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

c++ comparison-operators c++20 spaceship-operator
iBug picture iBug · Dec 31, 2017 · Viewed 11.2k times · Source

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, const char *b) {
    while (*a == *b && *a != 0 && *b != 0) {
        a++, b++;
    }
    // Version 1
    return *a - *b;
    // Version 2
    return *a <=> *b;
    // Version 3
    return ((*a > *b) - (*a < *b));
}

They have the same effect. I can't really understand the difference.

Answer

Sergey Kalinichenko picture Sergey Kalinichenko · Dec 31, 2017

The operator solves the problem with numeric overflow that you get with subtraction: if you subtract a large positive number from a negative that is close to INT_MIN, you get a number that cannot be represented as an int, thus causing undefined behavior.

Although version 3 is free from this problem, it utterly lacks readability: it would take some time to understand by someone who has never seen this trick before. <=> operator fixes the readability problem, too.

This is only one problem addressed by the new operator. Section 2.2.3 of Herb Sutter's Consistent comparison paper talks about the use of <=> with other data types of the language where subtraction may produce inconsistent results.

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