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# core::traits::Felt252DictValue

A trait that must be implemented for any type that will be stored as a value in a `Felt252Dict`.
When working with dictionaries in Cairo, we need a way to represent "empty" or "uninitialized"
slots. This trait provides a zero-like default value that is returned when accessing a key
that hasn't been explicitly set.

<Info>
  The `Felt252Dict` implementation needs to handle cases where a key hasn't been assigned a value
  yet.
  Instead of using `Option` or similar constructs, it uses a zero-like value specific to each
  type.
  This trait is only implemented for primitive scalar types and `Nullable`. It cannot be
  implemented manually.
  Instead, if you want to use a custom type as a value in a dictionary, you can wrap your type in
  a [`Nullable`](./core-nullable-Nullable), which implements `Felt252DictValue` for any wrapped type.
</Info>

## Signature

```rust theme={null}
pub trait Felt252DictValue
```

## Examples

```rust theme={null}
use core::dict::Felt252Dict;

#[derive(Copy, Drop, Default)]
struct Counter {
    value: u32,
}

 // u8 already implements Felt252DictValue
 let mut dict: Felt252Dict = Default::default();
 assert!(dict.get(123) == 0);

 // Counter is wrapped in a Nullable, as it doesn't implement Felt252DictValue
 let mut counters: Felt252Dict> = Default::default();

 // If the key is not set, `deref` would panic. `deref_or` returns the default value.
 let maybe_counter: Nullable = counters.get(123);
 assert!(maybe_counter.deref_or(Default::default()).value == 0);
```

## Trait functions

### zero\_default

Returns the default value for this type when used in a `Felt252Dict`.
This value should be logically equivalent to zero or an "empty" state.

#### Signature

```rust theme={null}
fn zero_default() -> T
```