Packed Structs

By default, all struct fields in Zig are naturally aligned to that of @alignOf(FieldType) (the ABI size), but without a defined layout. Sometimes you may want to have struct fields with a defined layout that do not conform to your C ABI. packed structs allow you to have extremely precise control of your struct fields, allowing you to place your fields on a bit-by-bit basis.

Inside packed structs, Zig’s integers take their bit-width in space (i.e. a u12 has an @bitSizeOf of 12, meaning it will take up 12 bits in the packed struct). Bools also take up 1 bit, meaning you can implement bit flags easily.

const MovementState = packed struct {
    running: bool,
    crouching: bool,
    jumping: bool,
    in_air: bool,
};

test "packed struct size" {
    try expect(@sizeOf(MovementState) == 1);
    try expect(@bitSizeOf(MovementState) == 4);
    const state = MovementState{
        .running = true,
        .crouching = true,
        .jumping = true,
        .in_air = true,
    };
    _ = state;
}