Formatting for derived classes

[ukreator]

Hi,

What is the correct way to provide the formatter for a derived class that uses the formatter of the base class? Something like this doesn't work:

#include <fmt/format.h>

namespace A
{
struct X {
    int x = 1;
};
struct Y: public X {
};
}

namespace fmt
{
template<>
struct formatter<A::X> {
   template<typename ParseContext>
   constexpr auto parse(ParseContext& ctx) {
       return ctx.begin();
   }

   template<typename FormatContext>
   auto format(const A::X& val, FormatContext& ctx) {
       return format_to(ctx.begin(), "{}", val.x);
   }
};
}

int main() {
    auto s = fmt::format("{}", A::Y{});
}

Compilation error:

In file included from <source>:1:
In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:54:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:541:56: error: implicit instantiation of undefined template 'fmt::v5::formatter<A::Y, char, void>'
    typename Context::template formatter_type<T>::type f;
                                                       ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:527:22: note: in instantiation of function template specialization 'fmt::v5::internal::value<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >::format_custom_arg<A::Y>' requested here
    custom.format = &format_custom_arg<T>;
                     ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:553:45: note: in instantiation of function template specialization 'fmt::v5::internal::value<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char> >::value<A::Y>' requested here
  FMT_CONSTEXPR typed_value(const T &val) : value<Context>(val) {}
                                            ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:960:10: note: in instantiation of function template specialization 'fmt::v5::internal::make_value<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>, A::Y, char>' requested here
  return make_value<Context>(value);
         ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1013:23: note: in instantiation of function template specialization 'fmt::v5::internal::make_arg<true, fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>, A::Y>' requested here
    : data_{internal::make_arg<IS_PACKED, Context>(args)...} {}
                      ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:1262:45: note: in instantiation of member function 'fmt::v5::format_arg_store<fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<char> >, char>, A::Y>::format_arg_store' requested here
  format_arg_store<format_context, Args...> as{args...};
                                            ^
<source>:29:19: note: in instantiation of function template specialization 'fmt::v5::format<A::Y>' requested here
    auto s = fmt::format("{}", A::Y{});
                  ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/core.h:315:8: note: template is declared here
struct formatter;

[ukreator]

My colleague suggested to use something like this:

template<typename T, typename Char>
struct formatter<T, Char,  std::enable_if_t<std::is_convertible<T*, A::X*>::value>> {
    // ...
}

which seem to be working. Closing this issue.

[vitaut]

My colleague suggested to use something like this

Yep, that's the way to do it.