Enum maybe_owned::MaybeOwned

pub enum MaybeOwned<'a, T: 'a> {
    Owned(T),
    Borrowed(&'a T),
}

This type provides a way to store data to which you either have a reference to or which you do own.

It provides From<T>, From<&'a T> implementations and, in difference to Cow does not require ToOwned to be implemented which makes it compatible with non cloneable data, as a draw back of this it does not know about ToOwned. As a consequence of it can’t know that &str should be the borrowed version of String and not &String this is especially bad wrt. Box as the borrowed version of Box<T> would be &Box<T>.

While this crate has some drawbacks compared to Cow is has the benefit, that it works with Types which neither implement Clone nor ToOwned. Another benefit lies in the ability to write API functions which accept a generic parameter E: Into<MaybeOwned<'a, T>> as the API consumer can pass T, &'a T and MaybeOwned<'a, T> as argument, without requiring a explicit Cow::Onwed or a split into two functions one accepting owed and the other borrowed values.

Alternatives

If you mainly have values implementing ToOwned like &str/String, Path/PathBuf or &[T]/Vec<T> using std::borrow::Cow might be preferable.

If you want to be able to treat &T, &mut T, Box<T> and Arc<T> the same consider using reffers::rbma::RBMA (through not all types/platforms are supported because as it relies on the fact that for many pointers the lowest two bits are 0, and stores the discriminant in them, nevertheless this is can only be used with 32bit-aligned data, e.g. using a &u8 might fail). RBMA also allows you to recover a &mut T if it was created from Box<T>, &mut T or a unique Arc.

Examples

struct PseudoBigData(u8);
fn pseudo_register_fn<'a, E>(_val: E) where E: Into<MaybeOwned<'a, PseudoBigData>> { }

let data = PseudoBigData(12);
let data2 = PseudoBigData(13);

pseudo_register_fn(&data);
pseudo_register_fn(&data);
pseudo_register_fn(data2);
pseudo_register_fn(MaybeOwned::Owned(PseudoBigData(111)));

#[repr(C)]
struct OpaqueFFI {
    ref1:  * const u8
    //we also might want to have PhantomData etc.
}

// does not work as it does not implement `ToOwned`
// let _ = Cow::Owned(OpaqueFFI { ref1: 0 as *const u8});

// ok, MaybeOwned can do this (but can't do &str<->String as tread of)
let _ = MaybeOwned::Owned(OpaqueFFI { ref1: 0 as *const u8 });

use std::collections::HashMap;

#[derive(Serialize, Deserialize)]
struct SerializedData<'a> {
    data: MaybeOwned<'a, HashMap<String, i32>>,
}

let mut map = HashMap::new();
map.insert("answer".to_owned(), 42);

// serializing can use borrowed data to avoid unnecessary copying
let bytes = serde_json::to_vec(&SerializedData { data: (&map).into() }).unwrap();

// deserializing creates owned data
let deserialized: SerializedData = serde_json::from_slice(&bytes).unwrap();
assert_eq!(deserialized.data["answer"], 42);

Transitive std::ops implementations

There are transitive implementations for most operator in std::ops.

A Op between a MaybeOwned<L> and MaybeOwned<R> is implemented if:

• L impl the Op with R
• L impl the Op with &R
• &L impl the Op with R
• &L impl the Op with &R
• the Output of all aboves implementations is the same type

The Neg (- prefix) op is implemented for V if:

• V impl Neg
• &V impl Neg
• both have the same Output

The Not (! prefix) op is implemented for V if:

• V impl Not
• &V impl Not
• both have the same Output

Adding implementations for Ops which add a MaybeOwned to a non MaybeOwned value (like MaybeOwned<T> + T) requires far reaching specialization in rust and is therefore not done for now.

Variants

Owned(T)

owns T

Borrowed(&'a T)

has a reference to T

Implementations

impl<'a, T> MaybeOwned<'a, T>

pub fn is_owned(&self) -> bool

returns true if the data is owned else false

impl<'a, T> MaybeOwned<'a, T>

where T: Clone,

pub fn to_mut(&mut self) -> &mut T

Aquires a mutable reference to owned data.

Clones data if it is not already owned.

Example

use maybe_owned::MaybeOwned;

#[derive(Clone, Debug, PartialEq, Eq)]
struct PseudoBigData(u8);

let data = PseudoBigData(12);

let mut maybe: MaybeOwned<PseudoBigData> = (&data).into();
assert_eq!(false, maybe.is_owned());

{
    let reference = maybe.to_mut();
    assert_eq!(&mut PseudoBigData(12), reference);
}
assert!(maybe.is_owned());

pub fn into_owned(self) -> T

Extracts the owned data.

If the data is borrowed it is cloned before being extracted.

Trait Implementations

impl<'min, L, R, OUT: 'min> Add<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Add<R, Output = OUT> + Add<&'min R, Output = OUT>, &'min L: Add<R, Output = OUT> + Add<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the + operator.

fn add(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the + operation.

impl<'min, L, R> AddAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + AddAssign<R> + AddAssign<&'min R>,

fn add_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the += operation.

impl<'a, T> AsRef<T> for MaybeOwned<'a, T>

fn as_ref(&self) -> &T

Converts this type into a shared reference of the (usually inferred) input type.

impl<'min, L, R, OUT: 'min> BitAnd<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: BitAnd<R, Output = OUT> + BitAnd<&'min R, Output = OUT>, &'min L: BitAnd<R, Output = OUT> + BitAnd<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the & operator.

fn bitand(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the & operation.

impl<'min, L, R> BitAndAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + BitAndAssign<R> + BitAndAssign<&'min R>,

fn bitand_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the &= operation.

impl<'min, L, R, OUT: 'min> BitOr<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: BitOr<R, Output = OUT> + BitOr<&'min R, Output = OUT>, &'min L: BitOr<R, Output = OUT> + BitOr<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the | operator.

fn bitor(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the | operation.

impl<'min, L, R> BitOrAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + BitOrAssign<R> + BitOrAssign<&'min R>,

fn bitor_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the |= operation.

impl<'min, L, R, OUT: 'min> BitXor<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: BitXor<R, Output = OUT> + BitXor<&'min R, Output = OUT>, &'min L: BitXor<R, Output = OUT> + BitXor<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the ^ operator.

fn bitxor(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the ^ operation.

impl<'min, L, R> BitXorAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + BitXorAssign<R> + BitXorAssign<&'min R>,

fn bitxor_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the ^= operation.

impl<'a, T> Borrow<T> for MaybeOwned<'a, T>

fn borrow(&self) -> &T

Immutably borrows from an owned value.

impl<'a, T> Clone for MaybeOwned<'a, T>

where T: Clone,

fn clone(&self) -> MaybeOwned<'a, T>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'a, T: Debug + 'a> Debug for MaybeOwned<'a, T>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'a, T> Default for MaybeOwned<'a, T>

where T: Default,

fn default() -> Self

Returns the “default value” for a type.

impl<'a, T> Deref for MaybeOwned<'a, T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &T

Dereferences the value.

impl<'a, T> Display for MaybeOwned<'a, T>

where T: Display,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'min, L, R, OUT: 'min> Div<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Div<R, Output = OUT> + Div<&'min R, Output = OUT>, &'min L: Div<R, Output = OUT> + Div<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the / operator.

fn div(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the / operation.

impl<'min, L, R> DivAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + DivAssign<R> + DivAssign<&'min R>,

fn div_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the /= operation.

impl<'a, T> From<&'a T> for MaybeOwned<'a, T>

fn from(v: &'a T) -> MaybeOwned<'a, T>

Converts to this type from the input type.

impl<'a, T> From<Cow<'a, T>> for MaybeOwned<'a, T>

where T: ToOwned<Owned = T>,

fn from(cow: Cow<'a, T>) -> MaybeOwned<'a, T>

Converts to this type from the input type.

impl<'a, T> From<T> for MaybeOwned<'a, T>

fn from(v: T) -> MaybeOwned<'a, T>

Converts to this type from the input type.

impl<'a, T> FromStr for MaybeOwned<'a, T>

where T: FromStr,

type Err = <T as FromStr>::Err

The associated error which can be returned from parsing.

fn from_str(s: &str) -> Result<Self, Self::Err>

Parses a string s to return a value of this type.

impl<'a, T> Hash for MaybeOwned<'a, T>

where T: Hash,

fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<'a, T> Into<Cow<'a, T>> for MaybeOwned<'a, T>

where T: ToOwned<Owned = T>,

fn into(self) -> Cow<'a, T>

Converts this type into the (usually inferred) input type.

impl<'min, L, R, OUT: 'min> Mul<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Mul<R, Output = OUT> + Mul<&'min R, Output = OUT>, &'min L: Mul<R, Output = OUT> + Mul<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the * operator.

fn mul(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the * operation.

impl<'min, L, R> MulAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + MulAssign<R> + MulAssign<&'min R>,

fn mul_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the *= operation.

impl<'l, V, OUT> Neg for MaybeOwned<'l, V>

where V: Neg<Output = OUT>, &'l V: Neg<Output = OUT>,

type Output = OUT

The resulting type after applying the - operator.

fn neg(self) -> Self::Output

Performs the unary - operation.

impl<'l, V, OUT> Not for MaybeOwned<'l, V>

where V: Not<Output = OUT>, &'l V: Not<Output = OUT>,

type Output = <V as Not>::Output

The resulting type after applying the ! operator.

fn not(self) -> Self::Output

Performs the unary ! operation.

impl<'a, T> Ord for MaybeOwned<'a, T>

where T: Ord,

fn cmp(&self, other: &MaybeOwned<'a, T>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized + PartialOrd,

Restrict a value to a certain interval.

impl<'a, 'b, A, B> PartialEq<MaybeOwned<'b, B>> for MaybeOwned<'a, A>

where A: PartialEq<B>,

fn eq(&self, other: &MaybeOwned<'b, B>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'a, T> PartialOrd for MaybeOwned<'a, T>

where T: PartialOrd,

fn partial_cmp(&self, other: &MaybeOwned<'a, T>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'min, L, R, OUT: 'min> Shl<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Shl<R, Output = OUT> + Shl<&'min R, Output = OUT>, &'min L: Shl<R, Output = OUT> + Shl<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the << operator.

fn shl(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the << operation.

impl<'min, L, R> ShlAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + ShlAssign<R> + ShlAssign<&'min R>,

fn shl_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the <<= operation.

impl<'min, L, R, OUT: 'min> Shr<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Shr<R, Output = OUT> + Shr<&'min R, Output = OUT>, &'min L: Shr<R, Output = OUT> + Shr<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the >> operator.

fn shr(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the >> operation.

impl<'min, L, R> ShrAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + ShrAssign<R> + ShrAssign<&'min R>,

fn shr_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the >>= operation.

impl<'min, L, R, OUT: 'min> Sub<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Sub<R, Output = OUT> + Sub<&'min R, Output = OUT>, &'min L: Sub<R, Output = OUT> + Sub<&'min R, Output = OUT>,

type Output = OUT

The resulting type after applying the - operator.

fn sub(self, rhs: MaybeOwned<'min, R>) -> Self::Output

Performs the - operation.

impl<'min, L, R> SubAssign<MaybeOwned<'min, R>> for MaybeOwned<'min, L>

where L: Clone + SubAssign<R> + SubAssign<&'min R>,

fn sub_assign(&mut self, rhs: MaybeOwned<'min, R>)

Performs the -= operation.

impl<'a, T> Eq for MaybeOwned<'a, T>

where T: Eq,