Custom type for numeric values

[chghehe]

Can I use custom type for numeric values?

For example, that might make sense in financial applications and I would like to use boost::multiprecision::cpp_dec_float_50 instead of double or any other builtin types (like float or long double, they are base-2 floating point numbers as well).

I tried to achieve that using corresponding template parameter of json<> class, but that did not work out of the box.

[nlohmann]

What was the corresponding error message? What did you try?

[chghehe]

The custom json instantiation:

using json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, boost::multiprecision::cpp_dec_float_50 >;
json j;

Error (clang-10):

/usr/local/include/nlohmann/json.hpp:16434: error: call to implicitly-deleted copy constructor of 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, boost::multiprecision::number<boost::multiprecision::backends::cpp_dec_float<50, int, void>, boost::multiprecision::et_on>, std::allocator, adl_serializer>::json_value'
In file included from /Users/asu/Code/test/main.cpp:1:
/usr/local/include/nlohmann/json.hpp:16434:11: error: call to implicitly-deleted copy constructor of 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, boost::multiprecision::number<boost::multiprecision::backends::cpp_dec_float<50, int, void>, boost::multiprecision::et_on>, std::allocator, adl_serializer>::json_value'
          m_value(std::move(other.m_value))
          ^       ~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/nlohmann/json.hpp:15565:28: note: in instantiation of member function 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, boost::multiprecision::number<boost::multiprecision::backends::cpp_dec_float<50, int, void>, boost::multiprecision::et_on>, std::allocator, adl_serializer>::basic_json' requested here
                basic_json current_item(std::move(stack.back()));
                           ^
/usr/local/include/nlohmann/json.hpp:16505:17: note: in instantiation of member function 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, boost::multiprecision::number<boost::multiprecision::backends::cpp_dec_float<50, int, void>, boost::multiprecision::et_on>, std::allocator, adl_serializer>::json_value::destroy' requested here
        m_value.destroy(m_type);
                ^
/Code/test/main.cpp:284:8: note: in instantiation of member function 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, boost::multiprecision::number<boost::multiprecision::backends::cpp_dec_float<50, int, void>, boost::multiprecision::et_on>, std::allocator, adl_serializer>::~basic_json' requested here
  json j;
       ^
/usr/local/include/nlohmann/json.hpp:15429:24: note: copy constructor of 'json_value' is implicitly deleted because variant field 'number_float' has a non-trivial copy constructor
        number_float_t number_float;
                       ^

[chghehe]

json_value is an union and

If a union contains a non-static data member with a non-trivial special member function (copy/move constructor, copy/move assignment, or destructor), that function is deleted by default in the union and needs to be defined explicitly by the programmer.

By the way, why std::variant is not used instead of union types?

[nlohmann]

Because std::variant is C++17, and the library targets C++11.

[chghehe]

Sure, I forgot about that. Well, that might be partially implemented using C++11 std::aligned_union<>:

struct json_value
{
  size_t id;  //< or enum?
  typename std::aligned_union<8, object_type, array_type, ... >::type storage;

  json_value(object_type const& object) : id(0) { 
    new (&storage) object_type(object); 
  }
  json_value(array_type const& object) : id(1) { 
    new (&storage) array_type(object); 
  }

  ...

  ~json_value() {
    this->destroy();
  }

private:
  void destroy() {
    if (id == 0)
      reinterpret_cast<object_type*>(&storage)->~object_type();

    ...
  }
};

Current implementation does the same things actually but in a different way, and some types must satisfy TrivialCopyable requirements. Not the modern C++ :)