Class cannot serialize itself using boost::json

Question

I wrote a tiny class which relies on boost::json to serialize itself. It fails to compile since compiler sounds to fail finding tag_invoke() functions which implement json::value<=>myClass translation :

/usr/include/boost/json/value_from.hpp:87:35: error: no matching function for call to ‘value_from_impl(CSpicaFTConfig&, std::remove_reference<boost::json::storage_ptr&>::type)’
 return detail::value_from_impl(
        ~~~~~~~~~~~~~~~~~~~~~~~^
     std::forward<T>(t), std::move(sp));

An implementation of those 2 functions is given:

CSpicaFTConfig CSpicaFTConfig::tag_invoke(value_to_tag<CSpicaFTConfig>, value const &jv);
void CSpicaFTConfig::tag_invoke(value_from_tag, value &jv, CSpicaFTConfig const &cfg);

But compiler seems to be unable to find them.

Any hints are welcome ! Thanks

Answer 1

Actually, the proper way to implement tag_invoke() functions is to define them as free functions, friends of the object class (here it is CSpicaFTConfig ) to serialize:

  // CSpicaFTConfig => boost::json::value
  friend void tag_invoke(value_from_tag, value &jv, CSpicaFTConfig const &cfg);
  // boost::json::value => CSpicaFTConfig
  friend CSpicaFTConfig tag_invoke(value_to_tag<CSpicaFTConfig>, value const &jv);

Thanks for your support.