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Cannot use std::format on nholman::json objects #4261
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User-defined types must opt in to being formatted by
I assume that |
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 |
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. |
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. |
Yes, this library has |
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 :(. |
To implement a formatter which would work from fmt 10.0.0 onwards it would suffice to implement namespace nlohmann {
auto format_as(const json& j) { return j.dump(); }
} Godbolt: https://godbolt.org/z/vEEad83h3 |
Description
I've opened a json file and parsed it with this library and I wrote this:
However I get this error:
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
Error messages
Compiler and operating system
clang 17, linux
Library version
3.11.2
Validation
develop
branch is used.The text was updated successfully, but these errors were encountered: