require - load in external functions from a library at runtime
require EXPR
require
Demands some semantics specified by
EXPR, or by $_
if
EXPR is not supplied. If
EXPR is numeric, demands that the current version of Perl (
$]
or
$PERL_VERSION) be equal or greater than
EXPR.
Otherwise, demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of eval(). Has semantics similar to the following subroutine:
sub require { my($filename) = @_; return 1 if $INC{$filename}; my($realfilename,$result); ITER: { foreach $prefix (@INC) { $realfilename = "$prefix/$filename"; if (-f $realfilename) { $result = do $realfilename; last ITER; } } die "Can't find $filename in \@INC"; } die $@ if $@; die "$filename did not return true value" unless $result; $INC{$filename} = $realfilename; return $result; }
Note that the file will not be included twice under the same specified name. The file must return
TRUE as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with ``
1;
'' unless you're sure it'll return
TRUE otherwise. But it's better just to put the ``1;
'', in case you add more statements.
If EXPR is a bareword, the require assumes a ``.pm'' extension and replaces ``::'' with ``/'' in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the ``Foo/Bar.pm'' file in the directories specified in the @INC
array.
But if you try this:
$class = 'Foo::Bar'; require $class; # $class is not a bareword #or require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the ``Foo::Bar'' file in the @INC
array and will complain about not finding
``Foo::Bar'' there. In this case you can do:
eval "require $class";
For a yet-more-powerful import facility, see use and the perlmod manpage.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.