Macro std::format_args
1.0.0 · source · macro_rules! format_args { ($fmt:expr) => { ... }; ($fmt:expr, $($args:tt)*) => { ... }; }
Expand description
Constructs parameters for the other string-formatting macros.
This macro functions by taking a formatting string literal containing
{}
for each additional argument passed. format_args!
prepares the
additional parameters to ensure the output can be interpreted as a string
and canonicalizes the arguments into a single type. Any value that implements
the Display
trait can be passed to format_args!
, as can any
Debug
implementation be passed to a {:?}
within the formatting string.
This macro produces a value of type fmt::Arguments
. This value can be
passed to the macros within std::fmt
for performing useful redirection.
All other formatting macros (format!
, write!
, println!
, etc) are
proxied through this one. format_args!
, unlike its derived macros, avoids
heap allocations.
You can use the fmt::Arguments
value that format_args!
returns
in Debug
and Display
contexts as seen below. The example also shows
that Debug
and Display
format to the same thing: the interpolated
format string in format_args!
.
let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
assert_eq!("1 foo 2", display);
assert_eq!(display, debug);
RunSee the formatting documentation in std::fmt
for details of the macro argument syntax, and further information.
§Examples
use std::fmt;
let s = fmt::format(format_args!("hello {}", "world"));
assert_eq!(s, format!("hello {}", "world"));
Run