pub struct Arguments<'a> {
pieces: &'a [&'static str],
fmt: Option<&'a [Placeholder]>,
args: &'a [Argument<'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);Fields§
§pieces: &'a [&'static str]§fmt: Option<&'a [Placeholder]>§args: &'a [Argument<'a>]Implementations§
source§impl<'a> Arguments<'a>
impl<'a> Arguments<'a>
Used by the format_args!() macro to create a fmt::Arguments object.
pub fn new_const(pieces: &'a [&'static str]) -> Arguments<'a>
fmt_internals)sourcepub fn new_v1(
pieces: &'a [&'static str],
args: &'a [Argument<'a>]
) -> Arguments<'a>
🔬This is a nightly-only experimental API. (fmt_internals)
pub fn new_v1( pieces: &'a [&'static str], args: &'a [Argument<'a>] ) -> Arguments<'a>
fmt_internals)When using the format_args!() macro, this function is used to generate the Arguments structure.
sourcepub fn new_v1_formatted(
pieces: &'a [&'static str],
args: &'a [Argument<'a>],
fmt: &'a [Placeholder],
_unsafe_arg: UnsafeArg
) -> Arguments<'a>
🔬This is a nightly-only experimental API. (fmt_internals)
pub fn new_v1_formatted( pieces: &'a [&'static str], args: &'a [Argument<'a>], fmt: &'a [Placeholder], _unsafe_arg: UnsafeArg ) -> Arguments<'a>
fmt_internals)This function is used to specify nonstandard formatting parameters.
An rt::UnsafeArg is required because the following invariants must be held
in order for this function to be safe:
- The
piecesslice must be at least as long asfmt. - Every
rt::Placeholder::positionvalue withinfmtmust be a valid index ofargs. - Every
rt::Count::Paramwithinfmtmust contain a valid index ofargs.
sourcepub fn estimated_capacity(&self) -> usize
🔬This is a nightly-only experimental API. (fmt_internals)
pub fn estimated_capacity(&self) -> usize
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.
source§impl<'a> Arguments<'a>
impl<'a> Arguments<'a>
1.52.0 (const: unstable) · sourcepub fn as_str(&self) -> Option<&'static str>
pub fn as_str(&self) -> Option<&'static str>
Get the formatted string, if it has no arguments to be formatted at runtime.
This can be used to avoid allocations in some cases.
Guarantees
For format_args!("just a literal"), this function is guaranteed to
return Some("just a literal").
For most cases with placeholders, this function will return None.
However, the compiler may perform optimizations that can cause this
function to return Some(_) even if the format string contains
placeholders. For example, format_args!("Hello, {}!", "world") may be
optimized to format_args!("Hello, world!"), such that as_str()
returns Some("Hello, world!").
The behavior for anything but the trivial case (without placeholders) is not guaranteed, and should not be relied upon for anything other than optimization.
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!("{:?}", std::env::current_dir()).as_str(), None);