How to do a manual implementation for 'AccountId'

Question

I am working on creating a custom map which takes T::AccountId as a perimeter:

#[pallet::storage]
#[pallet::getter(fn declaration_map)]
pub(super) type Declarations<T: Config> = StorageMap
<
_,
Blake2_128Concat,
T::AccountId,
Amendment<T::AccountId>,
OptionQuery
>;

I pass it as an OptionQuery since AccountId does not have a default value implemented and therefore could be None value. However in my calls: (Example)

#[pallet::weight(100_000)]
pub fn add_amendment(origin: OriginFor<T>, statement: Vec<u8>, owners: Vec<T::AccountId>) -> DispatchResult {
    use crate::pallet::Event::AmendmentAdded;
    ensure_root(origin)?;
    let amendment = Amendment {
                  statement: statement.clone(), 
                  owners: owners
                };
    let next_id = Self::next_amendment();
    Declarations::<T>::insert(next_id, amendment);
    Self::deposit_event(AmendmentAdded(next_id.unwrap(), statement));
    NextAmendmentID::<T>::put(&next_id.unwrap() + 1);
    Ok(())
}

When I get a value from the map there is a potential that the value will be None and can not be used as a key. The simplest way to fix this in my mind is to implement a default value to AccountId. My question is how do I implement that? There is an this article: Why is there no `Default` implementation for `T::AccountId`? The OP however doesn't go into detail about how they implemented a default value. If anyone has any insight on this it would be very helpful.

Answer 1

The rule in the Rust programming language is:

If you implement trait U for type T, either T or U must live in your crate. In this case, neither do, so you cannot do that.

The standard fix for this, called the "newtype patter" is to create a new type that is a mere wrapper around T and therefore the above statement then holds.

struct MyAccountId(AccountId);

impl Default for MyAccountId { .. }

// this will allow you to use `MyAccountId` and `AccountId` often interchange-ably. 
impl Deref for MyAccountId {
  type Target = AccountId
}