Added [[nodiscard]] and made should_flush_ const

include/spdlog/logger.h

@@ -209,7 +209,7 @@ protected:
         }
     }
     virtual void flush_();
-    bool should_flush_(const details::log_msg &msg);
+    [[nodiscard]] bool should_flush_(const details::log_msg &msg) const;
 
     // handle errors during logging.
     // default handler prints the error to stderr at max rate of 1 message/sec.

src/logger.cpp

@@ -81,7 +81,7 @@ void logger::flush_() {
     }
 }
 
-bool logger::should_flush_(const details::log_msg &msg) {
+bool logger::should_flush_(const details::log_msg &msg) const {
     auto flush_level = flush_level_.load(std::memory_order_relaxed);
     return (msg.log_level >= flush_level) && (msg.log_level != level::off);
 }