In functional programming, the degenerate case is often called "unit". In C++, "void" is often the best analogue. However, because of the syntactic special-casing required for void, it is frequently a liability for template metaprogramming. So, instead of writing specializations to handle cases like SomeContainer<void>, a library author may instead rule that out and simply have library users use SomeContainer<Unit>. Contained values may be ignored. Much easier.
"void" is the type that admits of no values at all. It is not possible to construct a value of this type. "unit" is the type that admits of precisely one unique value. It is possible to construct a value of this type, but it is always the same value every time, so it is uninteresting.
Definition at line 36 of file Unit.h.
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