# Custom Data Structures
The `ink_storage` crate provides useful utilities and data structures to organize and manipulate the contract's storage. However, contract authors should know that they can also create their own custom data structures.
### Using custom types on storage[](https://use.ink/datastructures/custom-datastructure#using-custom-types-on-storage)
Any custom type wanting to be compatible with ink! storage must implement the [`Storable`](https://docs.rs/ink_storage_traits/4.0.0/ink_storage_traits/trait.Storable.html) trait, so it can be SCALE [`encoded`](https://docs.rs/parity-scale-codec/3.2.2/parity_scale_codec/trait.Encode.html) and [`decoded`](https://docs.rs/parity-scale-codec/3.2.2/parity_scale_codec/trait.Decode.html). Additionally, the traits [`StorageLayout`](https://docs.rs/ink_storage_traits/4.0.0/ink_storage_traits/trait.StorageLayout.html) and [`TypeInfo`](https://docs.rs/scale-info/2.3.1/scale_info/trait.TypeInfo.html) are required as well. But don't worry, usually these traits can just be derived:
```rust
/// A custom type that we can use in our contract storage
#[derive(scale::Decode, scale::Encode)]
#[cfg_attr(
feature = "std",
derive(scale_info::TypeInfo, ink::storage::traits::StorageLayout)
)]
pub struct Inner {
value: bool,
}
#[ink(storage)]
pub struct ContractStorage {
inner: Inner,
}
```
Even better: there is a macro [`#[ink::storage_item]`](https://docs.rs/ink_macro/4.0.0/ink_macro/attr.storage_item.html), which derives all necessary traits for you. If there is no need to implement any special behavior, the above code example can be simplified further as follows:
```rust
/// A custom type that we can use in our contract storage
#[ink::storage_item]
pub struct Inner {
value: bool,
}
#[ink(storage)]
pub struct ContractStorage {
inner: Inner,
}
```
Naturally, you can as well implement any required trait manually. Please directly refer to the relevant trait documentations for more information.
NOTE
The `#[ink::storage_item]` macro is responsible for storage key calculation of non-[`Packed`](https://docs.rs/ink_storage_traits/4.0.0/ink_storage_traits/trait.Packed.html) types. Without it, the key for non-`Packed` fields will be zero. Using this macro is necessary if you don't plan to use a [`ManualKey`](https://docs.rs/ink_storage_traits/4.0.0/ink_storage_traits/struct.ManualKey.html) on a non-`Packed` type.
Types with custom implementations of the ink! storage traits can still use this macro only for key calculation by disabling the derives: `#[ink::storage_item(derive = false)].`
### Generic storage fields[](https://use.ink/datastructures/custom-datastructure#generic-storage-fields)
It is possible to use generic data types in your storage, as long as any generic type satisfies the required storage trait bounds. In fact, we already witnessed this in the previous sections about the [`Mapping`](https://docs.rs/ink_storage/4.0.0/ink_storage/struct.Mapping.html).
Let's say you want a mapping where accessing a non-existent key should just return it's default value, akin to how mappings work in Solidity. Additionally, you want to know how many values there are in the mapping (its length). This could be implemented as a thin wrapper around the ink! `Mapping` as follows:
```rust
/// Values for this map need to implement the `Default` trait.
/// Naturally, they also must be compatible with contract storage.
/// Note that the underlying `Mapping` type only supports `Packed` values.
#[ink::storage_item]
pub struct DefaultMap {
values: Mapping,
length: u32,
}
impl DefaultMap {
/// Accessing non-existent keys will return the default value.
pub fn get(&self, key: &K) -> V {
self.values.get(key).unwrap_or_default()
}
/// Inserting into the map increases its length by one.
pub fn set(&mut self, key: I, value: &U)
where
I: scale::EncodeLike,
E: scale::EncodeLike + Storable,
{
if self.values.insert(key, value).is_none() {
self.length += 1
}
}
/// Removing a value from the map decreases its length by one.
pub fn remove(&mut self, key: &K) {
if self.values.take(key).is_some() {
self.length -= 1
}
}
/// Return how many values the mapping contains
pub fn len(&self) -> u32 {
self.length
}
}
/// `DefaultMap` is compatible with contract storage.
#[ink(storage)]
pub struct MyContract {
my_map: DefaultMap,
}
```
CAUTION
Generic data types may substantially increase your contracts overall code size, making it more costly to store on-chain.
The reason for this is [Rust's monomorphization](https://rustwasm.github.io/twiggy/concepts/generic-functions-and-monomorphization.html).