UTC Time

[ThePhD]

This issue addresses #215 in the way @gabime stated he'd like it to be handled: with a simple flag that can be passed to pattern_formatter's constructor. It's a smaller, simpler solution that propagates its changes to the virtual functions _set_pattern and set_pattern in spdlog::logger and spdlog::async_logger.

The implementation is made as an enumeration simply because there may be more efficient time abstractions that spdlog takes advantage of in the future: the enumeration allows for seamless transition to these while maintaining backwards compatibility by ensuring that all defaulted loggers and pattern formatters use spdlog::pattern_time::local by default.

Anyone who has derived from logger and async_logger will have to change their code to handle the new parameter for the virtual set_pattern call, but I believe this is a negligible burden in 100% of the cases where people have actually customized spdlog to this degree.

I have not measured the impact of a switch statement, but have done my best to use my general knowledge to ensure it will remain more or less swift, especially in the default case.

Small question: did you mean to never initialize the member variable _pattern in spdlog::pattern_formatter ? It never gets initialized (and you never use it anywhere?). I would imagine it's accidentally an artifact of slowly changing the pattern formatter to work in a compiled form and actually no longer needing it, so I don't set it in my code, but I do set the _time member variable. Shouldn't it be removed, if that's the case? If so, I will be happy to remove it in this branch.

Let me know if you need anything else.

[gabime]

1. R_formatter should do HH:MM only

2. I am not sure what's the point in the lambda. Why not simple

if (ptime == pattern_time::local) 
  return details::os::localtime(log_clock::to_time_t(msg.time));
else 
  return details::os::gmtime(log_clock::to_time_t(msg.time));

3. Please change the pattern_time enum name to pattern_time_type

4. Please change the ptime variable name to time_type

[ThePhD]

1. (Fixed) Oops, that was a mistake from something else.

2. (Fixed) I'll change it to the if statement. I use a lambda so I could early-return and I only used it once, but I can just make it a private member function instead.
   2a. (Rationale Explanation, Not Actually Done) If I remove the "default" statement and use a switch statement, then gcc/clang will warn if the switch statement doesn't use all the enum values of pattern_time_type. This might be useful for catching unupdated code in the future if more pattern_time_type values are added. But, I'm still leaving it as the if-statement as you requested!

3 & 4. (Fixed) I was wondering what would be a good way to name them. Changing to pattern_time_type frees up the variable name pattern_time: done!