Trait std::convert::From

1.0.0 · source · [−]

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

Expand description

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. The From trait is intended for perfect conversions. If the conversion can fail or is not perfect, use TryFrom.

Generic Implementations

From<T> for U implies Into for T
From is reflexive, which means that From<T> for T is 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)
}

Trait std::convert::From

Generic Implementations

Examples

Required Methods

const fn from(value: T) -> Self

Implementors

impl From<&str> for Box<str, Global>

impl From<&str> for Box<dyn Error + 'static, Global>

impl From<&str> for Rc<str>

impl From<&str> for String

impl From<&str> for Arc<str>

impl From<&str> for Vec<u8, Global>

impl From<&CStr> for Box<CStr, Global>

impl From<&CStr> for CString

impl From<&CStr> for Rc<CStr>

impl From<&CStr> for Arc<CStr>

impl From<&OsStr> for Box<OsStr>

impl From<&OsStr> for Rc<OsStr>

impl From<&OsStr> for Arc<OsStr>

impl From<&Path> for Box<Path>

impl From<&Path> for Rc<Path>

impl From<&Path> for Arc<Path>

impl From<&String> for String

impl From<&mut str> for String

impl From<Cow<'_, str>> for Box<str, Global>

impl From<Cow<'_, CStr>> for Box<CStr, Global>

impl From<Cow<'_, OsStr>> for Box<OsStr>

impl From<Cow<'_, Path>> for Box<Path>

impl From<TryReserveErrorKind> for TryReserveError

impl From<ErrorKind> for Error

impl From<Infallible> for TryFromSliceError

impl From<Infallible> for TryFromIntError

impl From<bool> for i8

impl From<bool> for i16

impl From<bool> for i32

impl From<bool> for i64

impl From<bool> for i128

impl From<bool> for isize

impl From<bool> for u8

impl From<bool> for u16

impl From<bool> for u32

impl From<bool> for u64

impl From<bool> for u128

impl From<bool> for usize

impl From<bool> for AtomicBool

impl From<char> for u32

impl From<char> for u64

impl From<char> for u128

impl From<char> for String

impl From<f32> for f64

impl From<i8> for f32

impl From<i8> for f64

impl From<i8> for i16

impl From<i8> for i32

impl From<i8> for i64

impl From<i8> for i128

impl From<i8> for isize

impl From<i8> for AtomicI8

impl From<i16> for f32

impl From<i16> for f64

impl From<i16> for i32

impl From<i16> for i64

impl From<i16> for i128

impl From<i16> for isize

impl From<i16> for AtomicI16

impl From<i32> for f64

impl From<i32> for i64

impl From<i32> for i128

impl From<i32> for AtomicI32

impl From<i64> for i128

impl From<i64> for AtomicI64

impl From<isize> for AtomicIsize

impl From<!> for Infallible

impl From<!> for TryFromIntError

impl From<u8> for char

impl From<u8> for f32

impl From<u8> for f64

impl From<u8> for i16

impl From<u8> for i32

impl From<u8> for i64