How is the three-way comparison operator different from subtraction?

Question

There\'s a new comparison operator <=> in C++20. However I think in most cases a simple subtraction works well:

int my_strcmp(const char *a, c         


        

Answer 1

Here are some cases that subtraction won't work for:

1. unsigned types.
2. Operands that cause integer overflow.
3. User-defined types that don't define operator - (perhaps because it's not meaningful - one may define an order without defining a notion of distance).

I suspect this list is non-exhaustive.

Of course, one can come up with workarounds for at least #1 and #2. But the intent of operator <=> is to encapsulate that ugliness.