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Re^2: Alias of hash-values?

by Athanasius (Monsignor)
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"; }

Output:

12:02 >perl 541_SoPW.pl (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,


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Re^3: Alias of hash-values?
by LanX (Canon) 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

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