How to write format_as for enums of external namespaces in fmt 10.0

Question

For those who don't know, in fmt 10.0, they removed implicit conversions for enums for std::format. Instead you need to bring a generic format_as function to your own namespace. However having this function in your namespace alone does not fix everything because this function does not catch enums that exist other namespaces.

I tried implicit instantiation for enums that exist in other namespaces, however it failed. I tried to import external enums into my namespace using using ExternalEnum = ExternalNS::Enum;, it failed. I tried using namespace ExternalNS;, it also failed.

The only solutions I found are extending the external namespace by adding the same generic format_as function or implicitly casting every enum.

Is there any other solutions or if not which one is the best solution?

Answer 1

With C++20, you can provide a formatter for enums:

template <typename EnumType>
requires std::is_enum_v<EnumType>
struct fmt::formatter<EnumType> : fmt::formatter<std::underlying_type_t<EnumType>>
{
    // Forwards the formatting by casting the enum to its underlying type
    auto format(const EnumType& enumValue, format_context& ctx) const
    {
        return fmt::formatter<std::underlying_type_t<EnumType>>::format(
            static_cast<std::underlying_type_t<EnumType>>(enumValue), ctx);
    }
};

https://godbolt.org/z/ebj1EndG1