DebugMap in core::fmt - Rust
Description: A struct to help with `fmt::Debug` implementations.

# Struct DebugMap

1.6.0 · Source

    pub struct DebugMap<'a, 'b: 'a> { /* private fields */ }

Expand description

A struct to help with `fmt::Debug` implementations.

This is useful when you wish to output a formatted map as a part of your `Debug::fmt` implementation.

This can be constructed by the `Formatter::debug_map` method.

## §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_map().entries(self.0.iter().map(|&(ref k, ref v)| (k, v))).finish()
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
        r#"{"A": 10, "B": 11}"#,
    );

## Implementations§

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### impl<'a, 'b: 'a> DebugMap<'a, 'b>

1.2.0 · Source

#### pub fn entry(&mut self, key: &dyn Debug, value: &dyn Debug) -> &mut Self

Adds a new entry to the map output.

##### §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_map()
               .entry(&"whole", &self.0) // We add the "whole" entry.
               .finish()
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
        r#"{"whole": [("A", 10), ("B", 11)]}"#,
    );

1.42.0 · Source

#### pub fn key(&mut self, key: &dyn Debug) -> &mut Self

Adds the key part of a new entry to the map output.

This method, together with `value`, is an alternative to `entry` that can be used when the complete entry isn’t known upfront. Prefer the `entry` method when it’s possible to use.

##### §Panics

`key` must be called before `value` and each call to `key` must be followed by a corresponding call to `value`. Otherwise this method will panic.

##### §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_map()
               .key(&"whole").value(&self.0) // We add the "whole" entry.
               .finish()
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
        r#"{"whole": [("A", 10), ("B", 11)]}"#,
    );

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#### pub fn key_with<F>(&mut self, key_fmt: F) -> &mut Self
where F: FnOnce(&mut Formatter<'_>) -> Result,

🔬This is a nightly-only experimental API. (`debug_closure_helpers` #117729)

Adds the key part of a new entry to the map output.

This method is equivalent to `DebugMap::key`, but formats the key using a provided closure rather than by calling `Debug::fmt`.

1.42.0 · Source

#### pub fn value(&mut self, value: &dyn Debug) -> &mut Self

Adds the value part of a new entry to the map output.

This method, together with `key`, is an alternative to `entry` that can be used when the complete entry isn’t known upfront. Prefer the `entry` method when it’s possible to use.

##### §Panics

`key` must be called before `value` and each call to `key` must be followed by a corresponding call to `value`. Otherwise this method will panic.

##### §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_map()
               .key(&"whole").value(&self.0) // We add the "whole" entry.
               .finish()
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
        r#"{"whole": [("A", 10), ("B", 11)]}"#,
    );

Source

#### pub fn value_with<F>(&mut self, value_fmt: F) -> &mut Self
where F: FnOnce(&mut Formatter<'_>) -> Result,

🔬This is a nightly-only experimental API. (`debug_closure_helpers` #117729)

Adds the value part of a new entry to the map output.

This method is equivalent to `DebugMap::value`, but formats the value using a provided closure rather than by calling `Debug::fmt`.

1.2.0 · Source

#### pub fn entries<K, V, I>(&mut self, entries: I) -> &mut Self
where K: Debug, V: Debug, I: IntoIterator<Item = (K, V)>,

Adds the contents of an iterator of entries to the map output.

##### §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_map()
               // We map our vec so each entries' first field will become
               // the "key".
               .entries(self.0.iter().map(|&(ref k, ref v)| (k, v)))
               .finish()
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
        r#"{"A": 10, "B": 11}"#,
    );

1.83.0 · Source

#### pub fn finish_non_exhaustive(&mut self) -> Result

Marks the map as non-exhaustive, indicating to the reader that there are some other entries that are not shown in the debug representation.

##### §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            // Print at most two elements, abbreviate the rest
            let mut f = fmt.debug_map();
            let mut f = f.entries(self.0.iter().take(2).map(|&(ref k, ref v)| (k, v)));
            if self.0.len() > 2 {
                f.finish_non_exhaustive()
            } else {
                f.finish()
            }
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![
            ("A".to_string(), 10),
            ("B".to_string(), 11),
            ("C".to_string(), 12),
        ])),
        r#"{"A": 10, "B": 11, ..}"#,
    );

1.2.0 · Source

#### pub fn finish(&mut self) -> Result

Finishes output and returns any error encountered.

##### §Panics

`key` must be called before `value` and each call to `key` must be followed by a corresponding call to `value`. Otherwise this method will panic.

##### §Examples

    use std::fmt;

    struct Foo(Vec<(String, i32)>);

    impl fmt::Debug for Foo {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_map()
               .entries(self.0.iter().map(|&(ref k, ref v)| (k, v)))
               .finish() // Ends the map formatting.
        }
    }

    assert_eq!(
        format!("{:?}", Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
        r#"{"A": 10, "B": 11}"#,
    );

## Auto Trait Implementations§

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### impl<'a, 'b> Freeze for DebugMap<'a, 'b>

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### impl<'a, 'b> !RefUnwindSafe for DebugMap<'a, 'b>

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### impl<'a, 'b> !Send for DebugMap<'a, 'b>

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### impl<'a, 'b> !Sync for DebugMap<'a, 'b>

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### impl<'a, 'b> Unpin for DebugMap<'a, 'b>

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### impl<'a, 'b> !UnwindSafe for DebugMap<'a, 'b>

## Blanket Implementations§

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### impl<T> Any for T
where 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 T
where 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 T
where 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 T
where 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, U> TryFrom<U> for T
where 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 T
where 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.