The singleton pattern has come a long way since early 1990x. The programmer interview question "Write a singleton" in 1995 has evolved to "Write a lazy initialized singleton" in 2000x, and finished by "Why we do not use a singleton?" in 2010x.
Indeed, the unproved using of multiple singletons should be considered as antipattern. However, we still need singletons in some cases.
Do not use static_cast when you cast from a basic class to a derived one. This may lead to undefined behavior. To handle the polymorphism, a virtual inheritance or a multiple inheritance case always use dynamic_cast instead.
The following short example shows the undefined behavior cases. This works with GCC, VC and online C++ shells:
This is the revised translation of my article published in "ITWeek" magazine January 24, 2019.
An expert tries to know more and more about less and less until he knows absolutely everything about nothing. And a philosopher tries to know less and less about more and more until he knows nothing about everything.
Daniil Granin, "Going Inside a Storm"