Decouple MemoryPool from VariantSlot

src/ArduinoJson/Collection/CollectionFunctions.hpp

@@ -12,7 +12,7 @@ inline VariantData* collectionAddElement(CollectionData* array,
                                          MemoryPool* pool) {
   if (!array)
     return nullptr;
-  auto slot = pool->allocVariant();
+  auto slot = new (pool) VariantSlot();
   if (!slot)
     return nullptr;
   array->add(slot);
@@ -24,7 +24,7 @@ inline VariantData* collectionAddMember(CollectionData* obj, TAdaptedString key,
                                         MemoryPool* pool) {
   ARDUINOJSON_ASSERT(!key.isNull());
   ARDUINOJSON_ASSERT(obj != nullptr);
-  auto slot = pool->allocVariant();
+  auto slot = new (pool) VariantSlot();
   if (!slot)
     return nullptr;
   if (key.isLinked())

src/ArduinoJson/Collection/CollectionImpl.hpp

@@ -96,7 +96,7 @@ inline void CollectionData::movePointers(ptrdiff_t variantDistance) {
   movePointer(head_, variantDistance);
   movePointer(tail_, variantDistance);
   for (VariantSlot* slot = head_; slot; slot = slot->next())
-    slot->movePointers(variantDistance);
+    slot->data()->movePointers(variantDistance);
 }
 
 ARDUINOJSON_END_PRIVATE_NAMESPACE

src/ArduinoJson/Document/JsonDocument.hpp

@@ -86,7 +86,8 @@ class JsonDocument : public detail::VariantOperators<const JsonDocument&> {
   // Reduces the capacity of the memory pool to match the current usage.
   // https://arduinojson.org/v6/api/JsonDocument/shrinktofit/
   void shrinkToFit() {
-    pool_.shrinkToFit(data_);
+    auto offset = pool_.shrinkToFit();
+    data_.movePointers(offset);
   }
 
   // Reclaims the memory leaked when removing and replacing values.

src/ArduinoJson/Json/JsonDeserializer.hpp

@@ -279,7 +279,7 @@ class JsonDeserializer {
           auto savedKey = stringBuilder_.save();
 
           // Allocate slot in object
-          slot = pool_->allocVariant();
+          slot = new (pool_) VariantSlot();
           if (!slot)
             return DeserializationError::NoMemory;
 

src/ArduinoJson/Memory/MemoryPool.hpp

@@ -10,22 +10,11 @@
 #include <ArduinoJson/Polyfills/assert.hpp>
 #include <ArduinoJson/Polyfills/mpl/max.hpp>
 #include <ArduinoJson/Strings/StringAdapters.hpp>
-#include <ArduinoJson/Variant/VariantSlot.hpp>
 
 #include <string.h>  // memmove
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 
-// Returns the size (in bytes) of an array with n elements.
-constexpr size_t sizeofArray(size_t n) {
-  return n * sizeof(VariantSlot);
-}
-
-// Returns the size (in bytes) of an object with n members.
-constexpr size_t sizeofObject(size_t n) {
-  return n * sizeof(VariantSlot);
-}
-
 class MemoryPool {
  public:
   MemoryPool(size_t capa, Allocator* allocator = DefaultAllocator::instance())
@@ -88,11 +77,13 @@ class MemoryPool {
     return overflowed_;
   }
 
-  VariantSlot* allocVariant() {
-    auto slot = allocRight<VariantSlot>();
-    if (slot)
-      slot->clear();
-    return slot;
+  void* allocFromPool(size_t bytes) {
+    if (!canAlloc(bytes)) {
+      overflowed_ = true;
+      return 0;
+    }
+    right_ -= bytes;
+    return right_;
   }
 
   template <typename TAdaptedString>
@@ -198,10 +189,10 @@ class MemoryPool {
     return p;
   }
 
-  void shrinkToFit(VariantData& variant) {
+  ptrdiff_t shrinkToFit() {
     ptrdiff_t bytes_reclaimed = squash();
     if (bytes_reclaimed == 0)
-      return;
+      return 0;
 
     void* old_ptr = begin_;
     void* new_ptr = allocator_->reallocate(old_ptr, capacity());
@@ -210,8 +201,7 @@ class MemoryPool {
         static_cast<char*>(new_ptr) - static_cast<char*>(old_ptr);
 
     movePointers(ptr_offset);
-    reinterpret_cast<VariantSlot&>(variant).movePointers(ptr_offset -
-                                                         bytes_reclaimed);
+    return ptr_offset - bytes_reclaimed;
   }
 
  private:
@@ -251,20 +241,6 @@ class MemoryPool {
     }
   }
 
-  template <typename T>
-  T* allocRight() {
-    return reinterpret_cast<T*>(allocRight(sizeof(T)));
-  }
-
-  void* allocRight(size_t bytes) {
-    if (!canAlloc(bytes)) {
-      overflowed_ = true;
-      return 0;
-    }
-    right_ -= bytes;
-    return right_;
-  }
-
   void allocPool(size_t capa) {
     auto buf = capa ? reinterpret_cast<char*>(allocator_->allocate(capa)) : 0;
     begin_ = buf;

src/ArduinoJson/MsgPack/MsgPackDeserializer.hpp

@@ -495,7 +495,7 @@ class MsgPackDeserializer {
         // Save key in memory pool.
         auto savedKey = stringBuilder_.save();
 
-        VariantSlot* slot = pool_->allocVariant();
+        VariantSlot* slot = new (pool_) VariantSlot();
         if (!slot)
           return DeserializationError::NoMemory;
 

src/ArduinoJson/Variant/VariantAttorney.hpp

@@ -6,6 +6,7 @@
 
 #include <ArduinoJson/Polyfills/attributes.hpp>
 #include <ArduinoJson/Polyfills/type_traits.hpp>
+#include <ArduinoJson/Variant/VariantData.hpp>
 #include <ArduinoJson/Variant/VariantTo.hpp>
 #include "JsonVariantConst.hpp"
 

src/ArduinoJson/Variant/VariantData.hpp

@@ -10,6 +10,7 @@
 #include <ArduinoJson/Strings/JsonString.hpp>
 #include <ArduinoJson/Strings/StringAdapters.hpp>
 #include <ArduinoJson/Variant/VariantContent.hpp>
+#include <ArduinoJson/Variant/VariantSlot.hpp>
 
 ARDUINOJSON_BEGIN_PRIVATE_NAMESPACE
 

src/ArduinoJson/Variant/VariantFunctions.hpp

@@ -190,7 +190,7 @@ inline NO_INLINE VariantData* variantGetOrAddElement(VariantData* var,
   if (!slot)
     index++;
   while (index > 0) {
-    slot = pool->allocVariant();
+    slot = new (pool) VariantSlot();
     if (!slot)
       return nullptr;
     array->add(slot);

src/ArduinoJson/Variant/VariantSlot.hpp

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <ArduinoJson/Memory/MemoryPool.hpp>
 #include <ArduinoJson/Polyfills/integer.hpp>
 #include <ArduinoJson/Polyfills/limits.hpp>
 #include <ArduinoJson/Polyfills/type_traits.hpp>
@@ -25,11 +26,15 @@ class VariantSlot {
   const char* key_;
 
  public:
-  // Must be a POD!
-  // - no constructor
-  // - no destructor
-  // - no virtual
-  // - no inheritance
+  static void* operator new(size_t size, MemoryPool* pool) noexcept {
+    return pool->allocFromPool(size);
+  }
+
+  static void operator delete(void*, MemoryPool*) noexcept {
+    // we cannot release memory from the pool
+  }
+
+  VariantSlot() : flags_(0), next_(0), key_(0) {}
 
   VariantData* data() {
     return reinterpret_cast<VariantData*>(&content_);
@@ -93,21 +98,20 @@ class VariantSlot {
   bool ownsKey() const {
     return (flags_ & OWNED_KEY_BIT) != 0;
   }
-
-  void clear() {
-    next_ = 0;
-    flags_ = 0;
-    key_ = 0;
-  }
-
-  void movePointers(ptrdiff_t variantDistance) {
-    if (flags_ & COLLECTION_MASK)
-      content_.asCollection.movePointers(variantDistance);
-  }
 };
 
 inline VariantData* slotData(VariantSlot* slot) {
   return reinterpret_cast<VariantData*>(slot);
 }
 
+// Returns the size (in bytes) of an array with n elements.
+constexpr size_t sizeofArray(size_t n) {
+  return n * sizeof(VariantSlot);
+}
+
+// Returns the size (in bytes) of an object with n members.
+constexpr size_t sizeofObject(size_t n) {
+  return n * sizeof(VariantSlot);
+}
+
 ARDUINOJSON_END_PRIVATE_NAMESPACE