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Showing content from https://arduinojson.org/v6/api/basicjsondocument/ below:

BasicJsonDocument<T> | ArduinoJson 6

Description

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)

Examples DynamicJsonDocument

As as example here is how DynamicJsonDocument is defined:

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

  void deallocate(void* ptr) {
    free(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](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) {
    heap_caps_free(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?