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A flag bit is a specialized form of storing a number of boolean flags within a single integer value. This is a form of data storage that is known as a Bit Field. How it works is that, rather than store values within separate boolean fields, you instead leverage bit-shift operations to use the individual bits of an integer.

First, notice that each of the above values represent a certain power of two, going from 2⁰ (1) to 2³ (1073741824). The value for 2³¹ is also included, but due to the nature of signed integers, its value is -2147483648. Each of these flags represents a single 1 in the integer's binary value. For example, the binary value of 2⁵, or 32, in an integer would be 00000000000000000000000000100000 (a 1 in the sixth position).

Because of this, if you take different flags and add them together, they will result in a value that represents a 1 in each of those flag's position. For example, adding together eFlagBit_2, eFlagBit_7, eFlagBit_12, and eFlagBit_22 will result an integer that has 1's in the third, eighth, thirteenth, and twenty-third position. (00000000010000000001000010000100, or 4,198,532.)

In code, the way to combine different flags into a single value is pretty simple. All you do is use the bitwise-OR operator to combine each desired flag into a single value.

int lFlags = eFlagBit_2 | eFlagBit_7 | eFlagBit_12 | eFlagBit_22;

To check an integer for the presence of a flag, conversely you would use a bitwise-AND operator. Because of how the bitwise-AND works, if you perform it on an integer and a flag, the result will be the value of the flag if the integer holds that flag, or it will be zero if not.

bool bContainsFlag = (lFlags & eFlagBit_12) != 0;

Because bitwise operations are so fast and efficient, using a bit field in this way will result in much more efficient code than if you used standard booleans. The other upside is that a boolean variable only needs one bit to store its value (0 for false, 1 for true), but booleans still take up a full byte. Using a bit field, you can store up to 32 different flags in a single integer, meaning you are only using 4 bytes instead of 32.