On Thursday, 3 April 2014 at 21:42:18 UTC, bearophile wrote:
dnspies:
This doesn't work. It prints two different hashes for equal
objects. I meant how do I get the default hash which is used
by an associative array.
import std.stdio;
struct my_struct {
int[] arr;
}
void main() {
my_struct s1;
s1.arr = [1,2,3];
my_struct s2;
s2.arr = [1,2,3];
writeln(s1 == s2);
writeln(typeid(my_struct).getHash(&s1));
writeln(typeid(my_struct).getHash(&s2));
}
true
626617119
2124658624
Take a look at the output of this program:
import std.stdio;
struct MyStruct {
int[] arr;
}
void main() {
MyStruct s1;
s1.arr = [1,2,3];
MyStruct s2;
s2.arr = [1,2,3];
writeln(s1 == s2);
writeln(typeid(MyStruct).getHash(&s1));
writeln(typeid(MyStruct).getHash(&s2));
int[MyStruct] aa;
aa[s1] = 10;
aa[s2] = 20;
writeln(aa);
}
I have filed this big problem four years ago or more.
Workarounds: define (carefully!) the three hash protocol
methods, or use a tuple.
Bye,
bearophile
Oh so the problem isn't that that ISN'T the default hash used by
an AA. It's worse. The problem is that that IS the default hash
used by an AA.
When you say "use a tuple", do you mean that the hash
implementation for Tuples is defined recursively and based on its
members' hashes? There doesn't seem to be an opHash for Tuples
AFAICT. If not, could you provide an example?
Also, if this problem isn't going to be fixed any time soon,
shouldn't it be documented directly on the AA page somewhere?
It's just the sort of surprise that I would have no hope of
figuring out when trying to debug my program. (I put this
wrapped_string into my AA, but when I try to fetch it again, it's
disappeared!!??)
What's worse, there's no link provided to TypeInfo.getHash,
instead it just says "If the KeyType is a struct or union type, a
default mechanism is used to compute the hash and comparisons of
it based on the binary data within the struct value" which sounds
as though they're saying "don't worry, we've handled everything"
which is the opposite of true.
