Trait VariantAccess

trait VariantAccess<'de>: Sized

VariantAccess is a visitor that is created by the Deserializer and passed to the Deserialize to deserialize the content of a particular enum variant.

Lifetime

The 'de lifetime of this trait is the lifetime of data that may be borrowed by the deserialized enum variant. 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 VariantAccess for a basic JSON data format.

Associated Types

type Error: TraitBound { trait_: Path { path: "Error", id: Id(217), args: None }, generic_params: [], modifier: None }

The error type that can be returned if some error occurs during deserialization. Must match the error type of our EnumAccess.

Required Methods

fn unit_variant(self: Self) -> Result<(), <Self as >::Error>

Called when deserializing a variant with no values.

If the data contains a different type of variant, the following invalid_type error should be constructed:

# use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
#
# struct X;
#
# impl<'de> VariantAccess<'de> for X {
#     type Error = value::Error;
#
fn unit_variant(self) -> Result<(), Self::Error> {
    // What the data actually contained; suppose it is a tuple variant.
    let unexp = Unexpected::TupleVariant;
    Err(de::Error::invalid_type(unexp, &"unit variant"))
}
#
#     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
#     where
#         T: DeserializeSeed<'de>,
#     { unimplemented!() }
#
#     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
#     where
#         V: Visitor<'de>,
#     { unimplemented!() }
#
#     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
#     where
#         V: Visitor<'de>,
#     { unimplemented!() }
# }

fn newtype_variant_seed<T>(self: Self, seed: T) -> Result<<T as >::Value, <Self as >::Error>
where
    T: DeserializeSeed<'de>

Called when deserializing a variant with a single value.

Deserialize implementations should typically use VariantAccess::newtype_variant instead.

If the data contains a different type of variant, the following invalid_type error should be constructed:

# use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
#
# struct X;
#
# impl<'de> VariantAccess<'de> for X {
#     type Error = value::Error;
#
#     fn unit_variant(self) -> Result<(), Self::Error> {
#         unimplemented!()
#     }
#
fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
where
    T: DeserializeSeed<'de>,
{
    // What the data actually contained; suppose it is a unit variant.
    let unexp = Unexpected::UnitVariant;
    Err(de::Error::invalid_type(unexp, &"newtype variant"))
}
#
#     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
#     where
#         V: Visitor<'de>,
#     { unimplemented!() }
#
#     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
#     where
#         V: Visitor<'de>,
#     { unimplemented!() }
# }

fn tuple_variant<V>(self: Self, len: usize, visitor: V) -> Result<<V as >::Value, <Self as >::Error>
where
    V: Visitor<'de>

Called when deserializing a tuple-like variant.

The len is the number of fields expected in the tuple variant.

If the data contains a different type of variant, the following invalid_type error should be constructed:

# use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
#
# struct X;
#
# impl<'de> VariantAccess<'de> for X {
#     type Error = value::Error;
#
#     fn unit_variant(self) -> Result<(), Self::Error> {
#         unimplemented!()
#     }
#
#     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
#     where
#         T: DeserializeSeed<'de>,
#     { unimplemented!() }
#
fn tuple_variant<V>(self, _len: usize, _visitor: V) -> Result<V::Value, Self::Error>
where
    V: Visitor<'de>,
{
    // What the data actually contained; suppose it is a unit variant.
    let unexp = Unexpected::UnitVariant;
    Err(de::Error::invalid_type(unexp, &"tuple variant"))
}
#
#     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
#     where
#         V: Visitor<'de>,
#     { unimplemented!() }
# }

fn struct_variant<V>(self: Self, fields: &'static [&'static str], visitor: V) -> Result<<V as >::Value, <Self as >::Error>
where
    V: Visitor<'de>

Called when deserializing a struct-like variant.

The fields are the names of the fields of the struct variant.

If the data contains a different type of variant, the following invalid_type error should be constructed:

# use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, Unexpected};
#
# struct X;
#
# impl<'de> VariantAccess<'de> for X {
#     type Error = value::Error;
#
#     fn unit_variant(self) -> Result<(), Self::Error> {
#         unimplemented!()
#     }
#
#     fn newtype_variant_seed<T>(self, _: T) -> Result<T::Value, Self::Error>
#     where
#         T: DeserializeSeed<'de>,
#     { unimplemented!() }
#
#     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
#     where
#         V: Visitor<'de>,
#     { unimplemented!() }
#
fn struct_variant<V>(
    self,
    _fields: &'static [&'static str],
    _visitor: V,
) -> Result<V::Value, Self::Error>
where
    V: Visitor<'de>,
{
    // What the data actually contained; suppose it is a unit variant.
    let unexp = Unexpected::UnitVariant;
    Err(de::Error::invalid_type(unexp, &"struct variant"))
}
# }

Provided Methods

fn newtype_variant<T>(self: Self) -> Result<T, <Self as >::Error>
where
    T: Deserialize<'de>

Called when deserializing a variant with a single value.

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