std::convert::From - Rust
Description: API documentation for the Rust `From` trait in crate `std`.
Keywords: rust, rustlang, rust-lang, From

# Trait std::convert::From 1.0.0 [−] [src]

    pub trait From<T> {
        fn from(T) -> Self;
    }

Simple and safe type conversions in to `Self`. It is the reciprocal of `Into`.

This trait is useful when performing error handling as described by the book and is closely related to the `?` operator.

When constructing a function that is capable of failing the return type will generally be of the form `Result<T, E>`.

The `From` trait allows for simplification of error handling by providing a means of returning a single error type that encapsulates numerous possible erroneous situations.

This trait is not limited to error handling, rather the general case for this trait would be in any type conversions to have an explicit definition of how they are performed.

Note: this trait must not fail. If the conversion can fail, use `TryFrom` or a dedicated method which returns an `Option<T>` or a `Result<T, E>`.

# Generic Implementations

- `From<T> for U` implies `Into<U>``for T`
- `from` is reflexive, which means that `From<T> for T` is implemented

# Examples

`String` implements `From<&str>`:

    let string = "hello".to_string();
    let other_string = String::from("hello");

    assert_eq!(string, other_string);Run

An example usage for error handling:

    use std::io::{self, Read};
    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 file = std::fs::File::open("test")?;
        let mut contents = String::new();
        file.read_to_string(&mut contents)?;
        let num: i32 = contents.trim().parse()?;
        Ok(num)
    }Run

## Required Methods

### `fn from(T) -> Self`

Performs the conversion.

## Implementors

- `impl From<u64> for u128`
- `impl From<u64> for i128`
- `impl<T> From<T> for Cell<T>`
- `impl From<i8> for i64`
- `impl From<u32> for i64`
- `impl From<u8> for i32`
- `impl<T> From<T> for T`
- `impl From<i16> for f32`
- `impl From<i32> for i64`
- `impl From<char> for u32`
- `impl From<i8> for i128`
- `impl From<u8> for u16`
- `impl From<u16> for i32`
- `impl From<u32> for u128`
- `impl From<i16> for i64`
- `impl From<i16> for f64`
- `impl From<u32> for u64`
- `impl From<u16> for f64`
- `impl From<u16> for u128`
- `impl From<u8> for i16`
- `impl From<u16> for u64`
- `impl From<u8> for f64`
- `impl From<u8> for u32`
- `impl<T> From<T> for RefCell<T>`
- `impl From<i8> for f64`
- `impl From<u16> for u32`
- `impl From<u32> for i128`
- `impl From<i16> for i128`
- `impl From<u8> for i64`
- `impl From<f32> for f64`
- `impl From<i8> for i16`
- `impl From<i8> for i32`
- `impl From<u16> for i64`
- `impl From<u8> for u128`
- `impl From<u8> for char`
- `impl From<u8> for usize`
- `impl From<i16> for i32`
- `impl From<u32> for f64`
- `impl From<i32> for f64`
- `impl From<u8> for u64`
- `impl From<u8> for i128`
- `impl<T> From<T> for Option<T>`
- `impl From<i8> for isize`
- `impl From<i32> for i128`
- `impl From<u16> for i128`
- `impl From<u8> for f32`
- `impl From<i64> for i128`
- `impl From<u16> for f32`
- `impl<T> From<T> for UnsafeCell<T>`
- `impl From<i8> for f32`
- `impl From<Box<str>> for Box<[u8]>`
- `impl From<String> for Box<str>`
- `impl<'a> From<&'a str> for String`
- `impl<T> From<T> for Arc<T>`
- `impl<'a> From<&'a str> for Cow<'a, str>`
- `impl<'a, T> From<&'a [T]> for Vec<T> where
  T: Clone,`
- `impl<T> From<T> for Box<T>`
- `impl From<Box<str>> for String`
- `impl<'a, T> From<&'a mut [T]> for Vec<T> where
  T: Clone,`
- `impl<T> From<Box<[T]>> for Vec<T>`
- `impl<'a> From<Cow<'a, str>> for String`
- `impl<T> From<Vec<T>> for VecDeque<T>`
- `impl<T> From<BinaryHeap<T>> for Vec<T>`
- `impl<'a> From<String> for Cow<'a, str>`
- `impl<T> From<T> for Rc<T>`
- `impl<'a, T> From<Vec<T>> for Cow<'a, [T]> where
  T: Clone,`
- `impl<'a, T> From<&'a [T]> for Cow<'a, [T]> where
  T: Clone,`
- `impl<'a, T> From<&'a [T]> for Box<[T]> where
  T: Copy,`
- `impl<T> From<VecDeque<T>> for Vec<T>`
- `impl<'a, T> From<Cow<'a, [T]>> for Vec<T> where
  [T]: ToOwned,
  <[T] as ToOwned>::Owned == Vec<T>,`
- `impl<'a> From<&'a str> for Box<str>`
- `impl<'a> From<&'a str> for Vec<u8>`
- `impl<T> From<Vec<T>> for BinaryHeap<T> where
  T: Ord,`
- `impl From<String> for Vec<u8>`
- `impl<T> From<Vec<T>> for Box<[T]>`
- `impl<'a, E: Error + 'a> From<E> for Box<Error + 'a>`
- `impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<Error + Send + Sync + 'a>`
- `impl From<String> for Box<Error + Send + Sync>`
- `impl From<String> for Box<Error>`
- `impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a>`
- `impl<'a> From<&'a str> for Box<Error>`
- `impl From<CString> for Vec<u8>`
- `impl<'a> From<&'a CStr> for Box<CStr>`
- `impl From<Box<CStr>> for CString`
- `impl From<CString> for Box<CStr>`
- `impl From<NulError> for Error`
- `impl<'a> From<&'a CStr> for CString`
- `impl From<String> for OsString`
- `impl<'a, T: ?Sized + AsRef<OsStr>> From<&'a T> for OsString`
- `impl<'a> From<&'a OsStr> for Box<OsStr>`
- `impl From<Box<OsStr>> for OsString`
- `impl From<OsString> for Box<OsStr>`
- `impl<W> From<IntoInnerError<W>> for Error`
- `impl From<ErrorKind> for Error`
- `impl From<Ipv4Addr> for IpAddr`
- `impl From<Ipv6Addr> for IpAddr`
- `impl From<Ipv4Addr> for u32`
- `impl From<u32> for Ipv4Addr`
- `impl From<[u8; 4]> for Ipv4Addr`
- `impl From<[u8; 4]> for IpAddr`
- `impl From<Ipv6Addr> for u128`
- `impl From<u128> for Ipv6Addr`
- `impl From<[u8; 16]> for Ipv6Addr`
- `impl From<[u16; 8]> for Ipv6Addr`
- `impl From<[u8; 16]> for IpAddr`
- `impl From<[u16; 8]> for IpAddr`
- `impl From<SocketAddrV4> for SocketAddr`
- `impl From<SocketAddrV6> for SocketAddr`
- `impl<I: Into<IpAddr>> From<(I, u16)> for SocketAddr`
- `impl<'a> From<&'a Path> for Box<Path>`
- `impl From<Box<Path>> for PathBuf`
- `impl From<PathBuf> for Box<Path>`
- `impl<'a, T: ?Sized + AsRef<OsStr>> From<&'a T> for PathBuf`
- `impl From<OsString> for PathBuf`
- `impl From<PathBuf> for OsString`
- `impl From<String> for PathBuf`
- `impl<'a> From<&'a Path> for Cow<'a, Path>`
- `impl<'a> From<PathBuf> for Cow<'a, Path>`
- `impl From<ChildStdin> for Stdio`
- `impl From<ChildStdout> for Stdio`
- `impl From<ChildStderr> for Stdio`
- `impl From<File> for Stdio`
- `impl<T> From<PoisonError<T>> for TryLockError<T>`

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