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 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.

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<&str> for Alphabet

type Error = ParseAlphabetError

impl TryFrom<&str> for regex::regex::bytes::Regex

type Error = Error

impl TryFrom<&str> for regex::regex::string::Regex

type Error = Error

impl TryFrom<&SystemTime> for GeneralizedTime

type Error = Error

impl TryFrom<&SystemTime> for DateTime

type Error = Error

impl TryFrom<&DateTime> for UtcTime

type Error = Error

impl TryFrom<&AffinePoint> for elliptic_curve::public_key::PublicKey<Secp256k1>

type Error = Error

impl TryFrom<&ProjectivePoint> for elliptic_curve::public_key::PublicKey<Secp256k1>

type Error = Error

impl TryFrom<&PrivateKeyInfo<'_>> for SecretDocument

type Error = Error

impl TryFrom<&EncodedPoint<<Secp256k1 as Curve>::FieldBytesSize>> for AffinePoint

type Error = Error

impl TryFrom<&EcPrivateKey<'_>> for SecretDocument

type Error = Error

impl TryFrom<&[u8]> for ObjectIdentifier

type Error = Error

impl TryFrom<&[u8]> for CompressedEdwardsY

type Error = TryFromSliceError

impl TryFrom<&[u8]> for CompressedRistretto

type Error = TryFromSliceError

impl TryFrom<&[u8]> for Document

type Error = Error

impl TryFrom<&[u8]> for SecretDocument

type Error = Error

impl TryFrom<&[u8]> for ed25519_dalek::signing::SigningKey

type Error = Error

impl TryFrom<&[u8]> for ed25519_dalek::verifying::VerifyingKey

type Error = Error

impl TryFrom<&[u8]> for ed25519_zebra::signing_key::SigningKey

type Error = Error

impl TryFrom<&[u8]> for VerificationKey

type Error = Error

impl TryFrom<&[u8]> for VerificationKeyBytes

type Error = Error

impl TryFrom<&[u8]> for ed25519::Signature

type Error = Error

impl TryFrom<MultiSignature> for CryptoBytes<subsoil::::core::ecdsa::GenericSignature::{constant#0}, (SignatureTag, CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>)>

type Error = ()

impl TryFrom<MultiSignature> for CryptoBytes<subsoil::::core::ed25519::Signature::{constant#0}, (SignatureTag, Ed25519Tag)>

type Error = ()

impl TryFrom<MultiSignature> for CryptoBytes<subsoil::::core::sr25519::Signature::{constant#0}, (SignatureTag, Sr25519Tag)>

type Error = ()

impl TryFrom<MultiSigner> for CryptoBytes<subsoil::::core::ecdsa::GenericPublic::{constant#0}, (PublicTag, EcdsaTag)>

type Error = ()

impl TryFrom<MultiSigner> for CryptoBytes<subsoil::::core::ed25519::Public::{constant#0}, (PublicTag, Ed25519Tag)>

type Error = ()

impl TryFrom<Error> for Format

type Error = DifferentVariant

impl TryFrom<Error> for InvalidFormatDescription

type Error = DifferentVariant

impl TryFrom<Error> for ComponentRange

type Error = DifferentVariant

impl TryFrom<Error> for ConversionRange

type Error = DifferentVariant

impl TryFrom<Error> for DifferentVariant

type Error = DifferentVariant

impl TryFrom<Error> for InvalidVariant

type Error = DifferentVariant

impl TryFrom<Format> for Error

type Error = DifferentVariant

impl TryFrom<Format> for ComponentRange

type Error = DifferentVariant

impl TryFrom<BorrowedFormatItem<'_>> for Component

Trait TryFrom