How could this program work?

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I’ll throw in something ala Scott Meyers here that helps clarify the use cases around this. I made a topic about using @constCast a while ago: Method overloading and deduplication using @constCast - #3 by AndrewCodeDev

I’ve linked to the post with this example:

fn MatchPtrType(comptime Parent: type, comptime Child: type) type {
    return if (comptime isConstPtr(Parent)) *const Child else *Child;
}

// @constCast can propgate through optional pointers... ?*const T -> ?*T

pub fn last(self: anytype) ?MatchPtrType(@TypeOf(self), Node) {
    if (comptime isConstPtr(@TypeOf(self))) {
         
        return if (self.items.len > 0)
            &self.items[self.items.len - 1] else null;
         
    } else {
        return @constCast(asConst(@This(), self).last());
    }
}

The pattern that works is:

non-const x -> *const x -> @constCast(ptr)

You do not want to go in this direction:

const x -> *const x -> @constCast(ptr)

The reason being is that the compiler has made assumptions about x because it begins as const. You’re invalidating those assumptions. If you start with non-const, go up to const, and then back down, everything is fine.

  • Edit -

I should probably make a Doc on this…

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