667: Mainline

The reason a problem emerges is that the destructor runs whenever the memory for a class object is reclaimed:

• As you know, memory is reclaimed when the delete operator is called explicitly.
• Memory is also reclaimed whenever you return from a function with a call-by-value class-object argument.

In the example, the destructor runs because the memory for a railroad-car copy is reclaimed, but, unfortunately, the destructor further reclaims the memory chunk allocated for the serial number, which is shared with the original object:

                  <-- Retained    |   Reclaimed --> 
                                  | 
                                  | 
Memory reserved for               |      Memory reserved for r, 
railroad_car argument             |      the railroad_car parameter 
 |                                |       | 
 v                                |       v 
*-----*               Copy        |      *-----* 
|     |          ------------->   |      |     | 
|-----|                           |      |-----| 
|     |                           |      |     | 
|-----|                           |      |-----| 
|     | ---------------* *-------------- |     | 
|-----|                | |        |      *-----* 
|     |                | |        | 
|            *-------- | | -------* 
             |         v v 
             |        *---*---*---*---*---*---*---*---*---* 
             |        |   |   |   |   |   |   |   |   |   | 
             |        *---*---*---*---*---*---*---*---*---*