use custom input iterator for serialization

[dhebbeker]

I have data in an incompatible, proprietary container which I would like to serialize. I thought of using a custom iterator which would use that container and provide data in a compatible/serializable format. That is, de-referencing the iterator would convert the respective element on-demand.

I hoped I could simply pass begin and end iterator to the json constructor. But when I tried this with a well defined container, compilation fails:

#include <nlohmann/json.hpp>
#include <vector>
#include <iterator>

int main() {
  std::vector<int> v;
  nlohmann::json j(std::begin(v), std::end(v));
}

Is there a way to achieve this?

[nlohmann]

Do you have more details like the source of your container or the error message?

[dhebbeker]

I have a T1 of std::map which can not be automatically serialized by json. But I have an input iterator type which is constructed with a T1::iterator and which has a T2 operator*() overload. T2 can be serialized through a custom to_json().

The code I am working on is public, if you really want to dig into the details (WIP!).

As I was not succeeding, I though of trying with a simplified example (see above). Checking if it works with C++ Standard Library iterators. But this fails with clang 15 (see full output on Compiler Explorer). I think the relevant error message is:

<source>:7:18: error: no matching constructor for initialization of 'nlohmann::json' (aka 'basic_json<>')
  nlohmann::json j(std::begin(v), std::end(v));
                 ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
...
/opt/compiler-explorer/libs/nlohmann_json/trunk/single_include/nlohmann/json.hpp:20335:5: note: candidate template ignored: requirement 'std::is_same<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int>>>, nlohmann::detail::iter_impl<nlohmann::basic_json<std::map, std::vector, std::basic_string<char, std::char_traits<char>, std::allocator<char>>, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char>>, void>>>::value || std::is_same<__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator<int>>>, nlohmann::detail::iter_impl<const nlohmann::basic_json<std::map, std::vector, std::basic_string<char, std::char_traits<char>, std::allocator<char>>, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char>>, void>>>::value' was not satisfied [with InputIT = __gnu_cxx::__normal_iterator<int *, std::vector<int>>]
    basic_json(InputIT first, InputIT last)

[dhebbeker]

I was hoping to use constructor overload number 7 from the documentation: https://json.nlohmann.me/api/basic_json/basic_json/