Crate bitflags

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Expand description

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

The bitflags! macro generates structs that manage a set of flags. The type of those flags must be some primitive integer.

Examples

use bitflags::bitflags;

bitflags! {
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    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.

Visibility

The bitflags! macro supports visibility, just like you’d expect when writing a normal Rust struct:

mod example {
    use bitflags::bitflags;

    bitflags! {
        #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
        pub struct Flags1: u32 {
            const A = 0b00000001;
        }

        #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
        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 flags types and their constants as normal.

Representation

It’s valid to add a #[repr(C)] or #[repr(transparent)] attribute to a generated flags type. The generated flags type is always guaranteed to be a newtype where its only field has the same ABI as the underlying integer type.

In this example, Flags has the same ABI as u32:

use bitflags::bitflags;

bitflags! {
    #[repr(transparent)]
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}

Extending

Generated flags types belong to you, so you can add trait implementations to them outside of what the bitflags! macro gives:

use std::fmt;

use bitflags::bitflags;

bitflags! {
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
    }
}

impl Flags {
    pub fn clear(&mut self) {
        *self.0.bits_mut() = 0;
    }
}

fn main() {
    let mut flags = Flags::A | Flags::B;

    flags.clear();
    assert!(flags.is_empty());

    assert_eq!(format!("{:?}", Flags::A | Flags::B), "Flags(A | B)");
    assert_eq!(format!("{:?}", Flags::B), "Flags(B)");
}

What’s implemented by bitflags!

The bitflags! macro adds some trait implementations and inherent methods to generated flags types, but leaves room for you to choose the semantics of others.

Iterators

The following iterator traits are implemented for generated flags types:

  • Extend: adds the union of the instances iterated over.
  • FromIterator: calculates the union.
  • IntoIterator: iterates over set flag values.

Formatting

The following formatting traits are implemented for generated flags types:

  • Binary.
  • LowerHex and UpperHex.
  • Octal.

Also see the Debug and Display section for details about standard text representations for flags types.

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_retain: 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.

What’s not implemented by bitflags!

Some functionality is not automatically implemented for generated flags types by the bitflags! macro, even when it reasonably could be. This is so callers have more freedom to decide on the semantics of their flags types.

Clone and Copy

Generated flags types are not automatically copyable, even though they can always derive both Clone and Copy.

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, Clone, Copy, Debug, PartialEq, Eq, Hash)]
    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! {
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    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));
}

Debug and Display

The Debug trait can be derived for a reasonable implementation. This library defines a standard text-based representation for flags that generated flags types can use. For details on the exact grammar, see the parser module.

PartialEq and PartialOrd

Equality and ordering can be derived for a reasonable implementation, or implemented manually for different semantics.

Edge cases

Zero Flags

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

use bitflags::bitflags;

bitflags! {
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    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.

The BitFlags trait

This library defines a BitFlags trait that’s implemented by all generated flags types. The trait makes it possible to work with flags types generically:

fn count_unset_flags<F: bitflags::BitFlags>(flags: &F) -> usize {
    // Find out how many flags there are in total
    let total = F::all().iter().count();

    // Find out how many flags are set
    let set = flags.iter().count();

    total - set
}

use bitflags::bitflags;

bitflags! {
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
    }
}

assert_eq!(2, count_unset_flags(&Flags::B));

Modules

  • example_generated
    This module shows an example of code generated by the macro. IT MUST NOT BE USED OUTSIDE THIS CRATE.
  • parser
    Parsing flags from text.

Macros

  • bitflags
    The macro used to generate the flag structure.

Traits

  • BitFlags
    A trait that is automatically implemented for all bitflags.