Errors in data stored in the memory of a computer are detected prior to use of the data. A protected data type is declared in writing a program in which errors in critical data must be detected before the data are used. The invention is preferably implemented on a personal computer system (10) or in a microcontroller for a device. When a protected data item is initialized, both the protected data item and its corresponding bit-inverted form are stored in memory. A constructor function is used to determine the bit-inverted form. Any time that the protected data item is subsequently accessed for use by an application program, either the stored protected data item or its corresponding bit-inverted form are inverted for comparison to the other stored data item. Any difference detected in this comparison indicates that a change has occurred in either or both of the protected data item and its corresponding bit-inverted form within memory. Depending upon the application program in which the invention is used, an appropriate action is taken upon detecting such an error, such as providing an indication of the error and terminating further execution of the program.