CODEWITHSUNDEEP
perl
Thomson
Thomson

Reputation: 21625

Access package variable after its name is removed from symbol table in Perl?

I tried delete the name of a package variable from the main:: symbol table as below, but seems later code could still reference the variable, why it can be referenced after deletion? What's the effect of removing a name from symbol table?

$n = 10;
delete $main::{'n'};
print $n;

Upvotes: 2

Views: 148

Answers (1)

ysth
ysth

Reputation: 98398

When perl code is compiled, globs for package variables/symbols are looked up (and created as necessary) and referenced directly from the compiled code.

So if you delete a symbol table entry $pkg::{n}, all the already compiled code that used $pkg::n, @pkg::n, etc., still use the original glob. But do/require'd code or eval STRING or symbolic references won't.

This code ends up with two *n globs; all the code compiled before the delete executes will reference the original one; all the code compiled after will reference the new one (and symbolic references will get whichever is in the symbol table at that point in runtime).

$n = 123;
delete $::{n};
eval '$n=456';
print $n;
eval 'print $n';

Another example:

$n = 123;
sub get_n { $n }
BEGIN { delete $::{n} }
$n = 456;
print get_n();
print $n;

Upvotes: 5

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