parity_scale_codec::Encode is not implemented for ink_storage::Mapping<u32, [u8; 32]>

Question

I am trying to create structure for ink smart contract but it fails on build giving error

Error msg

-the trait parity_scale_codec::WrapperTypeEncode is not implemented for ink_storage::Mapping<u32, [u8; 32]

-required because of the requirements on the impl of parity_scale_codec::Encode for ink_storage::Mapping<u32, [u8; 32]>

Struct details

#[derive(Default, Encode, Decode, Debug, SpreadLayout, SpreadAllocate)]
#[cfg_attr(feature = "std", derive(StorageLayout))]
pub struct Sample {
    pub roots: Mapping<u32, [u8; 32]>
}

Do I need to implement parity_scale_codec::Encode? Any advice or help here is much appreciated.

Thank you!!

Answer 1

Why do you want to Encode and Decode for a structure that contains Mapping? If you're going to implement PackedLayout - you can't. If you want to accept it as input - you can't. If you're going to return it - you can't=)

Encode and Decode are not implemented for Mapping because Mapping doesn't contain any information inside. It only provides get and insert methods to manipulate the storage.

Answer 2

You can derive this trait implementation automatically:

#[derive(scale::Encode, scale::Decode)]
struct my_struct {
  // …
}

If it still doesn't work it would be great if you could create a minimal contract that produces this error on https://ink-playground.substrate.io/ and share a link to that code.