Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, here's an XS question this time regarding read-only variables...

In one of my "classes" my blessed thingy happens to be an HV which hold information about a distinguished name (canonical format, abbreviated, surname, given, common name, etc...).

i.e: print $name->{Common};

I'd like to be able to make that hash read-only and tried it (using SvREADONLY_on(HV)) but you can still do this:

$name->{valueNotInHashInitially} = "Some Value";

You can do that even though Devel::Peek show the hash as having the READONLY flag. I did set all the entries (SVs) in the hash to READONLY and that worked.

ie: $name->{Common} = "New Value"; raises an error and dies.

Is there anything I can do so Perl also raise the same error when a user tries to create new entries in my ReadOnly hash?


Christian Cloutier

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Re: XS ReadOnly HVs
by PodMaster (Abbot) on Apr 02, 2003 at 07:06 UTC
    I don't think you can do it in XS, at least not with normal hashes .... anyway, see Readonly and ex::constant::vars cause they do that.

    update: Yup, magic is needed (man that sounds funny).

    MJD says you can't just make shit up and expect the computer to know what you mean, retardo!
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Re: XS ReadOnly HVs
by robin (Chaplain) on Apr 03, 2003 at 10:37 UTC
    This is a very good question, and has been raised on p5p in the past. As far as I know (which might not be far enough - I'm doing more category theory than programming these days) there is still no simple way to do what you want.

    The most concerted attempt to change this was made by Jeff Friedl in 2001. Take a look at the threads starting here, here and here. As you can see, the necessary code has all been written, but doesn't seem to have been incorporated into perl. You could try asking p5p about the status of this.

    Update: On closer inspection, it seems that this has been incorporated into Perl 5.8. Look at the module Hash::Util. It seems that, in Perl 5.8, the SvREADONLY flag does prevent new keys from being added to a hash. What version of Perl are you using?



      Thanks a lot for pointing me in the right direction...
      I am currently using 5.6.1 but will most certainly upgrade to 5.8.x.

      So the only way of enforcing that behavior for hashes will be to require 5.8.0; in my modules?
      That will prevent users with older perl from using it...

      Any thoughts?

        Yes, I think you're right. On the other hand, you haven't explained why you rejected the low-tech alternative of using a tied hash. If it was for speed reasons, and compatibility is important to you, you could fall back to a tied hash implementation on older perls.

        Then at least it would work on 5.6.1, albeit slightly more slowly.