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We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

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What is "tieing"?

Posted by admin (Graham Ellis), 11 September 2002
What happens to a scalar when you leave your program?  "Its contents are lost" you say; that's the conventional logic, and, yes, it does usually work that way.  But you can change this.

With a scalar variable, deep in the bowels of Perl there are just four functions - create, destroy, read and write.  If you choose to, you can provide your own subroutines to perform each of these tasks for a specific variable, and thus alter the behaviour of a variable.   This is know as "tieing"; as an example of its use, you could tie a scalar to the contents of a file so that every read or write is a file read or write ... slow, but the variable would persist from one running of your program to the next!

Tieing is also implemented for hashes (it turns out that these are the most common use of the facility, where a hash is linked to a database table) and also in recent versions of Perl for lists and even for code.

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