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730: Mainline

At this point, you are ready to learn about an implementation of lists using external pointers. This implementation involves not only a link class, but also a header class. Each header object, one per list, contains a pointer to the first link object. If the list is empty, the link pointer is the null pointer, as in the following illustration:

*-------*  A header object 
|       |   
|-------| 
|   0   |  A null pointer to first link object 
*-------* 

The following illustration shows a list that contains three cars:

*-------*  A header object 
|       | 
|-------| 
|       | 
*-------* 
    | 
    v 
*-------*       *-------*       *-------*  Three link objects 
|       | ----> |       | ----> |   0   |   
|-------|       |-------|       |-------| 
|       |       |       |       |       | 
*-------*       *-------*       *-------*  
    |               |               | 
    v               v               v 
*-------*       *-------*       *-------*  Three railroad_car objects 
|       |       |       |       |       | 
|-------|       |-------|       |-------| 
|       |       |       |       |       |