format in alloc - Rust (original) (raw)

Macro format

1.36.0 · Source

macro_rules! format {
    ($($arg:tt)*) => { ... };
}

Expand description

Creates a String using interpolation of runtime expressions.

The first argument format! receives is a format string. This must be a string literal. The power of the formatting string is in the {}s contained. Additional parameters passed to format! replace the {}s within the formatting string in the order given unless named or positional parameters are used.

See the formatting syntax documentation in std::fmtfor details.

A common use for format! is concatenation and interpolation of strings. The same convention is used with print! and write! macros, depending on the intended destination of the string; all these macros internally use format_args!.

To convert a single value to a string, use the to_string method. This will use the Display formatting trait.

To concatenate literals into a &'static str, use the concat! macro.

§Panics

format! panics if a formatting trait implementation returns an error. This indicates an incorrect implementation since fmt::Write for String never returns an error itself.

§Examples

format!("test");                             // => "test"
format!("hello {}", "world!");               // => "hello world!"
format!("x = {}, y = {val}", 10, val = 30);  // => "x = 10, y = 30"
let (x, y) = (1, 2);
format!("{x} + {y} = 3");                    // => "1 + 2 = 3"