Crate bitflags

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Expand description

A typesafe bitmask flag generator useful for sets of C-style bitmask flags. It can be used for creating typesafe wrappers around C APIs.

The bitflags! macro generates structs that manage a set of flags. The flags should only be defined for integer types, otherwise unexpected type errors may occur at compile time.

Example

use bitflags::bitflags;

bitflags! {
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
        const ABC = Self::A.bits | Self::B.bits | Self::C.bits;
    }
}

fn main() {
    let e1 = Flags::A | Flags::C;
    let e2 = Flags::B | Flags::C;
    assert_eq!((e1 | e2), Flags::ABC);   // union
    assert_eq!((e1 & e2), Flags::C);     // intersection
    assert_eq!((e1 - e2), Flags::A);     // set difference
    assert_eq!(!e2, Flags::A);           // set complement
}

See example_generated::Flags for documentation of code generated by the above bitflags! expansion.

The generated structs can also be extended with type and trait implementations:

use std::fmt;

use bitflags::bitflags;

bitflags! {
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
    }
}

impl Flags {
    pub fn clear(&mut self) {
        self.bits = 0;  // The `bits` field can be accessed from within the
                        // same module where the `bitflags!` macro was invoked.
    }
}

impl fmt::Display for Flags {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "hi!")
    }
}

fn main() {
    let mut flags = Flags::A | Flags::B;
    flags.clear();
    assert!(flags.is_empty());
    assert_eq!(format!("{}", flags), "hi!");
    assert_eq!(format!("{:?}", Flags::A | Flags::B), "A | B");
    assert_eq!(format!("{:?}", Flags::B), "B");
}

Visibility

The generated structs and their associated flag constants are not exported out of the current module by default. A definition can be exported out of the current module by adding pub before struct:

mod example {
    use bitflags::bitflags;

    bitflags! {
        pub struct Flags1: u32 {
            const A = 0b00000001;
        }

        struct Flags2: u32 {
            const B = 0b00000010;
        }
    }
}

fn main() {
    let flag1 = example::Flags1::A;
    let flag2 = example::Flags2::B; // error: const `B` is private
}

Attributes

Attributes can be attached to the generated structs by placing them before the struct keyword.

Representations

It’s valid to add a #[repr(C)] or #[repr(transparent)] attribute to a type generated by bitflags!. In these cases, the type is guaranteed to be a newtype.

use bitflags::bitflags;

bitflags! {
    #[repr(transparent)]
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}

Trait implementations

The Copy, Clone, PartialEq, Eq, PartialOrd, Ord and Hash traits are automatically derived for the structs using the derive attribute. Additional traits can be derived by providing an explicit derive attribute on struct.

The Extend and FromIterator traits are implemented for the structs, too: Extend adds the union of the instances of the struct iterated over, while FromIterator calculates the union.

The Binary, Debug, LowerHex, Octal and UpperHex traits are also implemented by displaying the bits value of the internal struct.

Operators

The following operator traits are implemented for the generated structs:

  • BitOr and BitOrAssign: union
  • BitAnd and BitAndAssign: intersection
  • BitXor and BitXorAssign: toggle
  • Sub and SubAssign: set difference
  • Not: set complement

Methods

The following methods are defined for the generated structs:

  • empty: an empty set of flags
  • all: the set of all defined flags
  • bits: the raw value of the flags currently stored
  • from_bits: convert from underlying bit representation, unless that representation contains bits that do not correspond to a defined flag
  • from_bits_truncate: convert from underlying bit representation, dropping any bits that do not correspond to defined flags
  • from_bits_unchecked: convert from underlying bit representation, keeping all bits (even those not corresponding to defined flags)
  • is_empty: true if no flags are currently stored
  • is_all: true if currently set flags exactly equal all defined flags
  • intersects: true if there are flags common to both self and other
  • contains: true if all of the flags in other are contained within self
  • insert: inserts the specified flags in-place
  • remove: removes the specified flags in-place
  • toggle: the specified flags will be inserted if not present, and removed if they are.
  • set: inserts or removes the specified flags depending on the passed value
  • intersection: returns a new set of flags, containing only the flags present in both self and other (the argument to the function).
  • union: returns a new set of flags, containing any flags present in either self or other (the argument to the function).
  • difference: returns a new set of flags, containing all flags present in self without any of the flags present in other (the argument to the function).
  • symmetric_difference: returns a new set of flags, containing all flags present in either self or other (the argument to the function), but not both.
  • complement: returns a new set of flags, containing all flags which are not set in self, but which are allowed for this type.

Default

The Default trait is not automatically implemented for the generated structs.

If your default value is equal to 0 (which is the same value as calling empty() on the generated struct), you can simply derive Default:

use bitflags::bitflags;

bitflags! {
    // Results in default value with bits: 0
    #[derive(Default)]
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}

fn main() {
    let derived_default: Flags = Default::default();
    assert_eq!(derived_default.bits(), 0);
}

If your default value is not equal to 0 you need to implement Default yourself:

use bitflags::bitflags;

bitflags! {
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}

// explicit `Default` implementation
impl Default for Flags {
    fn default() -> Flags {
        Flags::A | Flags::C
    }
}

fn main() {
    let implemented_default: Flags = Default::default();
    assert_eq!(implemented_default, (Flags::A | Flags::C));
}

Zero Flags

Flags with a value equal to zero will have some strange behavior that one should be aware of.

use bitflags::bitflags;

bitflags! {
    struct Flags: u32 {
        const NONE = 0b00000000;
        const SOME = 0b00000001;
    }
}

fn main() {
    let empty = Flags::empty();
    let none = Flags::NONE;
    let some = Flags::SOME;

    // Zero flags are treated as always present
    assert!(empty.contains(Flags::NONE));
    assert!(none.contains(Flags::NONE));
    assert!(some.contains(Flags::NONE));

    // Zero flags will be ignored when testing for emptiness
    assert!(none.is_empty());
}

Users should generally avoid defining a flag with a value of zero.

Macros

  • bitflags
    The macro used to generate the flag structures.