Symbol table dereference

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

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Re: Symbol table dereference by ysth (Canon) on Jul 28, 2004 at 23:10 UTC
If $package_name is just a top level package, this is easy: First of all, the main symbol table %:: (aka %main::) is a hash; you want to look up the key named $package_name."::" (assuming $package_name doesn't already have the all-important trailing ::). This is just $::{$package_name."::"}. Now, the values in a symbol table are globs (with the exception of declared-but-not-defined subs, which are scalars giving prototype information); you want to get the hash reference out of the glob. This is done with the FOO{THING} syntax (enclosing our FOO in {} since its an expression that evaluates to a glob, not a simple glob name): {$::{$package_name."::"}}{HASH}. Voila: `use strict; use Data::Dumper; my $package_name = "Data"; print Dumper {$::{$package_name."::"}}{HASH};` [download] gives: `$VAR1 = { 'Dumper::' => {'Data::Dumper::'} };` [download] You can then go ahead and use that hash ref to get a nested package, look up a glob in it, and extract a hash reference from it: `use strict; use Data::Dumper; my $top_level = "Data"; my $second_level = "Dumper"; print Dumper {${{$::{$top_level."::"}}{HASH}}{$second_level."::"}}{H +ASH}` [download] gives: $VAR1 = { 'Seen' => Data::Dumper::Seen, 'post' => Data::Dumper::post, 'Toaster' => Data::Dumper::Toaster, 'Varname' => Data::Dumper::Varname, 'pad' => Data::Dumper::pad, 'Values' => Data::Dumper::Values, 'Dumpf' => Data::Dumper::Dumpf, 'new' => Data::Dumper::new, '_sortkeys' => Data::Dumper::_sortkeys, 'Pair' => Data::Dumper::Pair, 'DESTROY' => Data::Dumper::DESTROY, 'EXPORT_OK' => Data::Dumper::EXPORT_OK, 'Pad' => Data::Dumper::Pad, 'confess' => Data::Dumper::confess, 'Deepcopy' => Data::Dumper::Deepcopy, 'EXPORT_FAIL' => Data::Dumper::EXPORT_FAIL, 'Sortkeys' => Data::Dumper::Sortkeys, 'Freezer' => Data::Dumper::Freezer, 'ISA' => Data::Dumper::ISA, 'Terse' => Data::Dumper::Terse, 'Maxdepth' => Data::Dumper::Maxdepth, 'Dumpp' => Data::Dumper::Dumpp, 'Useperl' => Data::Dumper::Useperl, 'Dumpxs' => Data::Dumper::Dumpxs, 'bootstrap' => Data::Dumper::bootstrap, '_dump' => Data::Dumper::_dump, 'qquote' => Data::Dumper::qquote, 'Useqq' => Data::Dumper::Useqq, 'BEGIN' => Data::Dumper::BEGIN, 'Purity' => Data::Dumper::Purity, 'Names' => Data::Dumper::Names, 'Deparse' => Data::Dumper::Deparse, 'EXPORT' => Data::Dumper::EXPORT, 'DumperX' => Data::Dumper::DumperX, 'croak' => Data::Dumper::croak, 'Dump' => Data::Dumper::Dump, 'import' => Data::Dumper::import, 'Indent' => Data::Dumper::Indent, 'Dumper' => Data::Dumper::Dumper, 'Quotekeys' => Data::Dumper::Quotekeys, 'Bless' => Data::Dumper::Bless, 'carp' => Data::Dumper::carp, 'VERSION' => Data::Dumper::VERSION, 'Reset' => Data::Dumper::Reset, 'Dumpperl' => Data::Dumper::Dumpperl }; [download] and you can look up a particular symbol in that and use it. To call Data::Dumper::Dumper, for instance, you can say: `use Data::Dumper; my $top_level = "Data"; my $second_level = "Dumper"; my $subname = "Dumper"; print &{{${{${{$::{$top_level."::"}}{HASH}}{$second_level."::"}}{HA +SH}}{$subname}}{CODE}}(\%Data::);` [download] calls Data::Dumper::Dumper via the symbol table and prints: `$VAR1 = { 'Dumper::' => *{'Data::Dumper::'} };` [download] Actually, a lot of the above can be much simplified, because of an odd rule that says you can put a glob where a reference of a particular type is wanted, and it automatically produces the reference to that part of the glob: `use strict; use Data::Dumper; my $package_name = "Data"; print Dumper \%{$::{$package_name."::"}}; use strict; use Data::Dumper; my $top_level = "Data"; my $second_level = "Dumper"; print Dumper \%{${$::{$top_level."::"}}{$second_level."::"}}; use Data::Dumper; my $top_level = "Data"; my $second_level = "Dumper"; my $subname = "Dumper"; print &{${${$::{$top_level."::"}}{$second_level."::"}}{$subname}}(\%Da +ta::);` [download]	[reply] [d/l] [select]
Re: Symbol table dereference by Joost (Canon) on Jul 28, 2004 at 19:20 UTC
Hmmm this should work: `{ no strict 'refs'; my $package_name = "main::bla::"; # construct dynamic name here print Dumper(\%$package_name); }` [download] "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l]
Re^2: Symbol table dereference by shemp (Deacon) on Jul 28, 2004 at 19:23 UTC
Yep. Thanks much, i've been going nuts for hours!	[reply]
Re: Symbol table dereference by broquaint (Abbot) on Jul 29, 2004 at 03:06 UTC
I'd recommend looking into `qualify_to_ref` from `Symbol` e.g `use strict; use Symbol 'qualify_to_ref'; use Data::Dumper; my $tbl = qualify_to_ref('Data::Dumper::'); print Dumper $tbl{HASH}; __output__ $VAR1 = { 'ISA' => Data::Dumper::ISA, 'Useperl' => Data::Dumper::Useperl, 'Sortkeys' => Data::Dumper::Sortkeys, 'EXPORT_FAIL' => Data::Dumper::EXPORT_FAIL, 'new' => Data::Dumper::new, ...` [download] This neatly side steps the necessity of turning off `strict 'refs'` and jumping through various hoops to deal with nested package names. HTH `_________ broquaint`	[reply] [d/l]
Re: Symbol table dereference by stvn (Monsignor) on Jul 28, 2004 at 19:25 UTC
You have to do it under `no strict 'refs'`, and you make it basically a soft reference (variable resolved from a string). Also you need to watch how `$package` is in the string. This should work for you. `my $package_name = 'some_package'; { no strict 'refs'; print Dumper(\%{"main::${package_name}::"}); }` [download] -stvn	[reply] [d/l] [select]
Re: Symbol table dereference by gaal (Parson) on Jul 28, 2004 at 19:18 UTC
`print +Dumper(\%{ $package_name . "::"});`	[reply] [d/l]