Perl Cd Bookshelf [Electronic resources] نسخه متنی

12.16.1. Problem

A package declaration has meaning at compile time. If you don't know
the name of the package or variable until runtime, you'll have to
resort to symbolic references for direct access to the package symbol
table. Assuming you normally run with use
strict in effect, you must disable part of it to
use symbolic references. Once you've used the no
strict "refs" directive in that
block, build up a string with the fully qualified name of the
variable or function you're interested in. Then dereference this name
as though it were a proper Perl reference.

During the prehistoric eras (before Perl 5), programmers were forced
to use an eval for this kind of thing:

eval "package $packname; \$'$val = \$$varname"; # set $main'val
die if $@;

As you see, this approach makes quoting difficult. It's also
comparatively slow. Fortunately, you never need to do this just to
access variables indirectly by name. Symbolic references are a
necessary compromise.

Similarly, eval could be used to define functions
on the fly. Suppose you wanted to be able to get the base 2 or base
10 logs of numbers:

printf "log2 of 100 is %.2f\n", log2(100);
printf "log10 of 100 is %.2f\n", log10(100);

Perl has only the natural log function. Here's how one could use
eval to create these functions at runtime. Here
we'll create functions named log2 up through
log999:

$packname = "main";
for ($i = 2; $i < 1000; $i++) {
$logN = log($i);
eval "sub ${packname}::log$i { log(shift) / $logN }";
die if $@;
}