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COMPUTING SCIENCE

Identity Crisis

Brian Hayes

More of the Same

There is a third way of exploring issues of identity, but it lies outside the realm of nondestructive testing. If Alex spills ketchup on his tie, does Baxter's tie also acquire a stain? If Baxter steps on his teacher's toe, does Alex's teacher have a sore foot? The principle being suggested here is that two things are the selfsame thing if changing one of them changes the other in the same way.

In computing, this process is encountered most often in the unexpected and unpleasant discovery that two variables are "aliases" referring to the same value or object. For example, if the variable designating Alex's grade-point average and the variable for Baxter's average both point to the same location in memory, then any change in one value will also be reflected in the other. This is probably not the desired behavior of the school's grading system.

In principle, deliberate alteration of memory contents could serve as a test of identity: Just twiddle the bits of an object and see if the corresponding bits of another object flip. But the test is not foolproof, particularly in a computer with multiple threads of execution. There's always the chance that the same change might be made coincidentally in two different places; two independent ketchup stains are not an impossibility. If coincidence seems too unlikely, consider that there might be a process running whose purpose is to synchronize two variables, checking one of them at frequent intervals and changing the other one to match. Or, conversely, a background process might undo any change to a variable, restoring the original value whenever it is modified. Under these conditions, a bit-flipping identity test might find that an object is not even equal to itself.

The distinction between separate-but-equal objects and a selfsame object can be crucially important. When I make a bank deposit, I'd strongly prefer that the amount be credited to my own selfsame account, rather than to the account of someone who is separate-but-equal to me—perhaps someone with the same name and date of birth. The standard practice for maintaining identity in these circumstances is to issue a unique identifying number. These are the numbers that rule so much of modern life, the ones you find on your bank statement, your driver's license, your credit cards. The same technique can be used internally by a computer program to keep track of data objects. For example, the programming language Smalltalk tags all objects with individual serial numbers. (Smalltalk is also notable for having two equality operators: "=" for separate-but-equal objects and "==" for selfsame objects.)




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