When will a key shared between two hashes?

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

In hv.c is written, that keys (hek-structure) are shared for space optimization. But i don't find any example for that. If i use this hashes $rgb{rot}='aaa ';$cmyk{rot}='aaa'. Is the key 'rot' shared between them? If i look in the output of:

perl -MDevel::Peek -e '$rgb{rot}="aaa";$cmyk{rot}="aaa";Dump(\%rgb);Du
+mp(\%cmyk)'
[download]

I don't see any shared keys:

SV = RV(0x1351ba8) at 0x1351b98
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x136ecf0
  SV = PVHV(0x1357fb0) at 0x136ecf0
    REFCNT = 2
    FLAGS = (SHAREKEYS)
    ARRAY = 0x1392a30  (0:7, 1:1)
    hash quality = 100.0%
    KEYS = 1
    FILL = 1
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "rot" HASH = 0xaffb6cf6
    SV = PV(0x134fb78) at 0x1351d48
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x1369340 "aaa"\0
      CUR = 3
      LEN = 8
SV = RV(0x1351ba8) at 0x1351b98
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x1378990
  SV = PVHV(0x1357fd8) at 0x1378990
    REFCNT = 2
    FLAGS = (SHAREKEYS)
    ARRAY = 0x1368fa0  (0:7, 1:1)
    hash quality = 100.0%
    KEYS = 1
    FILL = 1
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "rot" HASH = 0xaffb6cf6
    SV = PV(0x134fc58) at 0x1351f88
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x13652c0 "aaa"\0
      CUR = 3
      LEN = 8
[download]

Comment on When will a key shared between two hashes? Select or Download Code

Replies are listed 'Best First'.
Re: When will a key shared between two hashes? by ikegami (Patriarch) on Dec 09, 2011 at 21:33 UTC
That doesn't show how or where the keys are stored, so I don't know how you can conclude a lack of shared keys. (I don't know whether they are shared or not.)	[reply]
Re^2: When will a key shared between two hashes? by spx2 (Deacon) on Dec 09, 2011 at 23:57 UTC
what does he mean by sharing ? I looked into the code(hv.c) and from what I gather, it's "sharing between threads". does he mean sharing 2 equal values from different hashes ?	[reply]
Re: When will a key shared between two hashes? by JavaFan (Canon) on Dec 09, 2011 at 22:08 UTC
AFAIK, keys are always shared. It's an implementation detail that isn't visible from the language.	[reply]
Re^2: When will a key shared between two hashes? by gyhor (Initiate) on Dec 10, 2011 at 11:34 UTC
If keys are always shared, they should use the same memory-address, or not?	[reply]
Re^3: When will a key shared between two hashes? by JavaFan (Canon) on Dec 10, 2011 at 13:40 UTC
If keys are always shared, they should use the same memory-address, or not? Sure, but you cannot get the memory address of a hash key from the language. Do note that hash keys are not SVs.	[reply]
Re^4: When will a key shared between two hashes? by gyhor (Initiate) on Dec 11, 2011 at 18:41 UTC
Re: When will a key shared between two hashes? by TJPride (Pilgrim) on Dec 09, 2011 at 23:00 UTC
What exactly is the purpose of this question? Are you interested in the theory, or are you actually trying to apply this to something in practice, and if the latter, can you please explain what exactly? I'm not really understanding what you're getting at.	[reply]
Re^2: When will a key shared between two hashes? by gyhor (Initiate) on Dec 10, 2011 at 11:38 UTC
It is for the theory. I was digging the last days in hv.c to understand the hash implementation for comparing it with the hash implementation in python. Everywhere (in the source, perl internals, etc.) is written, that keys are shared. but i didn't i find a way to "see" with a debugger or with Devel::Peek. So i am confused, if i understood it correctly.	[reply]
Re^3: When will a key shared between two hashes? by JavaFan (Canon) on Dec 11, 2011 at 20:33 UTC
What were you expecting to see in the output of Devel::Peek? The output already shows the SHAREKEYS flag -- what else were you looking for?	[reply]
Re^4: When will a key shared between two hashes? by gyhor (Initiate) on Dec 13, 2011 at 10:33 UTC