollectionData.hpp>
 #include <ArduinoJson/Memory/ResourceManager.hpp>
 #include <ArduinoJson/Variant/VariantData.hpp>
+#include <ArduinoJson/Variant/VariantSlot.hpp>
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-inline void ResourceManager::freeSlot(SlotWithId slot) {
-  slot->data()->setNull(this);
-  variantPools_.freeSlot(slot);
+inline VariantWithId ResourceManager::allocVariant() {
+  auto p = variantPools_.allocSlot(allocator_);
+  if (!p) {
+    overflowed_ = true;
+    return {};
+  }
+  return {new (&p->variant) VariantData, p.id()};
+}
+
+inline void ResourceManager::freeVariant(VariantWithId variant) {
+  variant->setNull(this);
+  variantPools_.freeSlot(variant);
+}
+
+inline VariantData* ResourceManager::getVariant(SlotId id) const {
+  return reinterpret_cast<VariantData*>(variantPools_.getSlot(id));
 }
 
 ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Memory/StringPool.hpp

@@ -12,7 +12,7 @@
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-class VariantSlot;
+union VariantSlot;
 class VariantPool;
 
 class StringPool {

src/ArduinoJson/Memory/VariantPool.hpp

@@ -10,7 +10,7 @@
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-class VariantSlot;
+union VariantSlot;
 using SlotId = uint_t<ARDUINOJSON_SLOT_ID_SIZE * 8>;
 using SlotCount = SlotId;
 const SlotId NULL_SLOT = SlotId(-1);
@@ -22,11 +22,15 @@ class SlotWithId {
     ARDUINOJSON_ASSERT((slot == nullptr) == (id == NULL_SLOT));
   }
 
+  explicit operator bool() const {
+    return slot_ != nullptr;
+  }
+
   SlotId id() const {
     return id_;
   }
 
-  operator VariantSlot*() {
+  VariantSlot* slot() const {
     return slot_;
   }
 

src/ArduinoJson/Memory/VariantPoolImpl.hpp

@@ -41,7 +41,7 @@ inline SlotWithId VariantPool::allocSlot() {
     return {};
   auto index = usage_++;
   auto slot = &slots_[index];
-  return {new (slot) VariantSlot, SlotId(index)};
+  return {slot, SlotId(index)};
 }
 
 inline VariantSlot* VariantPool::getSlot(SlotId id) const {
@@ -69,12 +69,12 @@ inline SlotWithId VariantPoolList::allocFromFreeList() {
   ARDUINOJSON_ASSERT(freeList_ != NULL_SLOT);
   auto id = freeList_;
   auto slot = getSlot(freeList_);
-  freeList_ = slot->next();
-  return {new (slot) VariantSlot, id};
+  freeList_ = slot->free.next;
+  return {slot, id};
 }
 
 inline void VariantPoolList::freeSlot(SlotWithId slot) {
-  slot->setNext(freeList_);
+  slot->free.next = freeList_;
   freeList_ = slot.id();
 }
 

src/ArduinoJson/Memory/VariantPoolList.hpp

@@ -138,7 +138,8 @@ class VariantPoolList {
     auto slot = pools_[poolIndex].allocSlot();
     if (!slot)
       return {};
-    return {slot, SlotId(poolIndex * ARDUINOJSON_POOL_CAPACITY + slot.id())};
+    return {slot.slot(),
+            SlotId(poolIndex * ARDUINOJSON_POOL_CAPACITY + slot.id())};
   }
 
   VariantPool* addPool(Allocator* allocator) {

src/ArduinoJson/MsgPack/MsgPackSerializer.hpp

@@ -61,8 +61,8 @@ class MsgPackSerializer : public VariantDataVisitor<size_t> {
 
     auto slotId = array.head();
     while (slotId != NULL_SLOT) {
-      auto slot = resources_->getSlot(slotId);
-      slot->data()->accept(*this);
+      auto slot = resources_->getVariant(slotId);
+      slot->accept(*this);
       slotId = slot->next();
     }
 
@@ -83,8 +83,8 @@ class MsgPackSerializer : public VariantDataVisitor<size_t> {
 
     auto slotId = object.head();
     while (slotId != NULL_SLOT) {
-      auto slot = resources_->getSlot(slotId);
-      slot->data()->accept(*this);
+      auto slot = resources_->getVariant(slotId);
+      slot->accept(*this);
       slotId = slot->next();
     }
 

src/ArduinoJson/Object/ObjectImpl.hpp

@@ -51,20 +51,20 @@ inline void ObjectData::removeMember(TAdaptedString key,
 template <typename TAdaptedString>
 inline VariantData* ObjectData::addMember(TAdaptedString key,
                                           ResourceManager* resources) {
-  auto keySlot = resources->allocSlot();
+  auto keySlot = resources->allocVariant();
   if (!keySlot)
     return nullptr;
 
-  auto valueSlot = resources->allocSlot();
+  auto valueSlot = resources->allocVariant();
   if (!valueSlot)
     return nullptr;
 
-  if (!keySlot->data()->setString(key, resources))
+  if (!keySlot->setString(key, resources))
     return nullptr;
 
   CollectionData::appendPair(keySlot, valueSlot, resources);
 
-  return valueSlot->data();
+  return valueSlot.data();
 }
 
 ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Variant/VariantData.hpp

@@ -5,12 +5,12 @@
 #pragma once
 
 #include <ArduinoJson/Memory/StringNode.hpp>
+#include <ArduinoJson/Memory/VariantPool.hpp>
 #include <ArduinoJson/Misc/SerializedValue.hpp>
 #include <ArduinoJson/Numbers/convertNumber.hpp>
 #include <ArduinoJson/Strings/JsonString.hpp>
 #include <ArduinoJson/Strings/StringAdapters.hpp>
 #include <ArduinoJson/Variant/VariantContent.hpp>
-#include <ArduinoJson/Variant/VariantSlot.hpp>
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
@@ -20,9 +20,25 @@ T parseNumber(const char* s);
 class VariantData {
   VariantContent content_;  // must be first to allow cast from array to variant
   uint8_t type_;
+  SlotId next_;
 
  public:
-  VariantData() : type_(VALUE_IS_NULL) {}
+  // Placement new
+  static void* operator new(size_t, void* p) noexcept {
+    return p;
+  }
+
+  static void operator delete(void*, void*) noexcept {}
+
+  VariantData() : type_(VALUE_IS_NULL), next_(NULL_SLOT) {}
+
+  SlotId next() const {
+    return next_;
+  }
+
+  void setNext(SlotId slot) {
+    next_ = slot;
+  }
 
   template <typename TVisitor>
   typename TVisitor::result_type accept(TVisitor& visit) const {
@@ -518,4 +534,20 @@ class VariantData {
   }
 };
 
+class VariantWithId : public SlotWithId {
+ public:
+  VariantWithId() {}
+  VariantWithId(VariantData* data, SlotId id)
+      : SlotWithId(reinterpret_cast<VariantSlot*>(data), id) {}
+
+  VariantData* data() {
+    return reinterpret_cast<VariantData*>(slot());
+  }
+
+  VariantData* operator->() {
+    ARDUINOJSON_ASSERT(data() != nullptr);
+    return data();
+  }
+};
+
 ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Variant/VariantSlot.hpp

@@ -7,52 +7,22 @@
 #include <ArduinoJson/Memory/ResourceManager.hpp>
 #include <ArduinoJson/Polyfills/limits.hpp>
 #include <ArduinoJson/Polyfills/type_traits.hpp>
-#include <ArduinoJson/Variant/VariantContent.hpp>
+#include <ArduinoJson/Variant/VariantData.hpp>
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
 struct StringNode;
 
-class VariantSlot {
-  // CAUTION: same layout as VariantData
-  // we cannot use composition because it adds padding
-  // (+20% on ESP8266 for example)
-  VariantContent content_;
-  uint8_t type_;
-  SlotId next_;
-
- public:
-  // Placement new
-  static void* operator new(size_t, void* p) noexcept {
-    return p;
-  }
-
-  static void operator delete(void*, void*) noexcept {}
-
-  VariantSlot() : type_(0), next_(NULL_SLOT) {
-    (void)type_;  // HACK: suppress Clang warning "private field is not used"
-  }
-
-  VariantData* data() {
-    return reinterpret_cast<VariantData*>(&content_);
-  }
-
-  const VariantData* data() const {
-    return reinterpret_cast<const VariantData*>(&content_);
-  }
-
-  SlotId next() const {
-    return next_;
-  }
-
-  void setNext(SlotId slot) {
-    next_ = slot;
-  }
+struct FreeSlot {
+  SlotId next;
 };
 
-inline VariantData* slotData(VariantSlot* slot) {
-  return reinterpret_cast<VariantData*>(slot);
-}
+union VariantSlot {
+  VariantSlot() {}
+
+  VariantData variant;
+  FreeSlot free;
+};
 
 // Returns the size (in bytes) of an array with n elements.
 constexpr size_t sizeofArray(size_t n) {