This is one of the oldest computer operators in the game: the arithmetic IF statement from FORTRAN.
It's useful for stable-sorting collections with a single test. Also, overloading <=> for a type, gives all comparison operators "for free": ==, !=, <, <=, >=, >
How would that apply to Fortran’s arithmetic IF statement? It goes to one label for a negative value, or to a second label for a zero, or to a third label for positive. A NaN is in none of these categories.
I mean maybe I'm missing something but it seems like it behaves exactly the same way as subtraction? At least for integers it's definitely the same, for floats I imagine it might handle equals better?
C++ has operator overloading, so you can define the spaceship for any class, and get every comparison operator from the fallback definitions, which use `<=>` in some obvious ways.
It's useful for stable-sorting collections with a single test. Also, overloading <=> for a type, gives all comparison operators "for free": ==, !=, <, <=, >=, >