Beefy Boxes and Bandwidth Generously Provided by pair Networks
Keep It Simple, Stupid

Re^2: Alias of hash-values?

by Athanasius (Chancellor)
on Feb 21, 2013 at 02:21 UTC ( #1019874=note: print w/replies, xml ) Need Help??

in reply to Re: Alias of hash-values?
in thread Alias of hash-values?

The thing that surprises me though is that trying to increment the hash keys doesn't die like this one:

I agree, it seems odd. Consider:

#! perl use strict; use warnings; use Data::Dump; my %h = (1 => 'Fred'); dd %h; fn(%h); dd %h; sub fn { print "BEGIN fn()\n"; dd @_; $_[0]++; $_[1] = 'Barney'; dd @_; print "-END- fn()\n"; }


12:02 >perl (1, "Fred") BEGIN fn() (1, "Fred") (2, "Barney") -END- fn() (1, "Barney") 12:02 >

The hash value is clearly aliased, since the assignment in sub fn persists in %h after the sub returns. But the hash key is not: it appears to be copied (passed by value), as $_[0] behaves as a variable local to sub fn. (Whereas a literal value like 42 or 'Wilma' is aliased, as evidenced by the Modification of a read-only value error message which results from trying to increment it or assign to it within the sub.)

An anomaly? Or a feature?

Athanasius <°(((><contra mundum Iustus alius egestas vitae, eros Piratica,

Replies are listed 'Best First'.
Re^3: Alias of hash-values?
by LanX (Chancellor) on Feb 21, 2013 at 02:32 UTC
    We had the discussion about "aliasing read-only falling back to copy" not long a year ago.

    ref to read-only alias ... why?

    Apparently it also depends on the perl-version, threading and optimization ...

    Cheers Rolf

Log In?

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://1019874]
[Corion]: I mostly wonder how I can add this "reversal" to my toolchest, and if it's worth it or just clever
[choroba]: It should greatly simplify the parsing code, no flags or additional reading of the next buffer needed
[Corion]: choroba: Yeah, that's why I like that approach; it seems even quite obvious in retrospect to me, but many things are in hindsight ;)

How do I use this? | Other CB clients
Other Users?
Others perusing the Monastery: (13)
As of 2016-12-06 15:36 GMT
Find Nodes?
    Voting Booth?
    On a regular basis, I'm most likely to spy upon:

    Results (109 votes). Check out past polls.