code refactoring: fix memory leaks, code style, etc.

singleton/Singleton.cpp

@@ -15,16 +15,28 @@
  * has private static variable to hold one instance of the class
  * and method which gives us a way to instantiate the class
  */
-class Singleton {
+class Singleton
+{
 public:
-  static Singleton *get() {
-    if (instance == NULL) {
+  static Singleton* get()
+  {
+    if ( !instance )
+    {
       instance = new Singleton();
-    }
+    }    
     return instance;
   }
-
-  void tell() {
+  
+  static void restart()
+  {
+    if ( instance )
+    {
+      delete instance;
+    }
+  }
+  
+  void tell()
+  {
     std::cout << "This is Singleton." << std::endl;
     // ...
   }
@@ -42,5 +54,7 @@ Singleton* Singleton::instance = nullptr;
 int main()
 {
   Singleton::get()->tell();
+  Singleton::restart();
+  
   return 0;
 }