How do I use the spaceship operator in a switch statement

February 11, 2024

The new <=> operator makes writing code more convenient and it can save some performance if the comparison algorithm is non-trivial, because it doesn’t need to be repeated two times to get the full ordering.

Or at least I though so when I learned about it.
However, when I try to use it in practice, in a switch statement, it doesn’t work.

This code doesn’t compile:

#include <iostream>

void compare_values(int x, int y)
{
    switch (x <=> y)
    {
    case std::strong_ordering::less:
        std::cout << "is less\n";
        break;
    case std::strong_ordering::greater:
        std::cout << "is greater\n";
        break;
    case std::strong_ordering::equal:
        std::cout << "is equal\n";
        break;
    }
}

The compiler shows an error suggesting that the value returned by <=> cannot be used in a switch:

<source>: In function 'void compare_values(int, int)':
<source>:5:15: error: switch quantity not an integer
    5 |     switch (x <=> y)
      |             ~~^~~~~
Compiler returned: 1

(live example)

I would guess that using the spaceship operator in switch is a pretty basic, obvious and common use case, so there is probably some trick to making it work. I cannot figure it out, though.

How can I fix this code?

>Solution :

The spaceship operator returns a std::strong_ordering which is not an integral type and therefore cannot be used in a switch-case statement.

You can use it in an if-else statement instead.

If you prefer to use a switch-case, you can use a trivial utility that converts the std::strong_ordering to an integral type with some predefined values.
Returning -1/0/1 in this case will be quite natural:

#include <iostream>

constexpr int strong_ordering_to_int(std::strong_ordering o)
{
    if (o == std::strong_ordering::less)    return -1;
    if (o == std::strong_ordering::greater) return 1;
    return 0;
}

void compare_values(int x, int y)
{
    switch (strong_ordering_to_int(x <=> y))
    {
    case -1:
        std::cout << "is less\n";
        break;
    case 1:
        std::cout << "is greater\n";
        break;
    case 0:
        std::cout << "is equal\n";
        break;
    }
}

int main()
{
    compare_values(2, 3);
    compare_values(3, 2);
    compare_values(3, 3);
}

Output: