REBOL 3 Concepts: Objects: Encapsulation

Pending Revision

This document was written for R2 and has yet to be revised for R3.

An object provides a good way to encapsulate a group of variables that should not appear at the global level. When function variables are defined as globals, they can unintentionally be modified by other functions.

The solution to this problem of global variables is to wrap an object around both the variables and the function. When that is done, the function can still access the variables, but the variables cannot be accessed globally. For example:

Bank: make object! [

    last-account: 89431
    bank-bonus: $10.00

    set 'make-account func [
        "Returns a new account object"
        f-name [string!] "First name"
        l-name [string!] "Last name"
        start-balance [money!] "Starting balance"
    ][
        last-account: last-account + 1
        make bank-account [
            first-name: f-name
            last-name: l-name
            account: last-account
            balance: start-balance + bank-bonus
        ]
    ]
]

In this example, the variables are safe from accidental modification. Notice that the make-account function was set to a variable using the set function, rather than using a variable definition. This was done to make it a global function. The function can be used in the same way as functions set with a variable definition, but does not require an object path:

bob: make-account "Bob" "Baker" $4000