Author jessymilare
2 Stars
Updated Last
2 Years Ago
Started In
October 2020


FlagSet.jl provides an Enum-like type for bit flag option values. The main motivations are:

  1. Flags have implicit numbering with incrementing powers of 2.
  2. Binary OR (|), AND (&) and XOR() operations are supported among members.
  3. Set operations like union, intersect, setdiff, in and issubset are also supported.
  4. Values are pretty-printed, showing the FlagSet type and each flag set.

This implementation is based on BitFlags, with some minor differences:

  1. Each flag set is treated as a set of flags, represented by Symbols, which is associated to an Integer (whose bits 1 corresponds to the flags in the set).
  2. The empty flag set (corresponding to 0) is always valid.
  3. The @flagset macro doesn't create a constant for each flag.

Basic usage

To create a new FlagSet type, use the @flagset macro, provide a name, an optional integer type, and a list of the flag names (and optional values). A new definition can be given in inline form:

@flagset FlagSetName[::BaseType] value1[=x] value2[=y]

or as a block definition:

@flagset FlagSetName[::BaseType] begin

Automatic numbering starts at 1. In the following example, we build an 8-bit FlagSet with no value for bit 3 (value of 4).

julia> @flagset FontFlags::UInt8 bold italic large=8

Instances can be created from integers or flag names and composed with bitwise operations

julia> FontFlags(1)
FontFlags(:bold) = 0x01

julia> FontFlags(:bold, :italic)
FontFlags(:bold, :italic) = 0x03

julia> FontFlags(3) | FontFlags(8)
FontFlags(:bold, :italic, :large) = 0x0b

Flag sets support iteration and other set operations

julia> for flag in FontFlags(3); println(flag) end

julia> :bold in FontFlags(3)

Conversion to and from integers is permitted, but only when the integer contains valid bits for the flag set type.

julia> Int(FontFlags(:bold))

julia> Integer(FontFlags(:italic))    # Abstract Integer uses native UInt8 type

julia> FontFlags(9)
FontFlags(:bold, :large) = 0x09

julia> FontFlags(4)
ERROR: ArgumentError: invalid value for FlagSet FontFlags: 4

The empty and the full set can be created with typemin and typemax (which is consistent with these function definitions).

julia> typemin(FontFlags)
FontFlags() = 0x00

julia> typemax(FontFlags)
FontFlags(:bold, :italic, :large) = 0x0b


Printing a FlagSet subtype shows usefull information about it:

julia> FontFlags
FlagSet FontFlags:
 bold = 0x01
 italic = 0x02
 large = 0x08

In a compact context (such as in multi-dimensional arrays), the pretty-printing takes on a shorter form:

julia> [FontFlags() FontFlags(:bold, :large)]
1×2 Array{FontFlags,2}:
 FontFlags(0x00)  FontFlags(0x09)

julia> show(IOContext(stdout, :compact => true), FontFlags(:bold, :large))

julia> print(IOContext(stdout, :compact => true), FontFlags(:bold, :large))


FlagSets support writing to and reading from streams as integers:

julia> io = IOBuffer();

julia> write(io, UInt8(9));

julia> seekstart(io);

julia> read(io, FontFlags)
FontFlags(:bold, :large) = 0x09