Added optional default logger setup possibility

User can now add default logger passed to set_default function and then use it across code base by just calling get_default function. This helps to avoid common typos when accessing logger via get function.

example/example.cpp

@@ -20,6 +20,7 @@ void user_defined_example();
 void err_handler_example();
 void syslog_example();
 void clone_example();
+void default_logger_example();
 
 #include "spdlog/spdlog.h"
 
@@ -38,6 +39,8 @@ int main(int, char *[])
 
         clone_example();
 
+        default_logger_example();
+
         // async logging using a backing thread pool
         async_example();
 
@@ -138,6 +141,13 @@ void clone_example()
     network_logger->info("Logging network stuff..");
 }
 
+void default_logger_example()
+{
+    auto logger = spdlog::get("console");
+    spdlog::set_default(logger);
+    spdlog::get_default()->warn("Default logger message...");
+}
+
 #include "spdlog/async.h"
 void async_example()
 {

include/spdlog/details/registry.h

@@ -72,6 +72,16 @@ public:
         tp_ = std::move(tp);
     }
 
+    void set_default(std::shared_ptr<logger> logger)
+    {
+        default_logger_ = std::move(logger);
+    }
+
+    std::shared_ptr<logger> get_default()
+    {
+        return default_logger_;
+    }
+
     std::shared_ptr<thread_pool> get_tp()
     {
         std::lock_guard<std::recursive_mutex> lock(tp_mutex_);
@@ -202,6 +212,7 @@ private:
     std::mutex logger_map_mutex_, flusher_mutex_;
     std::recursive_mutex tp_mutex_;
     std::unordered_map<std::string, std::shared_ptr<logger>> loggers_;
+    std::shared_ptr<logger> default_logger_;
     std::unique_ptr<formatter> formatter_;
     level::level_enum level_ = level::info;
     level::level_enum flush_level_ = level::off;

include/spdlog/spdlog.h

@@ -54,6 +54,23 @@ inline std::shared_ptr<logger> get(const std::string &name)
     return details::registry::instance().get(name);
 }
 
+// Return default logger instance when no explicitly defined
+// example: spdlog::get_default()->info("hello {}", "world");
+inline std::shared_ptr<logger> get_default()
+{
+    return details::registry::instance().get_default();
+}
+
+// Sets logger passed in parameter as default logger that will be returned
+// when calling get_default() function
+// example:
+// auto logger = spdlog::create<daily_file_sink_st>("logger_name", "dailylog_filename", 11, 59);
+// spdlog::set_default(logger);
+inline void set_default(std::shared_ptr<logger> logger)
+{
+    details::registry::instance().set_default(std::move(logger));
+}
+
 // Set global formatter. Each sink in each logger will get a clone of this object
 inline void set_formatter(std::unique_ptr<spdlog::formatter> formatter)
 {