FAQ 7.15 How do I create a static variable?

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7.15: How do I create a static variable?

    As with most things in Perl, TMTOWTDI. What is a "static variable" in
    other languages could be either a function-private variable (visible
    only within a single function, retaining its value between calls to that
    function), or a file-private variable (visible only to functions within
    the file it was declared in) in Perl.

    Here's code to implement a function-private variable:

        BEGIN {
            my $counter = 42;
            sub prev_counter { return --$counter }
            sub next_counter { return $counter++ }

    Now prev_counter() and next_counter() share a private variable $counter
    that was initialized at compile time.

    To declare a file-private variable, you'll still use a my(), putting the
    declaration at the outer scope level at the top of the file. Assume this
    is in file Pax.pm:

        package Pax;
        my $started = scalar(localtime(time()));

        sub begun { return $started }

    When "use Pax" or "require Pax" loads this module, the variable will be
    initialized. It won't get garbage-collected the way most variables going
    out of scope do, because the begun() function cares about it, but no one
    else can get it. It is not called $Pax::started because its scope is
    unrelated to the package. It's scoped to the file. You could conceivably
    have several packages in that same file all accessing the same private
    variable, but another file with the same package couldn't get to it.

    See "Persistent Private Variables" in perlsub for details.


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