# Struct alloc::fmt::Arguments

1.0.0 · source ·

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

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

## Implementations§

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### impl<'a> Arguments<'a>

1.52.0 (const: unstable) · source

#### 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());
        }
    }

Run

    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);

Run

## Trait Implementations§

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

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

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for Arguments<'_>

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

Formats the value using the given formatter. Read more
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### impl Display for Arguments<'_>

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

Formats the value using the given formatter. Read more
1.71.0 · source§

### impl ToString for Arguments<'_>

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#### fn to_string(&self) -> String

Converts the given value to a `String`. Read more
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### impl<'a> Copy for Arguments<'a>

## Auto Trait Implementations§

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### impl<'a> !RefUnwindSafe for Arguments<'a>

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### impl<'a> !Send for Arguments<'a>

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### impl<'a> !Sync for Arguments<'a>

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### impl<'a> Unpin for Arguments<'a>

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### impl<'a> !UnwindSafe for Arguments<'a>

## Blanket Implementations§

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### impl<T> Any for Twhere T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

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

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### impl<T> ToOwned for Twhere T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T> ToString for Twhere T: Display + ?Sized,

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#### default fn to_string(&self) -> String

Converts the given value to a `String`. Read more
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### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.