What is the most efficient way to store flag values?
A flag is a value used to make a decision between two or more options in the execution of a program. For instance, the /w flag on the MS-DOS dir command causes the command to display filenames in several columns across the screen instead of displaying them one per line. In which a flag is used to indicate which of two possible types is held in a union. Because a flag has a small number of values (often only two), it is tempting to save memory space by not storing each flag in its own int or char.
Efficiency in this case is a tradeoff between size and speed. The most memory-space efficient way to store a flag value is as single bits or groups of bits just large enough to hold all the possible values. This is because most computers cannot address individual bits in memory, so the bit or bits of interest must be extracted from the bytes that contain it.
The most time-efficient way to store flag values is to keep each in its own integer variable. Unfortunately, this method can waste up to 31 bits of a 32-bit variable, which can lead to very inefficient use of memory. If there are only a few flags, it doesn't matter how they are stored. If there are many flags, it might be advantageous to store them packed in an array of characters or integers. They must then be extracted by a process called bit masking, in which unwanted bits are removed from the ones of interest.
Sometimes it is possible to combine a flag with another value to save space. It might be possible to use high- order bits of integers that have values smaller than what an integer can hold. Another possibility is that some data is always a multiple of 2 or 4, so the low-order bits can be used to store a flag.