Trait FromStr

1.6.0 · Source
pub trait FromStr: Sized {
    type Err;

    // Required method
    fn from_str(s: &str) -> Result<Self, Self::Err>;
}
Expand description

Parse a value from a string

FromStr’s from_str method is often used implicitly, through str’s parse method. See parse’s documentation for examples.

FromStr does not have a lifetime parameter, and so you can only parse types that do not contain a lifetime parameter themselves. In other words, you can parse an i32 with FromStr, but not a &i32. You can parse a struct that contains an i32, but not one that contains an &i32.

§Input format and round-tripping

The input format expected by a type’s FromStr implementation depends on the type. Check the type’s documentation for the input formats it knows how to parse. Note that the input format of a type’s FromStr implementation might not necessarily accept the output format of its Display implementation, and even if it does, the Display implementation may not be lossless so the round-trip may lose information.

However, if a type has a lossless Display implementation whose output is meant to be conveniently machine-parseable and not just meant for human consumption, then the type may wish to accept the same format in FromStr, and document that usage. Having both Display and FromStr implementations where the result of Display cannot be parsed with FromStr may surprise users.

§Examples

Basic implementation of FromStr on an example Point type:

use std::str::FromStr;

#[derive(Debug, PartialEq)]
struct Point {
    x: i32,
    y: i32
}

#[derive(Debug, PartialEq, Eq)]
struct ParsePointError;

impl FromStr for Point {
    type Err = ParsePointError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let (x, y) = s
            .strip_prefix('(')
            .and_then(|s| s.strip_suffix(')'))
            .and_then(|s| s.split_once(','))
            .ok_or(ParsePointError)?;

        let x_fromstr = x.parse::<i32>().map_err(|_| ParsePointError)?;
        let y_fromstr = y.parse::<i32>().map_err(|_| ParsePointError)?;

        Ok(Point { x: x_fromstr, y: y_fromstr })
    }
}

let expected = Ok(Point { x: 1, y: 2 });
// Explicit call
assert_eq!(Point::from_str("(1,2)"), expected);
// Implicit calls, through parse
assert_eq!("(1,2)".parse(), expected);
assert_eq!("(1,2)".parse::<Point>(), expected);
// Invalid input string
assert!(Point::from_str("(1 2)").is_err());

Required Associated Types§

1.0.0 · Source

type Err

The associated error which can be returned from parsing.

Required Methods§

1.0.0 · Source

fn from_str(s: &str) -> Result<Self, Self::Err>

Parses a string s to return a value of this type.

If parsing succeeds, return the value inside Ok, otherwise when the string is ill-formatted return an error specific to the inside Err. The error type is specific to the implementation of the trait.

§Examples

Basic usage with i32, a type that implements FromStr:

use std::str::FromStr;

let s = "5";
let x = i32::from_str(s).unwrap();

assert_eq!(5, x);

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.

Implementors§

1.7.0 · Source§

impl FromStr for IpAddr

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impl FromStr for SocketAddr

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impl FromStr for Level

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impl FromStr for LevelFilter

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impl FromStr for bool

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impl FromStr for char

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impl FromStr for f16

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impl FromStr for f32

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impl FromStr for f64

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impl FromStr for i8

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impl FromStr for i16

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impl FromStr for i32

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impl FromStr for i64

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impl FromStr for i128

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impl FromStr for isize

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impl FromStr for u8

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impl FromStr for u16

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impl FromStr for u32

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impl FromStr for u64

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impl FromStr for u128

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impl FromStr for usize

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impl FromStr for String

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impl FromStr for Ipv4Addr

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impl FromStr for Ipv6Addr

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impl FromStr for SocketAddrV4

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impl FromStr for SocketAddrV6

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impl FromStr for NonZero<i8>

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impl FromStr for NonZero<i16>

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impl FromStr for NonZero<i32>

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impl FromStr for NonZero<i64>

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impl FromStr for NonZero<i128>

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impl FromStr for NonZero<isize>

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impl FromStr for NonZero<u8>

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impl FromStr for NonZero<u16>

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impl FromStr for NonZero<u32>

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impl FromStr for NonZero<u64>

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impl FromStr for NonZero<u128>

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impl FromStr for NonZero<usize>

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impl FromStr for ByteString

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impl FromStr for CString

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impl FromStr for OsString

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impl FromStr for PathBuf

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impl FromStr for BuildMetadata

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impl FromStr for Comparator

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impl FromStr for Prerelease

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impl FromStr for Version

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impl FromStr for VersionReq

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impl FromStr for Aarch64Architecture

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type Err = ()

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impl FromStr for Architecture

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type Err = ()

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impl FromStr for ArmArchitecture

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type Err = ()

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impl FromStr for BinaryFormat

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type Err = ()

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impl FromStr for Color

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type Err = ParseColorError

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impl FromStr for ColorChoice

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type Err = ColorChoiceParseError

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impl FromStr for Environment

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type Err = ()

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impl FromStr for Mips32Architecture

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type Err = ()

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impl FromStr for Mips64Architecture

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type Err = ()

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impl FromStr for OperatingSystem

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type Err = ()

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impl FromStr for Riscv32Architecture

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type Err = ()

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impl FromStr for Riscv64Architecture

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type Err = ()

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impl FromStr for Triple

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type Err = ParseError

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impl FromStr for Vendor

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type Err = ()

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impl FromStr for X86_32Architecture

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type Err = ()