SPDLOG_LOGGER_CALL with additional data and custom formatting

[AndyDralle]

Hi!

So, I have a logger that I want to produce output in JSON format.

If I call SPDLOG_LOGGER_CALL(myLogger, myLogLevel, "Some message"), I suppose I could give my logger a custom formatting string that will take the payload and produce a log message like this:
{payload: "Some message"}

But I would like to introduce some additional data, beyond just the message string. For example, I may have something like this:

enum Class Day {
Monday,
Tuesday,
Wednesday,
...
}

struct MessageContext {
Day day;
int count;
}

Is there some way for me to include a MessageContext object in my logger call, in addition to the payload string?
Like:

MessageContext context{Day::Monday, 5};

SPDLOG_LOGGER_CALL(myLogger, myLogLevel, "Some message", context);

And then have my logger produce final output like:

{payload: "Some message", day: "Monday", count: 5}

I believe this SPDLOG_LOGGER_CALL call compiles, but I'm having trouble figuring out what happens to my additional context, once it has become part of the __VA_ARGS__ in SPDLOG_LOGGER_CALL. Is there a way for me to use this additional data to produce my final log output, or is this simply outside the intended use case?

[tt4g]

Additional log arguments are formatted by the fmt library, so a custom data formatter for fmt must be defined.
See: https://fmt.dev/9.1.0/api.html#formatting-user-defined-types

However, spdlog does not have an API to output log data as JSON format.
This should be customized by the developer.
See: #1797

[AndyDralle]

Additional log arguments are formatted by the fmt library, so a custom data formatter for fmt must be defined. See: https://fmt.dev/9.1.0/api.html#formatting-user-defined-types

Does this mean that my message would need to include a {} placeholder like:
SPDLOG_LOGGER_CALL(myLogger, myLogLevel, "Some message {}", context);?

I suppose fmt does not do anything with additional arguments otherwise.

[tt4g]

Yes. {} is required.