Common Uses

Flags

Flags are single-bit independent data that are kept together in the same variable. As they are only one bit wide each, they are suitable for representing binary concepts like enabled/disabled, valid/invalid, etc.

We sometimes encounter such one-bit concepts in D modules that are based on C libraries.

Flags are usually defined as non-overlapping values of an enum type.

As an example, let’s consider a car racing game where the realism of the game is configurable:

• The fuel consumption is realistic or not.

• Collisions can damage the cars or not.

• Tires can deteriorate by use or not.

• Skid marks are left on the road surface or not.

These configuration options can be specified at run time by the following enum values:

enum Realism {
    fuelUse = 1 << 0, 
    bodyDamage = 1 << 1,
    tireUse = 1 << 2, 
    skidMarks = 1 << 3
}

Note that all of those values consist of single bits that do not conflict with each other. Each value is determined by left-shifting 1 by a different number of bits. The corresponding bit representations are the following:

fuelUse   : 0001
bodyDamage: 0010
tireUse   : 0100
skidMarks : 1000

Since their 1 bits do not match others’, these values can be combined by the | operator to be kept in the same variable. For example, the two configuration options that are related to tires can be combined as in the following code: