BasicJsonDocument<T> is the base class of DynamicJsonDocument. It’s a JsonDocument that allocates its memory pool using the allocator class T.

Use this class when you want to use a custom memory allocator; for example, when you want to use an external PSRAM found in many ESP32 boards. See examples below.

This class was added in ArduinoJson 6.10.0

The allocator class

The allocator class must implement two member functions:

struct Allocator {
  void* allocate(size_t);
  void deallocate(void*);
  void* reallocate(void*, size_t) ; // optional

allocate() is similar to malloc()

deallocate() is similar to free()

reallocate() is similar to realloc(). This member is optional and its only required if you call shrinkToFit().

Member functions

  • as<T>() casts the root to the specified type (e.g. JsonArray or JsonObject)
  • add() adds elements to the root array
  • capacity() returns the capacity of the memory pool
  • clear() empties the document and resets the memory pool
  • containsKey() tests if the root object contains the specified key
  • createNestedArray() creates a nested array attached to the root
  • createNestedObject() create a nested object attached to the root
  • garbageCollect() reclaims leaked memory blocks
  • operator[] gets or sets values in the document
  • overflowed() tells if the memory pool was large enough 🆕
  • is<T>() tests the type of the root
  • isNull() tells if the document is null or empty
  • memoryUsage() tells how many bytes are used in the memory pool
  • nesting() returns the number of nesting layers in the document
  • remove() removes an element (or member) at the specified index (or key)
  • set() replaces the root with the specified value
  • shrinkToFit() reduces the capacity of the memory pool to match the current usage
  • size() returns the number of elements (or members) that the root array (or object) contains
  • to<T>() clears the document and converts it to the specified type (e.g. JsonArray or JsonObject)



As as example here is how DynamicJsonDocument is defined:

struct DefaultAllocator {
  void* allocate(size_t size) {
    return malloc(size);

  void deallocate(void* ptr) {

  void* reallocate(void* ptr, size_t new_size) {
    return realloc(ptr, new_size);

typedef BasicJsonDocument<DefaultAllocator> DynamicJsonDocument;

External RAM on ESP32

To use the external SPI memory on an ESP32, you must call heap_caps_malloc(MALLOC_CAP_SPIRAM) instead of the usual malloc(). Such memory is available on the WROVER module.

The following SpiRamJsonDocument allocates its memory pool in the external RAM:

struct SpiRamAllocator {
  void* allocate(size_t size) {
    return heap_caps_malloc(size, MALLOC_CAP_SPIRAM);

  void deallocate(void* pointer) {

  void* reallocate(void* ptr, size_t new_size) {
    return heap_caps_realloc(ptr, new_size, MALLOC_CAP_SPIRAM);

using SpiRamJsonDocument = BasicJsonDocument<SpiRamAllocator>;

See: How to use external RAM on an ESP32?

See also