Trait core::convert::From 1.0.0[−][src]
Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
Into.
One should always prefer implementing From over Into
because implementing From automatically provides one with an implementation of Into
thanks to the blanket implementation in the standard library.
Only implement Into when targeting a version prior to Rust 1.41 and converting to a type
outside the current crate.
From was not able to do these types of conversions in earlier versions because of Rust’s
orphaning rules.
See Into for more details.
Prefer using Into over using From when specifying trait bounds on a generic function.
This way, types that directly implement Into can be used as arguments as well.
The From is also very useful when performing error handling. When constructing a function
that is capable of failing, the return type will generally be of the form Result<T, E>.
The From trait simplifies error handling by allowing a function to return a single error type
that encapsulate multiple error types. See the “Examples” section and the book for more
details.
Note: This trait must not fail. If the conversion can fail, use TryFrom.
Generic Implementations
From<T> for UimpliesInto<U> for TFromis reflexive, which means thatFrom<T> for Tis implemented
Examples
String implements From<&str>:
An explicit conversion from a &str to a String is done as follows:
let string = "hello".to_string(); let other_string = String::from("hello"); assert_eq!(string, other_string);Run
While performing error handling it is often useful to implement From for your own error type.
By converting underlying error types to our own custom error type that encapsulates the
underlying error type, we can return a single error type without losing information on the
underlying cause. The ‘?’ operator automatically converts the underlying error type to our
custom error type by calling Into<CliError>::into which is automatically provided when
implementing From. The compiler then infers which implementation of Into should be used.
use std::fs; use std::io; use std::num; enum CliError { IoError(io::Error), ParseError(num::ParseIntError), } impl From<io::Error> for CliError { fn from(error: io::Error) -> Self { CliError::IoError(error) } } impl From<num::ParseIntError> for CliError { fn from(error: num::ParseIntError) -> Self { CliError::ParseError(error) } } fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> { let mut contents = fs::read_to_string(&file_name)?; let num: i32 = contents.trim().parse()?; Ok(num) }Run
Required methods
Loading content...Implementors
impl From<!> for Infallible1.34.0[src]
impl From<!> for Infallible1.34.0[src]impl From<!> for TryFromIntError[src]
impl From<!> for TryFromIntError[src]fn from(never: !) -> TryFromIntError[src]
impl From<Infallible> for TryFromSliceError1.36.0[src]
impl From<Infallible> for TryFromSliceError1.36.0[src]fn from(x: Infallible) -> TryFromSliceError[src]
impl From<Infallible> for TryFromIntError1.34.0[src]
impl From<Infallible> for TryFromIntError1.34.0[src]fn from(x: Infallible) -> TryFromIntError[src]
impl From<NonZeroI8> for NonZeroI161.41.0[src]
impl From<NonZeroI8> for NonZeroI161.41.0[src]impl From<NonZeroI8> for NonZeroI321.41.0[src]
impl From<NonZeroI8> for NonZeroI321.41.0[src]impl From<NonZeroI8> for NonZeroI641.41.0[src]
impl From<NonZeroI8> for NonZeroI641.41.0[src]impl From<NonZeroI8> for NonZeroI1281.41.0[src]
impl From<NonZeroI8> for NonZeroI1281.41.0[src]impl From<NonZeroI8> for NonZeroIsize1.41.0[src]
impl From<NonZeroI8> for NonZeroIsize1.41.0[src]impl From<NonZeroI16> for NonZeroI321.41.0[src]
impl From<NonZeroI16> for NonZeroI321.41.0[src]fn from(small: NonZeroI16) -> Self[src]
Converts NonZeroI16 to NonZeroI32 losslessly.
impl From<NonZeroI16> for NonZeroI641.41.0[src]
impl From<NonZeroI16> for NonZeroI641.41.0[src]fn from(small: NonZeroI16) -> Self[src]
Converts NonZeroI16 to NonZeroI64 losslessly.
impl From<NonZeroI16> for NonZeroI1281.41.0[src]
impl From<NonZeroI16> for NonZeroI1281.41.0[src]fn from(small: NonZeroI16) -> Self[src]
Converts NonZeroI16 to NonZeroI128 losslessly.
impl From<NonZeroI16> for NonZeroIsize1.41.0[src]
impl From<NonZeroI16> for NonZeroIsize1.41.0[src]fn from(small: NonZeroI16) -> Self[src]
Converts NonZeroI16 to NonZeroIsize losslessly.
impl From<NonZeroI16> for i161.31.0[src]
impl From<NonZeroI16> for i161.31.0[src]fn from(nonzero: NonZeroI16) -> Self[src]
Converts a NonZeroI16 into an i16
impl From<NonZeroI32> for NonZeroI641.41.0[src]
impl From<NonZeroI32> for NonZeroI641.41.0[src]fn from(small: NonZeroI32) -> Self[src]
Converts NonZeroI32 to NonZeroI64 losslessly.
impl From<NonZeroI32> for NonZeroI1281.41.0[src]
impl From<NonZeroI32> for NonZeroI1281.41.0[src]fn from(small: NonZeroI32) -> Self[src]
Converts NonZeroI32 to NonZeroI128 losslessly.
impl From<NonZeroI32> for i321.31.0[src]
impl From<NonZeroI32> for i321.31.0[src]fn from(nonzero: NonZeroI32) -> Self[src]
Converts a NonZeroI32 into an i32
impl From<NonZeroI64> for NonZeroI1281.41.0[src]
impl From<NonZeroI64> for NonZeroI1281.41.0[src]fn from(small: NonZeroI64) -> Self[src]
Converts NonZeroI64 to NonZeroI128 losslessly.
impl From<NonZeroI64> for i641.31.0[src]
impl From<NonZeroI64> for i641.31.0[src]fn from(nonzero: NonZeroI64) -> Self[src]
Converts a NonZeroI64 into an i64
impl From<NonZeroI128> for i1281.31.0[src]
impl From<NonZeroI128> for i1281.31.0[src]fn from(nonzero: NonZeroI128) -> Self[src]
Converts a NonZeroI128 into an i128
impl From<NonZeroIsize> for isize1.31.0[src]
impl From<NonZeroIsize> for isize1.31.0[src]fn from(nonzero: NonZeroIsize) -> Self[src]
Converts a NonZeroIsize into an isize
impl From<NonZeroU8> for NonZeroI161.41.0[src]
impl From<NonZeroU8> for NonZeroI161.41.0[src]impl From<NonZeroU8> for NonZeroI321.41.0[src]
impl From<NonZeroU8> for NonZeroI321.41.0[src]impl From<NonZeroU8> for NonZeroI641.41.0[src]
impl From<NonZeroU8> for NonZeroI641.41.0[src]impl From<NonZeroU8> for NonZeroI1281.41.0[src]
impl From<NonZeroU8> for NonZeroI1281.41.0[src]impl From<NonZeroU8> for NonZeroIsize1.41.0[src]
impl From<NonZeroU8> for NonZeroIsize1.41.0[src]impl From<NonZeroU8> for NonZeroU161.41.0[src]
impl From<NonZeroU8> for NonZeroU161.41.0[src]impl From<NonZeroU8> for NonZeroU321.41.0[src]
impl From<NonZeroU8> for NonZeroU321.41.0[src]impl From<NonZeroU8> for NonZeroU641.41.0[src]
impl From<NonZeroU8> for NonZeroU641.41.0[src]impl From<NonZeroU8> for NonZeroU1281.41.0[src]
impl From<NonZeroU8> for NonZeroU1281.41.0[src]impl From<NonZeroU8> for NonZeroUsize1.41.0[src]
impl From<NonZeroU8> for NonZeroUsize1.41.0[src]impl From<NonZeroU16> for NonZeroI321.41.0[src]
impl From<NonZeroU16> for NonZeroI321.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroI32 losslessly.
impl From<NonZeroU16> for NonZeroI641.41.0[src]
impl From<NonZeroU16> for NonZeroI641.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroI64 losslessly.
impl From<NonZeroU16> for NonZeroI1281.41.0[src]
impl From<NonZeroU16> for NonZeroI1281.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroI128 losslessly.
impl From<NonZeroU16> for NonZeroU321.41.0[src]
impl From<NonZeroU16> for NonZeroU321.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroU32 losslessly.
impl From<NonZeroU16> for NonZeroU641.41.0[src]
impl From<NonZeroU16> for NonZeroU641.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroU64 losslessly.
impl From<NonZeroU16> for NonZeroU1281.41.0[src]
impl From<NonZeroU16> for NonZeroU1281.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroU128 losslessly.
impl From<NonZeroU16> for NonZeroUsize1.41.0[src]
impl From<NonZeroU16> for NonZeroUsize1.41.0[src]fn from(small: NonZeroU16) -> Self[src]
Converts NonZeroU16 to NonZeroUsize losslessly.
impl From<NonZeroU16> for u161.31.0[src]
impl From<NonZeroU16> for u161.31.0[src]fn from(nonzero: NonZeroU16) -> Self[src]
Converts a NonZeroU16 into an u16
impl From<NonZeroU32> for NonZeroI641.41.0[src]
impl From<NonZeroU32> for NonZeroI641.41.0[src]fn from(small: NonZeroU32) -> Self[src]
Converts NonZeroU32 to NonZeroI64 losslessly.
impl From<NonZeroU32> for NonZeroI1281.41.0[src]
impl From<NonZeroU32> for NonZeroI1281.41.0[src]fn from(small: NonZeroU32) -> Self[src]
Converts NonZeroU32 to NonZeroI128 losslessly.
impl From<NonZeroU32> for NonZeroU641.41.0[src]
impl From<NonZeroU32> for NonZeroU641.41.0[src]fn from(small: NonZeroU32) -> Self[src]
Converts NonZeroU32 to NonZeroU64 losslessly.
impl From<NonZeroU32> for NonZeroU1281.41.0[src]
impl From<NonZeroU32> for NonZeroU1281.41.0[src]fn from(small: NonZeroU32) -> Self[src]
Converts NonZeroU32 to NonZeroU128 losslessly.
impl From<NonZeroU32> for u321.31.0[src]
impl From<NonZeroU32> for u321.31.0[src]fn from(nonzero: NonZeroU32) -> Self[src]
Converts a NonZeroU32 into an u32
impl From<NonZeroU64> for NonZeroI1281.41.0[src]
impl From<NonZeroU64> for NonZeroI1281.41.0[src]fn from(small: NonZeroU64) -> Self[src]
Converts NonZeroU64 to NonZeroI128 losslessly.
impl From<NonZeroU64> for NonZeroU1281.41.0[src]
impl From<NonZeroU64> for NonZeroU1281.41.0[src]fn from(small: NonZeroU64) -> Self[src]
Converts NonZeroU64 to NonZeroU128 losslessly.
impl From<NonZeroU64> for u641.31.0[src]
impl From<NonZeroU64> for u641.31.0[src]fn from(nonzero: NonZeroU64) -> Self[src]
Converts a NonZeroU64 into an u64
impl From<NonZeroU128> for u1281.31.0[src]
impl From<NonZeroU128> for u1281.31.0[src]fn from(nonzero: NonZeroU128) -> Self[src]
Converts a NonZeroU128 into an u128
impl From<NonZeroUsize> for usize1.31.0[src]
impl From<NonZeroUsize> for usize1.31.0[src]fn from(nonzero: NonZeroUsize) -> Self[src]
Converts a NonZeroUsize into an usize
impl From<bool> for AtomicBool1.24.0[src]
impl From<bool> for AtomicBool1.24.0[src]impl From<i128> for AtomicI128[src]
impl From<i128> for AtomicI128[src]impl From<isize> for AtomicIsize1.23.0[src]
impl From<isize> for AtomicIsize1.23.0[src]impl From<u8> for char1.13.0[src]
impl From<u8> for char1.13.0[src]Maps a byte in 0x00..=0xFF to a char whose code point has the same value, in U+0000..=U+00FF.
Unicode is designed such that this effectively decodes bytes with the character encoding that IANA calls ISO-8859-1. This encoding is compatible with ASCII.
Note that this is different from ISO/IEC 8859-1 a.k.a. ISO 8859-1 (with one less hyphen), which leaves some “blanks”, byte values that are not assigned to any character. ISO-8859-1 (the IANA one) assigns them to the C0 and C1 control codes.
Note that this is also different from Windows-1252 a.k.a. code page 1252, which is a superset ISO/IEC 8859-1 that assigns some (not all!) blanks to punctuation and various Latin characters.
To confuse things further, on the Web
ascii, iso-8859-1, and windows-1252 are all aliases
for a superset of Windows-1252 that fills the remaining blanks with corresponding
C0 and C1 control codes.
impl From<u128> for AtomicU128[src]
impl From<u128> for AtomicU128[src]impl From<usize> for AtomicUsize1.23.0[src]
impl From<usize> for AtomicUsize1.23.0[src]impl<'a, T> From<&'a Option<T>> for Option<&'a T>1.30.0[src]
impl<'a, T> From<&'a Option<T>> for Option<&'a T>1.30.0[src]fn from(o: &'a Option<T>) -> Option<&'a T>[src]
Converts from &Option<T> to Option<&T>.
Examples
Converts an Option<String> into an Option<usize>, preserving the original.
The map method takes the self argument by value, consuming the original,
so this technique uses as_ref to first take an Option to a reference
to the value inside the original.
let s: Option<String> = Some(String::from("Hello, Rustaceans!")); let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len()); println!("Can still print s: {:?}", s); assert_eq!(o, Some(18));Run
impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>1.30.0[src]
impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>1.30.0[src]impl<T> From<!> for T1.34.0[src]
impl<T> From<!> for T1.34.0[src]Stability note: This impl does not yet exist, but we are “reserving space” to add it in the future. See rust-lang/rust#64715 for details.
impl<T> From<T> for UnsafeCell<T>1.12.0[src]
impl<T> From<T> for UnsafeCell<T>1.12.0[src]