[json, core] Remove stored RAMAllocator, make constructors constexpr

RAMAllocator with default flags is stateless — it's just a dispatch
wrapper over heap_caps_malloc/realloc/free. Remove the stored member
from SpiRamAllocator, using stack-local instances at each call site.

Also make RAMAllocator constructors constexpr so the compiler can
fully evaluate flag logic at compile time.

Note: SpiRamAllocator was initialized with RAMAllocator::NONE (0),
which is equivalent to default construction since the constructor
preserves the default ALLOC_INTERNAL | ALLOC_EXTERNAL flags when
no valid allocation flags are provided.

Co-Authored-By: J. Nick Koston <nick@koston.org>

esphome/components/json/json_util.h

@@ -18,7 +18,10 @@ namespace json {
 // Build an allocator for the JSON Library using the RAMAllocator class
 // This is only compiled when PSRAM is enabled
 struct SpiRamAllocator : ArduinoJson::Allocator {
-  void *allocate(size_t size) override { return allocator_.allocate(size); }
+  void *allocate(size_t size) override {
+    RAMAllocator<uint8_t> allocator;
+    return allocator.allocate(size);
+  }
 
   void deallocate(void *ptr) override {
     // ArduinoJson's Allocator interface doesn't provide the size parameter in deallocate.
@@ -31,11 +34,9 @@ struct SpiRamAllocator : ArduinoJson::Allocator {
   }
 
   void *reallocate(void *ptr, size_t new_size) override {
-    return allocator_.reallocate(static_cast<uint8_t *>(ptr), new_size);
+    RAMAllocator<uint8_t> allocator;
+    return allocator.reallocate(static_cast<uint8_t *>(ptr), new_size);
   }
-
- protected:
-  RAMAllocator<uint8_t> allocator_{RAMAllocator<uint8_t>::NONE};
 };
 #endif
 

esphome/core/helpers.h

@@ -1673,13 +1673,10 @@ template<class T> class RAMAllocator {
     ALLOW_FAILURE = 1 << 2,   // Does nothing. Kept for compatibility.
   };
 
-  RAMAllocator() = default;
-  RAMAllocator(uint8_t flags) {
-    // default is both external and internal
-    flags &= ALLOC_INTERNAL | ALLOC_EXTERNAL;
-    if (flags != 0)
-      this->flags_ = flags;
-  }
+  constexpr RAMAllocator() = default;
+  constexpr RAMAllocator(uint8_t flags)
+      : flags_((flags & (ALLOC_INTERNAL | ALLOC_EXTERNAL)) != 0 ? (flags & (ALLOC_INTERNAL | ALLOC_EXTERNAL))
+                                                                : (ALLOC_INTERNAL | ALLOC_EXTERNAL)) {}
   template<class U> constexpr RAMAllocator(const RAMAllocator<U> &other) : flags_{other.flags_} {}
 
   T *allocate(size_t n) { return this->allocate(n, sizeof(T)); }