spdlog is very fast, header only, C++ logging library.

Features:
- Very fast - performance is the primary goal (see benchmarks below).
- Headers only, just copy and use.
- Feature rich call style using the excellent fmt library.
- Extremely fast asynchronous mode (optional) - using lockfree queues
  and other tricks to reach millions of calls/sec.
- Custom formatting.
- Multi/Single threaded loggers.
- Various log targets:
 - Rotating log files.
 - Daily log files.
 - Console logging (colors supported).
 - syslog.
 - Windows debugger (OutputDebugString(..))
 - Easily extendable with custom log targets (just implement a single
   function in the sink interface).
- Severity based filtering - threshold levels can be modified in
  runtime as well as in compile time.