Consistent comparison with the operator <=>

The C++ language defines six relational operators that perform comparison: ==, !=, <, <=, >, and >=. Although != can be implemented in terms of ==, and <=, >=, and > in terms of <, you still have to implement both == and != if you want your user-defined type to support equality comparison, and <, <=, >, and >= if you want it to support ordering.

That means 6 functions if you want objects of your type—let’s call it T—to be comparable, 12 if you want them to be comparable with another type, U, 18 if you also want values of a U type to be comparable with your T type, and so on. The new C++20 standard reduces this number to either one or two, or multiple of these (depending on the comparison with other types) by introducing a new comparison operator, called the three-way comparison, which is designated with the symbol <=>, for which reason it is popularly known as the spaceship...