Short string optimization example for C.180

Extra blank lines removed

strcpy replaced with strcpy_s to get rid of Travis CI warnings

Note: strcpy_s  is defined in the Annex K of the C++11 standard.

Gabriel's comments incorporated

- No need for braces for a single statement.
- Comment to explain why buffer_size is 16.

Bracing style made consistent with the rest of the examples

Bracing style made consistent with the rest of the examples

braces made consistent with the rest of the examples

CppCoreGuidelines.md

@@ -7322,7 +7322,44 @@ But heed the warning: [Avoid "naked" `union`s](#Ru-naked)
 
 ##### Example
 
-    ??? short-string optimization; safe union without dscriminant ???
+    // Short string optimization
+    
+    constexpr size_t buffer_size = 16; // Slightly larger than the size of a pointer
+    
+    class Immutable_string {
+    public:
+        Immutable_string(const char* str) :
+            size(strlen(str))
+        {
+            if (size < buffer_size)
+                strcpy_s(string_buffer, buffer_size, str);
+            else {
+                string_ptr = new char[size + 1];
+                strcpy_s(string_ptr, size + 1, str);
+            }
+        }
+    
+        ~Immutable_string()
+        {
+            if (size >= buffer_size)
+                delete string_ptr;
+        }
+    
+        const char* get_str() const
+        {
+            return (size < buffer_size) ? string_buffer : string_ptr;
+        }
+    
+    private:
+        // If the string is short enough, we store the string itself
+        // instead of a pointer to the string.
+        union {
+            char* string_ptr;
+            char string_buffer[buffer_size];
+        };
+    
+        const size_t size;
+    };
 
 ##### Enforcement