Cannot use std::format on nholman::json objects

[AA1999]

Description

I've opened a json file and parsed it with this library and I wrote this:

std::string connection_config = std::format("host={} port={} dbname={} user={} password='{}'", db_config.at("host"),
												db_config.at("port"), db_config.at("dbname"), db_config.at("user"),
												db_config.at("password"));

However I get this error:

In template: call to deleted constructor of 'typename format_context::formatter_type<basic_json<std::map, std::vector, string, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, vector<unsigned char, allocator<unsigned char>>>>' (aka 'formatter<nlohmann::basic_json<std::map, std::vector, std::string, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char>>>, char>')

Note of interest: this only happens with std::format, fmt::format is fine

Reproduction steps

Open a file with std::ifstream
Pass the file object to nlohmann::json
Parse the json file
Try to format it using std::format

Expected vs. actual results

Expected result: The json array being formatted properly
Actual result: error

Minimal code example

auto db_config_file = std::ifstream {"~/.reacatio/database.json"};

	json db_config = json::parse(db_config_file);
	db_config_file.close();

	std::string connection_config = std::format("host={} port={} dbname={} user={} password='{}'", db_config.at("host"),
												db_config.at("port"), db_config.at("dbname"), db_config.at("user"),
												db_config.at("password"));

Error messages

In template: call to deleted constructor of 'typename format_context::formatter_type<basic_json<std::map, std::vector, string, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, vector<unsigned char, allocator<unsigned char>>>>' (aka 'formatter<nlohmann::basic_json<std::map, std::vector, std::string, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer, std::vector<unsigned char, std::allocator<unsigned char>>>, char>')

Compiler and operating system

clang 17, linux

Library version

3.11.2

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[gregmarr]

User-defined types must opt in to being formatted by std::format by specializing std::formatter. https://en.cppreference.com/w/cpp/utility/format/formatter

fmt::format is probably picking up implicit conversions, which means that it can get them very very wrong. What happens if you have a json file with numbers instead of strings? Does fmt::format display them properly? I would be very surprised, unless someone added support for this specific library.

I assume that std::format explicitly excludes implicit conversions to types that std::formatter is enabled for, which is much safer.

[AA1999]

For some reason now fmt is also now not working (I'm pretty sure it did before, maybe an earlier version)

In file included from test.cpp:4:
In file included from /usr/include/fmt/format.h:49:
/usr/include/fmt/core.h:2548:45: error: implicit instantiation of undefined template 'fmt::detail::type_is_unformattable_for<nlohmann::basic_json<>, char>'
    type_is_unformattable_for<T, char_type> _;
                                            ^
/usr/include/fmt/core.h:2611:23: note: in instantiation of function template specialization 'fmt::detail::parse_format_specs<nlohmann::basic_json<>, fmt::detail::compile_parse_context<char>>' requested here
        parse_funcs_{&parse_format_specs<Args, parse_context_type>...} {}
                      ^
/usr/include/fmt/core.h:2740:47: note: in instantiation of member function 'fmt::detail::format_string_checker<char, nlohmann::basic_json<>, nlohmann::basic_json<>>::format_string_checker' requested here
      detail::parse_format_string<true>(str_, checker(s));
                                              ^
test.cpp:9:27: note: in instantiation of function template specialization 'fmt::basic_format_string<char, nlohmann::basic_json<> &, nlohmann::basic_json<> &>::basic_format_string<char[6], 0>' requested here
        std::cout << fmt::format("{} {}", data.at("host"), data.at("port"));
                                 ^
/usr/include/fmt/core.h:1554:45: note: template is declared here
template <typename T, typename Char> struct type_is_unformattable_for;
                                            ^
test.cpp:9:27: error: call to consteval function 'fmt::basic_format_string<char, nlohmann::basic_json<> &, nlohmann::basic_json<> &>::basic_format_string<char[6], 0>' is not a constant expression
        std::cout << fmt::format("{} {}", data.at("host"), data.at("port"));
                                 ^
In file included from test.cpp:4:
In file included from /usr/include/fmt/format.h:49:
/usr/include/fmt/core.h:1576:63: error: implicit instantiation of undefined template 'fmt::detail::type_is_unformattable_for<nlohmann::basic_json<>, char>'
    type_is_unformattable_for<T, typename Context::char_type> _;
                                                              ^
/usr/include/fmt/core.h:1808:23: note: in instantiation of function template specialization 'fmt::detail::make_arg<true, fmt::basic_format_context<fmt::appender, char>, nlohmann::basic_json<>, 0>' requested here
        data_{detail::make_arg<is_packed, Context>(args)...} {
                      ^
/usr/include/fmt/core.h:1826:10: note: in instantiation of function template specialization 'fmt::format_arg_store<fmt::basic_format_context<fmt::appender, char>, nlohmann::basic_json<>, nlohmann::basic_json<>>::format_arg_store<nlohmann::basic_json<>, nlohmann::basic_json<>>' requested here
  return {args...};
         ^
/usr/include/fmt/core.h:2788:28: note: in instantiation of function template specialization 'fmt::make_format_args<fmt::basic_format_context<fmt::appender, char>, nlohmann::basic_json<>, nlohmann::basic_json<>>' requested here
  return vformat(fmt, fmt::make_format_args(args...));
                           ^
test.cpp:9:20: note: in instantiation of function template specialization 'fmt::format<nlohmann::basic_json<> &, nlohmann::basic_json<> &>' requested here
        std::cout << fmt::format("{} {}", data.at("host"), data.at("port"));
                          ^
/usr/include/fmt/core.h:1554:45: note: template is declared here
template <typename T, typename Char> struct type_is_unformattable_for;
                                            ^
/usr/include/fmt/core.h:1579:3: error: static assertion failed due to requirement 'formattable': Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
  static_assert(
  ^
4 errors generated.

Anyhow then ig nlohmann::json would need a std::formatter?

[gregmarr]

Anyhow then ig nlohmann::json would need a std::formatter?

Yes. I assume, but can't say for sure, that a PR would be welcome as long as it was sufficient protected such that it didn't affect the ability to build as C++11. There are other features that are conditionally available based on the C++ standard in use. @nlohmann would have the final say on that.

[AA1999]

As far as I'm aware there are micros to check C++ version and not define a function if it's not matching, so maybe this could be added only if c++20 flag is added.

[gregmarr]

Yes, this library has JSON_HAS_CPP_20 for that.

[heinemml]

I ran into this when updating dependencies.

For fmt you can observe this break in this godbolt example: https://godbolt.org/z/jx7fTen7Y

with fmt 9.1.0 it works, with 10.0.0 it still compiles but throws and with 10.1.1 it finally doesn't compile anymore.

I guess something like this would fix the issue for std::format

#if defined(JSON_HAS_CPP_20) && __has_include(<format>)

#include <format>

template <>
struct std::formatter<json> {

    constexpr auto parse(std::format_parse_context& ctx) {
        return ctx.begin();
    }

    auto format(const json& j, std::format_context& ctx) const {
        return std::format_to(ctx.out(), "{}", to_string(j));
    }
};

#endif

but this doesn't fix it for fmt :(.

[heinemml]

fmtlib/fmt#3648

[heinemml]

To implement a formatter which would work from fmt 10.0.0 onwards it would suffice to implement format_as as follows:

namespace nlohmann {

auto format_as(const json& j) { return j.dump(); }

}

Godbolt: https://godbolt.org/z/vEEad83h3