Symbol Tables

Each namespace -- and therefore, each module, class, or package -- has its own symbol table. A symbol table, in Perl, is a hash that holds all of the names defined in a namespace. All of the variable and function names can be found there. The hash for each namespace is named after the namespace with two colons. For example, the symbol table for the Foo namespace is called %Foo::.

The next example shows a program that displays all of the entries in the Foo:: namespace. It basically operates as follows:

• Define the dispSymbols() function.
• Get the hash reference that should be the first parameter.
• Declare local temporary variables.
• Initialize the %symbols variable. This is done to make the code easier to read. Initialize the @symbols variables. This variable is also used to make the code easier to read.
• Iterate over the symbols array displaying the key-value pairs of the symbol table.
• Call the dispSymbols() function to display the symbols for the Foo package.
• Start the Foo package.
• Initialize the $bar variable. This will place an entry into the symbol table. Define the baz() function.
• This will also create an entry into the symbol table.

The Perl code to do this is as follows, symbol.pl:

sub dispSymbols {

    my($hashRef) = shift;

    my(%symbols);

    my(@symbols);



    %symbols = %{$hashRef};

    @symbols = sort(keys(%symbols));



    foreach (@symbols) {

        printf("%-10.10s| %s\n", $_, $symbols{$_});

    }

}



dispSymbols(\%Foo::);



package Foo;

    $bar = 2;



    sub baz {

        $bar++;

    }

This program displays:

bar       | *Foo::bar
baz       | *Foo::baz

This example shows that there are only two things in the %Foo:: symbol table-only those things that the script placed there. This is not the case with the %main:: symbol table. When I display the entries in %main::, I see over 85 items. Part of the reason for the large number of names in the main package is that some variables are forced there. For example, STDIN, STDOUT, STDERR, @ARGV, @ARGVOUT, %ENV, @Inc, and %SIG are forced into the main namespace regardless of when they are used.