Macro core::writeln

1.0.0 · source · []
macro_rules! writeln {
    ($dst:expr $(,)?) => { ... };
    ($dst:expr, $($arg:tt)*) => { ... };
Expand description

Write formatted data into a buffer, with a newline appended.

On all platforms, the newline is the LINE FEED character (\n/U+000A) alone (no additional CARRIAGE RETURN (\r/U+000D).

For more information, see write!. For information on the format string syntax, see std::fmt.


use std::io::{Write, Result};

fn main() -> Result<()> {
    let mut w = Vec::new();
    writeln!(&mut w)?;
    writeln!(&mut w, "test")?;
    writeln!(&mut w, "formatted {}", "arguments")?;

    assert_eq!(&w[..], "\ntest\nformatted arguments\n".as_bytes());

A module can import both std::fmt::Write and std::io::Write and call write! on objects implementing either, as objects do not typically implement both. However, the module must import the traits qualified so their names do not conflict:

use std::fmt::Write as FmtWrite;
use std::io::Write as IoWrite;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut s = String::new();
    let mut v = Vec::new();

    writeln!(&mut s, "{} {}", "abc", 123)?; // uses fmt::Write::write_fmt
    writeln!(&mut v, "s = {:?}", s)?; // uses io::Write::write_fmt
    assert_eq!(v, b"s = \"abc 123\\n\"\n");