Trait std::convert::TryFrom

1.34.0 · 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 (essentially giving the i64’s value modulo i32::MAX) 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.

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))
        }
    }
}

Run

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());

Run

Required Associated Types§

source

type Error

The type returned in the event of a conversion error.

Required Methods§

source

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

Performs the conversion.

Implementors§

1.59.0 · source§

impl TryFrom<char> for u8

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

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

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type Error = TryFromCharError

1.74.0 · source§

impl TryFrom<char> for u16

Maps a char with code point in U+0000..=U+FFFF to a u16 in 0x0000..=0xFFFF with same value, failing if the code point is greater than U+FFFF.

This corresponds to the UCS-2 encoding, as specified in ISO/IEC 10646:2003.

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Trait std::convert::TryFrom

Generic Implementations

Examples

Required Associated Types§

type Error

Required Methods§

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

Implementors§

impl TryFrom<char> for u8

type Error = TryFromCharError

impl TryFrom<char> for u16

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 NonZeroI8

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 NonZeroI16

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 NonZeroI32

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

type Error = TryFromIntError

impl TryFrom<i64> for u128

type Error = TryFromIntError