Modern implementation of Singleton

With this implementation we also make the initialization of the Singleton instance thread-safe

src/actions.cpp

@@ -161,27 +161,18 @@ namespace {
 // *****************************************************************************
 // class member definitions
 namespace Action {
-    TaskFactory* TaskFactory::instance_ = nullptr;
-
     TaskFactory& TaskFactory::instance()
     {
-        /// \todo move the static instance here. It is the "modern" way to implement a singleton
-        if (nullptr == instance_) {
-            instance_ = new TaskFactory;
-        }
-        return *instance_;
-    } // TaskFactory::instance
+        static TaskFactory instance_;
+        return instance_;
+    }
 
     void TaskFactory::cleanup()
     {
-        if (instance_ != nullptr) {
-            for (auto&& i : registry_) {
-                delete i.second;
-            }
-            delete instance_;
-            instance_ = nullptr;
-        }
-    } //TaskFactory::cleanup
+         for (auto&& i : registry_) {
+            delete i.second;
+         }
+    }
 
     void TaskFactory::registerTask(TaskType type, Task::UniquePtr task)
     {
@@ -190,7 +181,7 @@ namespace Action {
             delete i->second;
         }
         registry_[type] = task.release();
-    } // TaskFactory::registerTask
+    }
 
     TaskFactory::TaskFactory()
     {

src/actions.hpp

@@ -140,12 +140,8 @@ namespace Action {
         //! Prevent construction other than through instance().
         TaskFactory();
 
-        //! Pointer to the one and only instance of this class.
-        static TaskFactory* instance_;
-        //! Type used to store Task prototype classes
-        using Registry = std::map<TaskType, Task*>;
         //! List of task types and corresponding prototypes.
-        Registry registry_;
+        std::map<TaskType, Task*> registry_;
     };
 
     //! %Print the Exif (or other metadata) of a file to stdout