Struct alloc::fmt::Arguments

pub struct Arguments<'a> { /* private fields */ }

This structure represents a safely precompiled version of a format string and its arguments. This cannot be generated at runtime because it cannot safely be done, so no constructors are given and the fields are private to prevent modification.

The format_args! macro will safely create an instance of this structure. The macro validates the format string at compile-time so usage of the write() and format() functions can be safely performed.

You can use the Arguments<'a> 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);

Implementations

impl<'a> Arguments<'a>

pub fn as_str(&self) -> Option<&'static str>

Get the formatted string, if it has no arguments to be formatted.

This can be used to avoid allocations in the most trivial case.

Examples

use std::fmt::Arguments;

fn write_str(_: &str) { /* ... */ }

fn write_fmt(args: &Arguments) {
    if let Some(s) = args.as_str() {
        write_str(s)
    } else {
        write_str(&args.to_string());
    }
}

assert_eq!(format_args!("hello").as_str(), Some("hello"));
assert_eq!(format_args!("").as_str(), Some(""));
assert_eq!(format_args!("{}", 1).as_str(), None);

Trait Implementations

impl<'a> Clone for Arguments<'a>

fn clone(&self) -> Arguments<'a>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Arguments<'_>

fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Display for Arguments<'_>

fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<'a> Copy for Arguments<'a>