# 1.2.0[−][src]Struct alloc::fmt::DebugMap

    #[must_use = "must eventually call `finish()` on Debug builders"]pub struct DebugMap<'a, 'b> where
        'b: 'a,  { /* fields omitted */ }

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)])),
        "{\"A\": 10, \"B\": 11}",
    );

Run

## Implementations

### `impl<'a, 'b> DebugMap<'a, 'b> where
'b: 'a,` [src]

#### `pub fn entry(
&mut self,
key: &dyn Debug,
value: &dyn Debug
) -> &mut DebugMap<'a, 'b>`[src]

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)])),
        "{\"whole\": [(\"A\", 10), (\"B\", 11)]}",
    );

Run

#### `pub fn key(&mut self, key: &dyn Debug) -> &mut DebugMap<'a, 'b>`1.42.0[src]

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)])),
        "{\"whole\": [(\"A\", 10), (\"B\", 11)]}",
    );

Run

#### `pub fn value(&mut self, value: &dyn Debug) -> &mut DebugMap<'a, 'b>`1.42.0[src]

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)])),
        "{\"whole\": [(\"A\", 10), (\"B\", 11)]}",
    );

Run

#### `pub fn entries<K, V, I>(&mut self, entries: I) -> &mut DebugMap<'a, 'b> where
I: IntoIterator<Item = (K, V)>,
K: Debug,
V: Debug,` [src]

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)])),
        "{\"A\": 10, \"B\": 11}",
    );

Run

#### `pub fn finish(&mut self) -> Result<(), Error>`[src]

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 struct formatting.
        }
    }

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

Run

## Auto Trait Implementations

### `impl<'a, 'b> !Send for DebugMap<'a, 'b>`[src]

### `impl<'a, 'b> !Sync for DebugMap<'a, 'b>`[src]

### `impl<'a, 'b> Unpin for DebugMap<'a, 'b> where
'b: 'a,` [src]

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T`[src]

#### `pub fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U`[src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

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

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.