Trait TryFrom

1.34.0 (const: unstable) · Source

pub trait TryFrom<T>: Sized {
    type Error;

    // Required method
    fn try_from(value: T) -> Result<Self, Self::Error>;
}

Expand description

Simple and safe type conversions that may fail in a controlled way under some circumstances. It is the reciprocal of TryInto.

This is useful when you are doing a type conversion that may trivially succeed but may also need special handling. For example, there is no way to convert an i64 into an i32 using the From trait, because an i64 may contain a value that an i32 cannot represent and so the conversion would lose data. This might be handled by truncating the i64 to an i32 or by simply returning i32::MAX, or by some other method. The From trait is intended for perfect conversions, so the TryFrom trait informs the programmer when a type conversion could go bad and lets them decide how to handle it.

§Generic Implementations

TryFrom<T> for U implies TryInto<U> for T
try_from is reflexive, which means that TryFrom<T> for T is implemented and cannot fail – the associated Error type for calling T::try_from() on a value of type T is Infallible. When the ! type is stabilized Infallible and ! will be equivalent.

Prefer using TryInto over TryFrom when specifying trait bounds on a generic function to ensure that types that only implement TryInto can be used as well.

TryFrom<T> can be implemented as follows:

struct GreaterThanZero(i32);

impl TryFrom<i32> for GreaterThanZero {
    type Error = &'static str;

    fn try_from(value: i32) -> Result<Self, Self::Error> {
        if value <= 0 {
            Err("GreaterThanZero only accepts values greater than zero!")
        } else {
            Ok(GreaterThanZero(value))
        }
    }
}

§Examples

As described, i32 implements TryFrom<i64>:

let big_number = 1_000_000_000_000i64;
// Silently truncates `big_number`, requires detecting
// and handling the truncation after the fact.
let smaller_number = big_number as i32;
assert_eq!(smaller_number, -727379968);

// Returns an error because `big_number` is too big to
// fit in an `i32`.
let try_smaller_number = i32::try_from(big_number);
assert!(try_smaller_number.is_err());

// Returns `Ok(3)`.
let try_successful_smaller_number = i32::try_from(3);
assert!(try_successful_smaller_number.is_ok());

Required Associated Types§

1.34.0 · Source

type Error

The type returned in the event of a conversion error.

Required Methods§

1.34.0 · Source

fn try_from(value: T) -> Result<Self, Self::Error>

Performs the conversion.

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors§

1.59.0 (const: unstable) · Source§

impl TryFrom<char> for u8

Maps a char with a code point from U+0000 to U+00FF (inclusive) to a byte in 0x00..=0xFF with the same value, failing if the code point is greater than U+00FF.

See impl From<u8> for char for details on the encoding.

Trait TryFrom Copy item path

§Generic Implementations

§Examples

Required Associated Types§

type Error

Required Methods§

fn try_from(value: T) -> Result<Self, Self::Error>

Dyn Compatibility§

Implementors§

impl TryFrom<char> for u8

type Error = TryFromCharError

impl TryFrom<char> for u16

type Error = TryFromCharError

impl TryFrom<char> for usize

type Error = TryFromCharError

impl TryFrom<i8> for u8

type Error = TryFromIntError

impl TryFrom<i8> for u16

type Error = TryFromIntError

impl TryFrom<i8> for u32

type Error = TryFromIntError

impl TryFrom<i8> for u64

type Error = TryFromIntError

impl TryFrom<i8> for u128

type Error = TryFromIntError

impl TryFrom<i8> for usize

type Error = TryFromIntError

impl TryFrom<i8> for NonZero<i8>

type Error = TryFromIntError

impl TryFrom<i16> for i8

type Error = TryFromIntError

impl TryFrom<i16> for u8

type Error = TryFromIntError

impl TryFrom<i16> for u16

type Error = TryFromIntError

impl TryFrom<i16> for u32

type Error = TryFromIntError

impl TryFrom<i16> for u64

type Error = TryFromIntError

impl TryFrom<i16> for u128

type Error = TryFromIntError

impl TryFrom<i16> for usize

type Error = TryFromIntError

impl TryFrom<i16> for NonZero<i16>

type Error = TryFromIntError

impl TryFrom<i32> for i8

type Error = TryFromIntError

impl TryFrom<i32> for i16

type Error = TryFromIntError

impl TryFrom<i32> for isize

type Error = TryFromIntError

impl TryFrom<i32> for u8

type Error = TryFromIntError

impl TryFrom<i32> for u16

type Error = TryFromIntError

impl TryFrom<i32> for u32

type Error = TryFromIntError

impl TryFrom<i32> for u64

type Error = TryFromIntError

impl TryFrom<i32> for u128

type Error = TryFromIntError

impl TryFrom<i32> for usize

type Error = TryFromIntError

impl TryFrom<i32> for NonZero<i32>

type Error = TryFromIntError

impl TryFrom<i64> for i8

type Error = TryFromIntError

impl TryFrom<i64> for i16

type Error = TryFromIntError

impl TryFrom<i64> for i32

type Error = TryFromIntError

impl TryFrom<i64> for isize

type Error = TryFromIntError

impl TryFrom<i64> for u8

type Error = TryFromIntError

impl TryFrom<i64> for u16

type Error = TryFromIntError

impl TryFrom<i64> for u32

type Error = TryFromIntError

impl TryFrom<i64> for u64

Trait TryFrom