Struct core::fmt::Arguments

1.0.0 · source · []
pub struct Arguments<'a> {
    pieces: &'a [&'static str],
    fmt: Option<&'a [Argument]>,
    args: &'a [ArgumentV1<'a>],
}
Expand description

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);
Run

Fields

pieces: &'a [&'static str]fmt: Option<&'a [Argument]>args: &'a [ArgumentV1<'a>]

Implementations

🔬 This is a nightly-only experimental API. (fmt_internals)

When using the format_args!() macro, this function is used to generate the Arguments structure.

🔬 This is a nightly-only experimental API. (fmt_internals)

This function is used to specify nonstandard formatting parameters.

An UnsafeArg is required because the following invariants must be held in order for this function to be safe:

  1. The pieces slice must be at least as long as fmt.
  2. Every rt::v1::Argument::position value within fmt must be a valid index of args.
  3. Every [Count::Param] within fmt must contain a valid index of args.
🔬 This is a nightly-only experimental API. (fmt_internals)

Estimates the length of the formatted text.

This is intended to be used for setting initial String capacity when using format!. Note: this is neither the lower nor upper bound.

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

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Formats the value using the given formatter. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.