The implementation of the singleton in the previous section works, at least as far as enforcing a unique single global object and providing access to it for the entire program. There are, however, other considerations that may influence the choice of the implementation. We should also point out that, over the years, there have been a wide variety of implementations created. At the time, many of them were valid implementations that dealt with specific challenges in various ways. However, today, most are obsolete and should not be considered.
When it comes to the implementation of the singleton, there are several different types of implementations to choose from. First of all, we can classify implementations by how the singleton is accessed in the program—while there is definitely only one instance of the singleton object itself, or at least of the data...