std::write - Rust
Description: API documentation for the Rust `write` macro in crate `std`.
Keywords: rust, rustlang, rust-lang, write

# 1.0.0[−][src]Macro std::write

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

Write formatted data into a buffer.

This macro accepts a format string, a list of arguments, and a 'writer'. Arguments will be formatted according to the specified format string and the result will be passed to the writer. The writer may be any value with a `write_fmt` method; generally this comes from an implementation of either the `std::fmt::Write` or the `std::io::Write` trait. The macro returns whatever the `write_fmt` method returns; commonly a `std::fmt::Result`, or an `io::Result`.

See `std::fmt` for more information on the format string syntax.

# Examples

    use std::io::Write;

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

    assert_eq!(w, b"testformatted arguments");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();
    write!(&mut s, "{} {}", "abc", 123).unwrap(); // uses fmt::Write::write_fmt
    write!(&mut v, "s = {:?}", s).unwrap(); // uses io::Write::write_fmt
    assert_eq!(v, b"s = \"abc 123\"");Run

Note: This macro can be used in `no_std` setups as well. In a `no_std` setup you are responsible for the implementation details of the components.

    use core::fmt::Write;

    struct Example;

    impl Write for Example {
        fn write_str(&mut self, _s: &str) -> core::fmt::Result {
             unimplemented!();
        }
    }

    let mut m = Example{};
    write!(&mut m, "Hello World").expect("Not written");Run

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