[qalmakka]

Thanks a lot to you for writing such an awesome library! :)

This is briefly how I use C++ Allocators with the ESP32 and Nlohmann/JSON (GCC8, C++17 mode):

I have a series of "extmem" headers which define aliases for STL containers which use my allocator (ext::allocator). The allocator is a simple allocator that just uses IDF's heap_caps_malloc() to allocate memory on the SPIRAM of the ESP-WROVER SoC.

I then define in <extmem/json.hpp>:

    namespace ext {
        using json = nlohmann::basic_json<std::map, std::vector, ext::string, bool, long long, unsigned long long, double, ext::allocator, nlohmann::adl_serializer>;
    }

where `ext::string` is just `std::basic_string<char, std::char_traits<char>, ext::allocator<char>>`. In order to be able to define generic from/to_json functions in an ergonomic way, I had to reexport the following internal macros in a separate header:

    #define JSON_TEMPLATE_PARAMS \
        template<typename, typename, typename...> class ObjectType,   \
        template<typename, typename...> class ArrayType,              \
        class StringType, class BooleanType, class NumberIntegerType, \
        class NumberUnsignedType, class NumberFloatType,              \
        template<typename> class AllocatorType,                       \
        template<typename, typename = void> class JSONSerializer

    #define JSON_TEMPLATE template<JSON_TEMPLATE_PARAMS>

    #define GENERIC_JSON                                            \
        nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType,             \
        NumberIntegerType, NumberUnsignedType, NumberFloatType,                \
        AllocatorType, JSONSerializer>

I am now able to just write stuff like the following:

    JSON_TEMPLATE
    inline void from_json(const GENERIC_JSON &j, my_type &t) {
        // ... 
    }

    JSON_TEMPLATE
    inline void to_json(GENERIC_JSON &j, const my_type &t) {
        // ...
    }

And it works fine with both nlohmann::json and ext::json.

In the rest of the code, everything stays the same; I simply use ext::json (and catch const ext::json::exception&) as if it were it's default version, and it works great. FYI, I'm currently using nlohmann/json v.3.9.1.