P is for Practical PerlMonks

### Re^3: Tell me how it works!

by nagalenoj (Friar)
 on Jan 28, 2009 at 15:13 UTC ( #739599=note: print w/replies, xml ) Need Help??

in reply to Re^2: Tell me how it works!
in thread Tell me how it works!

```*a=*b;

But, I read somewhere that assigning the typeglob(*a) will make both to point the same location(address/ memory).

So, When I undef the *a, how the *b remains the same?

Can you clear me in this?

Replies are listed 'Best First'.
Re^4: Tell me how it works!
by massa (Hermit) on Jan 28, 2009 at 15:28 UTC
undefing @a would've cleared it, affecting one of the things that the symbol "b" also points to:
```\$ perl -le '@x = qw(1 2 3); \$x = 1; *y = *x; undef @x; print "\$y [@y]"
+'
1 []
but undefing *a clears the entries for "a" in the symbol table, without affecting the things that the symbol "b" also points to:
```\$ perl -le '@x = qw(1 2 3); \$x = 1; *y = *x; undef *x; print "\$y [@y]"
+'
1 [1 2 3]
[]s, HTH, Massa (κς,πμ,πλ)
Re^4: Tell me how it works!
by gone2015 (Deacon) on Jan 28, 2009 at 16:56 UTC
But, I read somewhere that assigning the typeglob(*a) will make both to point the same location(address/ memory).

That is incomplete... it's not the typeglob that ends up pointing to the same location, but the contents of the typeglob that end up pointing to the same locations (one for each type of value).

Update: the more accurate answer is that yes, *a and *b end up as pointing to the same location, but what that location contains is a set of pointers, one for each type of value. (I'm sorry, I had previously left out one level of indirection.)

The typeglob *a implicitly points a structure which contains, amongst other things: a pointer to the SCALAR value \$a; a pointer to the ARRAY value @a; a pointer to the HASH value %a; etc.

When you use \$a you are implicitly using the SCALAR value pointed to by the respective pointer in the structure refered to by *a. (There is are two implicit dereferences.)

The assignment *b = *a makes them both point to the same set of pointers to values. So, \$a and \$b now implicitly refer to the same value (the same location in memory). So changing \$a changes \$b. And similarly for @a and @b, %a and %b, and so on.

So, undef *a wipes out its pointer to the structure containing the pointers to the values. It does not wipe out that structure or the values it points to. The glob *b still points at the structure, and hence to values that *a used to point to, so it and those values remain unchanged.

The typeglob *a contains, amongst other things: a pointer to the SCALAR value \$a; a pointer to the ARRAY value @a; a pointer to the HASH value %a; etc.

When you use \$a you are implicitly using the SCALAR value pointed to by the respective pointer in *a. (There is an implicit dereference.)

The assignment *b = *a copies the pointers. So, \$a and \$b now implicitly refer to the same value (the same location in memory). So changing \$a changes \$b. And similarly for @a and @b, %a and %b, and so on. Or, in other words, \$a and \$b are aliases of each other.

So, undef *a wipes out its pointers. It does not wipe out the values. The glob *b still points at the values that *a used to point to, so it and those values remain unchanged.

Re^4: Tell me how it works!
by nagalenoj (Friar) on Jan 29, 2009 at 05:43 UTC
Dear monks,

Now, I have another question, how can I clear the *b when undef the *a, in the above scenario?

That is, when doing

```*b = *a;
undef *a;
The contents of *b is not cleared. Is it possible to clear that too with the help of glob?

Yes, ofcourse without using undef *b; And also, it is understood that undef \$a; or undef @a will affect \$b and @b.

undef @a; would do it. @a = (); is better
undef @b; would do it. @b = (); is better

@\$glob = (); if you have a glob.

Create A New User
Node Status?
node history
Node Type: note [id://739599]
help
Chatterbox?
and all is quiet...

How do I use this? | Other CB clients
Other Users?
Others surveying the Monastery: (1)
As of 2018-05-23 01:19 GMT
Sections?
Information?
Find Nodes?
Leftovers?
Voting Booth?
World peace can best be achieved by:

Results (166 votes). Check out past polls.

Notices?