Recommended method for adding formatter for third-party custom type

[mikecrowe]

I'd like to add a formatter for std::chrono::duration in the style of #392 . This is straightforward to do:

template <typename Rep, typename Period>void format_arg(fmt::BasicFormatter<char> &f,                                                                                                                                             
                const char *&format_str, const std::chrono::duration<Rep, Period> &ms) {
    f.writer().write("{}s", std::chrono::duration<double>(ms).count());
}

But, in order for argument-dependent lookup to work, this function needs to either be in the fmt or std::chrono namespaces. I don't believe that I'm supposed to put it in the std namespace, so that leaves putting it in fmt. Is that acceptable, or is there a better way?

Thanks.

Mike.

[vitaut]

Is that acceptable, or is there a better way?

Defining extension functions in the fmt namespace is perfectly acceptable.

Note, however, that the extension API will change in the next major version to accommodate constexpr format string checks and output iterators per standards committee feedback. The API is still WIP, but will likely look as follows: http://fmtlib.net/dev/api.html#formatting-user-defined-types. The 4.x branch will remain supported for compatibility with older compilers so migration to the new API is optional.

[mikecrowe]

Thanks for the quick answer!

Mike.