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

There are some meaningful answers here on the difference, but Herb Sutter in his paper specifically says:

<=> is for type implementers: User code (including generic code) outside the implementation of an operator<=> should almost never invoke an <=> directly (as already discovered as a good practice in other languages);

So even if there was no difference, the point of the operator is different: to aid class writers to generate comparison operators.

The core difference between the subtraction operator and the "spaceship" operator (according to Sutter's proposal) is that overloading operator- gives you a subtraction operator, whereas overloading operator<=>:

• gives you the 6 core comparison operators (even if you declare the operator as default: no code to write!);
• declares whether your class is comparable, is sortable, and whether the order is total or partial (strong/weak in Sutter's proposal);
• allows for heterogeneous comparisons: you can overload it to compare your class to any other type.

Other differences are in the return value: operator<=> would return an enum of a class, the class specifies whether the type is sortable and whether the sort is strong or weak. The return value would convert to -1, 0 or 1 (though Sutter leaves room for the return type to also indicate distance, as strcmp does). In any case, assuming the -1, 0, 1 return value, we'll finally get a true signum function in C++! (signum(x) == x<=>0)