# Trait serde::de::MapAccess

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    pub trait MapAccess<'de> {
        type Error: Error;

        fn next_key_seed<K>(
            &mut self,
            seed: K
        ) -> Result<Option<K::Value>, Self::Error>
        where
            K: DeserializeSeed<'de>;
        fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error>
        where
            V: DeserializeSeed<'de>;

        fn next_entry_seed<K, V>(
            &mut self,
            kseed: K,
            vseed: V
        ) -> Result<Option<(K::Value, V::Value)>, Self::Error>
        where
            K: DeserializeSeed<'de>,
            V: DeserializeSeed<'de>,
        { ... }
        fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error>
        where
            K: Deserialize<'de>,
        { ... }
        fn next_value<V>(&mut self) -> Result<V, Self::Error>
        where
            V: Deserialize<'de>,
        { ... }
        fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error>
        where
            K: Deserialize<'de>,
            V: Deserialize<'de>,
        { ... }
        fn size_hint(&self) -> Option<usize> { ... }
    }

Expand description

Provides a `Visitor` access to each entry of a map in the input.

This is a trait that a `Deserializer` passes to a `Visitor` implementation.

## Lifetime

The `'de` lifetime of this trait is the lifetime of data that may be borrowed by deserialized map entries. See the page Understanding deserializer lifetimes for a more detailed explanation of these lifetimes.

## Example implementation

The example data format presented on the website demonstrates an implementation of `MapAccess` for a basic JSON data format.

## Required Associated Types

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#### type Error: Error

The error type that can be returned if some error occurs during deserialization.

## Required Methods

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#### fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error> where
K: DeserializeSeed<'de>,

This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)` if there are no more remaining entries.

`Deserialize` implementations should typically use `MapAccess::next_key` or `MapAccess::next_entry` instead.

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#### fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error> where
V: DeserializeSeed<'de>,

This returns a `Ok(value)` for the next value in the map.

`Deserialize` implementations should typically use `MapAccess::next_value` instead.

##### Panics

Calling `next_value_seed` before `next_key_seed` is incorrect and is allowed to panic or return bogus results.

## Provided Methods

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#### fn next_entry_seed<K, V>(
&mut self,
kseed: K,
vseed: V
) -> Result<Option<(K::Value, V::Value)>, Self::Error> where
K: DeserializeSeed<'de>,
V: DeserializeSeed<'de>,

This returns `Ok(Some((key, value)))` for the next (key-value) pair in the map, or `Ok(None)` if there are no more remaining items.

`MapAccess` implementations should override the default behavior if a more efficient implementation is possible.

`Deserialize` implementations should typically use `MapAccess::next_entry` instead.

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#### fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error> where
K: Deserialize<'de>,

This returns `Ok(Some(key))` for the next key in the map, or `Ok(None)` if there are no more remaining entries.

This method exists as a convenience for `Deserialize` implementations. `MapAccess` implementations should not override the default behavior.

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#### fn next_value<V>(&mut self) -> Result<V, Self::Error> where
V: Deserialize<'de>,

This returns a `Ok(value)` for the next value in the map.

This method exists as a convenience for `Deserialize` implementations. `MapAccess` implementations should not override the default behavior.

##### Panics

Calling `next_value` before `next_key` is incorrect and is allowed to panic or return bogus results.

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#### fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error> where
K: Deserialize<'de>,
V: Deserialize<'de>,

This returns `Ok(Some((key, value)))` for the next (key-value) pair in the map, or `Ok(None)` if there are no more remaining items.

This method exists as a convenience for `Deserialize` implementations. `MapAccess` implementations should not override the default behavior.

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#### fn size_hint(&self) -> Option<usize>

Returns the number of entries remaining in the map, if known.

## Implementations on Foreign Types

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### impl<'de, 'a, A: ?Sized> MapAccess<'de> for &'a mut A where
A: MapAccess<'de>,

#### type Error = <A as MapAccess<'de>>::Error

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#### fn next_key_seed<K>(&mut self, seed: K) -> Result<Option<K::Value>, Self::Error> where
K: DeserializeSeed<'de>,

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#### fn next_value_seed<V>(&mut self, seed: V) -> Result<V::Value, Self::Error> where
V: DeserializeSeed<'de>,

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#### fn next_entry_seed<K, V>(
&mut self,
kseed: K,
vseed: V
) -> Result<Option<(K::Value, V::Value)>, Self::Error> where
K: DeserializeSeed<'de>,
V: DeserializeSeed<'de>,

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#### fn next_entry<K, V>(&mut self) -> Result<Option<(K, V)>, Self::Error> where
K: Deserialize<'de>,
V: Deserialize<'de>,

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#### fn next_key<K>(&mut self) -> Result<Option<K>, Self::Error> where
K: Deserialize<'de>,

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#### fn next_value<V>(&mut self) -> Result<V, Self::Error> where
V: Deserialize<'de>,

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#### fn size_hint(&self) -> Option<usize>

## Implementors

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### impl<'de, I, E> MapAccess<'de> for MapDeserializer<'de, I, E> where
I: Iterator,
I::Item: Pair,
<I::Item as Pair>::First: IntoDeserializer<'de, E>,
<I::Item as Pair>::Second: IntoDeserializer<'de, E>,
E: Error,

#### type Error = E