Macro std::writeln []

macro_rules! writeln {
    ( $ dst : expr , $ fmt : expr ) => { ... };
$ dst : expr , $ fmt : expr , $ ( $ arg : tt ) * ) => { ... };

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).

This macro accepts a 'writer' (any value with a write_fmt method), a format string, and a list of arguments to format.

The write_fmt method usually comes from an implementation of std::fmt::Write or std::io::Write traits. The term 'writer' refers to an implementation of one of these two traits.

Passed arguments will be formatted according to the specified format string and the resulting string will be passed to the writer, along with the appended newline.

See std::fmt for more information on format syntax.

write! returns whatever the 'write_fmt' method returns.

Common return values include: fmt::Result, io::Result


use std::io::Write;

let mut w = Vec::new();
writeln!(&mut w, "test").unwrap();
writeln!(&mut w, "formatted {}", "arguments").unwrap();

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

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;

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