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<&DatexExpressionData> for StructuralTypeDefinition

type Error = ()

impl TryFrom<&DatexExpressionData> for ValueContainer

type Error = ()

impl TryFrom<&DatexExpressionData> for Type

type Error = ()

impl TryFrom<&PointerAddress> for CoreLibPointerId

type Error = String

impl TryFrom<&PrivateKeyDer<'_>> for KeyPair

type Error = Error

impl TryFrom<&str> for InterfaceDirection

type Error = ParseError

impl TryFrom<&str> for DecimalTypeVariant

type Error = ParseError

impl TryFrom<&str> for IntegerTypeVariant

type Error = ParseError

impl TryFrom<&str> for PointerAddress

type Error = &'static str

impl TryFrom<&str> for IpAddr

type Error = AddrParseError

impl TryFrom<&str> for Endpoint

type Error = InvalidEndpointError

impl TryFrom<&str> for Alphabet

type Error = ParseAlphabetError

impl TryFrom<&str> for fancy_regex::Regex

type Error = Error

impl TryFrom<&str> for LogSpecification

type Error = FlexiLoggerError

impl TryFrom<&str> for CodeActionTriggerKind

type Error = &'static str

impl TryFrom<&str> for CompletionItemKind

type Error = &'static str

impl TryFrom<&str> for CompletionItemTag

type Error = &'static str

impl TryFrom<&str> for CompletionTriggerKind

type Error = &'static str

impl TryFrom<&str> for InsertTextFormat

type Error = &'static str

impl TryFrom<&str> for InsertTextMode

type Error = &'static str

impl TryFrom<&str> for DocumentHighlightKind

type Error = &'static str

impl TryFrom<&str> for InlayHintKind

type Error = &'static str

impl TryFrom<&str> for PrepareSupportDefaultBehavior

type Error = &'static str

impl TryFrom<&str> for SignatureHelpTriggerKind

type Error = &'static str

impl TryFrom<&str> for DiagnosticSeverity

type Error = &'static str

impl TryFrom<&str> for DiagnosticTag

type Error = &'static str

impl TryFrom<&str> for FileChangeType

type Error = &'static str

impl TryFrom<&str> for SymbolKind

type Error = &'static str

impl TryFrom<&str> for SymbolTag

type Error = &'static str

impl TryFrom<&str> for TextDocumentSaveReason

type Error = &'static str

impl TryFrom<&str> for TextDocumentSyncKind

type Error = &'static str

impl TryFrom<&str> for MessageType

type Error = &'static str

impl TryFrom<&str> for rcgen::string_types::BmpString

type Error = Error

impl TryFrom<&str> for rcgen::string_types::Ia5String

type Error = Error

impl TryFrom<&str> for rcgen::string_types::PrintableString

type Error = Error

impl TryFrom<&str> for rcgen::string_types::TeletexString

Trait TryFrom