Member basic_json::operator<< (basic_json &j, std::istream &i): This stream operator is deprecated since 3.0.0 and will be removed in version 4.0.0 of the library. Please use operator>>(std::istream&, basic_json&) instead; that is, replace calls like j << i; with i >> j;.
Member basic_json::sax_parse (detail::span_input_adapter &&i, SAX *sax, input_format_t format=input_format_t::json, const bool strict=true, const bool ignore_comments=false): This function is deprecated since 3.8.0 and will be removed in version 4.0.0 of the library. Please use sax_parse(ptr, ptr + len) instead.