Examples

This page contains some examples on how to use @bitflags and @bitfields, though if you are familiar with regular structs, the usage should be familiar, as the interfaces are modeled after them.

Basic bitflags

Starting with @bitflags, which is used for tightly-packed boolean storage, which can be used like so:

julia> using FieldFlags
julia> @bitflags struct MyFlags
           flagA
           flagB
           _ # padding
           flagC
       end

The above defines a struct MyFlags with three fields, flagA, flagB and flagC. As the comment indicates, _ is for specifying padding. All fields specified in the struct take up a single bit - even the padding. The minimum size for the above is thus 4 bits. The fields are stored from least significant bit to most significant bit, starting with fieldA.

While the minimum bitsize for the above struct is 4 bits, due to an implementation detail/compiler requirement, all structsizes are rounded up to the next multiple of 8 bits. MyFlags is thus 8 bits, or 1 byte, large:

julia> sizeof(MyFlags)1

That is, an instance of MyFlags has these bits:

With the 4 bits higher than flagC being implicit padding as well.

@bitflags gives us two default constructors; a zero-arg constructor as well as an n-arg constructor.

The zero-arg constructor allows us to construct an instance of MyFlags with all fields set to false:

julia> mf = MyFlags()ERROR: MethodError: no method matching Main.MyFlags()

Closest candidates are:
  Main.MyFlags(!Matched::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}, !Matched::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}, !Matched::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8})
   @ Main none:0
  Main.MyFlags(!Matched::Main.MyFlags_fields)
   @ Main none:0
julia> mf.flagA == mf.flagB == mf.flagC == falseERROR: UndefVarError: `mf` not defined

As can be seen above, individual fields can be accessed with regular dot-syntax.

In contrast, the n-arg constructor takes one argument for each field:

julia> mf = MyFlags(true, false, true)Main.MyFlags(flagA: true, flagB: false, flagC: true)
julia> mf.flagA == mf.flagC == truetrue
julia> mf.flagB == falsetrue

Mutability

While immutability can be useful, sometimes it is more convenient to mutate a flag in-place. This can be achieved by marking the struct given to @bitflags as mutable:

julia> using FieldFlags
julia> @bitflags mutable struct MutableFlags
           a
           _
           b
           _
           c
       end

The above struct requires at least 5 bits, which means the bitlayout is like so:

The remaining upper 2 bits are once again implicit padding, while the overall size of the objects stay the same:

julia> sizeof(MutableFlags)1

The available constructors are also once again the same:

julia> methods(MutableFlags)# 2 methods for type constructor:
 [1] Main.MutableFlags(a::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}, b::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}, c::Union{Bool, Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8})
     @ none:0
 [2] Main.MutableFlags(t::Main.MutableFlags_fields)
     @ none:0

The only difference is that we are now able to set individual fields in an object:

julia> mutf = MutableFlags(false, false, false)Main.MutableFlags(a: false, b: false, c: false)
julia> mutf.a == falsetrue
julia> mutf.a = truetrue
julia> mutf.a == truetrue

which we weren't able to do earlier:

julia> mf.flagA = trueERROR: setfield!: immutable struct of type MyFlags cannot be changed

Subtyping

On top of mutability, we can also specify an abstract supertype as usual:

julia> using FieldFlags
julia> abstract type MyAbstract end
julia> @bitflags struct MyConcrete <: MyAbstract
           foo
           _
           bar
           baz
       end
julia> supertype(MyConcrete) == MyAbstracttrue

This allows for defining common fallback methods for @bitfield or @bitflags structs that may share some common fields or other invariants:

julia> @bitflags struct OtherConcrete <: MyAbstract
           foo
           _
           bak
       end
julia> fallback(ma::MyAbstract) = ma.foofallback (generic function with 1 method)
julia> fallback(MyConcrete(true, false, false)) == truetrue
julia> fallback(OtherConcrete(false, true)) == falsetrue

@bitfield structs

Structs defined with @bitfield are, in regards to mutability, bitsize and subtyping behavior, identical to those defined by @bitflags. The major difference is that while @bitflags structs only hold one bit per field, @bitfield can hold multiple bits per field:

julia> using FieldFlags
julia> @bitfield mutable struct MyField
           a:1
           _:2
           b:3
           _
           c:2
       end

The above defines a struct MyField, with three fields a, b and c, with sizes (in bits) 1, 3 and 2 respectively. There are also two definitions of explicit padding between fields, the first being 2 bits in size and the second one being 1 bit in size; taken implicitly from _ not having a size annotated. The layout of the above struct is like so:

With the additional padding bits, we come to a total of 9 bits. This is again rounded up to the next multiple of 8, which is 16 bits or 2 bytes:

julia> sizeof(MyField)2