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(Ovid -- constant internals)

by Ovid (Cardinal)
on Jan 16, 2002 at 03:14 UTC ( #139079=note: print w/replies, xml ) Need Help??


in reply to Re: (ar0n) Re: Accessing Constant Hashes
in thread Accessing Constant Hashes

As ar0n already replied, the constant pragma does create a subroutine. If you're curious, this is an inlined subroutine with a null prototype (one of the few places where a prototype is really useful). Looking in the constant module, the following code is where the constant is actually created:

{ no strict 'refs'; my $full_name = "${pkg}::$name"; $declared{$full_name}++; if (@_ == 1) { my $scalar = $_[0]; *$full_name = sub () { $scalar }; } elsif (@_) { my @list = @_; *$full_name = sub () { @list }; } else { *$full_name = sub () { }; } }

What the null prototype does is allow your constant to behave like a Perl built-in:

use constant FOO => 7; print FOO + 1;

That prints 8, just like you would expect. Contrast that to this:

sub FOO {7}; print FOO + 1'

That will print 7. Why? Because Perl will interpret the +1 as being an argument to &FOO, which is silently discarded as it is not used. Yuck! With the null prototype, Perl knows that nothing coming after FOO can be an argument, so everything parses as you expect. It's also nice to note, from the docs, that as of Perl 5.004, Perl will replace all instances of your constant with the returned value, thus saving you the overhead of a subroutine call.

Cheers,
Ovid

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