Added singleton example source code.

examples/creational/singleton.cpp

@@ -0,0 +1,31 @@
+#include <assert.h>
+
+class President
+{
+  public:
+    static President& getInstance()
+    {
+      static President instance;
+      return instance;
+    }
+
+  private:
+    President()
+    {
+    }
+
+    ~President()
+    {
+    }
+};
+
+int main()
+{
+  President& president1 = President::getInstance();
+  President& president2 = President::getInstance();
+
+  // There can only be 1 president, so they must be the same.
+  assert(&president1 == &president2);
+
+  return 0;
+}