Bitwise flags

Bitwise flags are sets of variables, usually simple number values, which can be used to enable or disable specific usages or features of a method or other code structure. They can do this quickly and efficiently because they operate at the bit level. Related flags in the same group are generally given complementary values representing different bit positions in a single value (e.g. hexadecimal), so that multiple flag settings can be represented by a single value.

For example, in the WebGPU API, a GPUBuffer object instance is created using the GPUDevice.createBuffer() method. When invoking this method, you define a usage property in the descriptor containing one or more flags that enable different allowed usages of that buffer.

usage: GPUBufferUsage.COPY_SRC | GPUBufferUsage.MAP_WRITE;

These values are defined inside the same namespace, and each one has a hexadecimal value:

When you query the GPUBuffer.usage property, you get a single decimal number returned, which is the sum of the different decimal values for the different usage flags. Returning to the above example, querying GPUBuffer.usage for the GPUBuffer created with the usage specified earlier would return the following:

• GPUBufferUsage.COPY_SRC's decimal equivalent, 4
• Add GPUBufferUsage.MAP_WRITE's decimal equivalent, 2
• Equals 6.

Because of the values chosen for the different flags, each combination of values is unique, so the program can tell at a glance which flags are set from a single value. In addition, you can easily test which flags are set in the combined value using the bitwise and operator:

if (buffer.usage & GPUBufferUsage.MAP_WRITE) {
  // Buffer has MAP_WRITE usage
}