Avoid variables with hidden meanings

Here are some negative examples that we should avoid:

• The value in the variable pageCount might represent the number of pages printed, unless it equals -1, in which case it indicates that an error has occurred.

• The variable customerId might represent a customer number, unless its value is greater than 500,000, in which case you subtract 500,000 to get the number of a delinquent account.

• The variable bytesWritten might be the number of bytes written to an output file, unless its value is negative, in which case it indicates the number of the disk drive used for the output.

From Code Complete 2nd edition page 256