A clear explanation will be appreciated as to why Decode trait is not implemented while encode is. What are the design reason for this?

1 Answer 1


What? It is definetly decodable. Code from here:

impl<T: Decode, S: Get<u32>> Decode for BoundedVec<T, S> {
    fn decode<I: codec::Input>(input: &mut I) -> Result<Self, codec::Error> {
        let inner = Vec::<T>::decode(input)?;
        if inner.len() > S::get() as usize {
            return Err("BoundedVec exceeds its limit".into())
        Ok(Self(inner, PhantomData))

    fn skip<I: codec::Input>(input: &mut I) -> Result<(), codec::Error> {

and a small test:

fn encode_decode_works() {
    let bounded: BoundedVec<u32, ConstU32<7>> = bounded_vec![1, 2, 3, 4, 5, 6];
    let encoded = bounded.encode();
    let decoded: BoundedVec<u32, ConstU32<7>> = Decode::decode(&mut &encoded[..]).unwrap();

    assert_eq!(bounded, decoded);
  • I Guess not the trait Decode is not implemented for sp_runtime::BoundedVec<<T as frame_system::Config>::AccountId, u32>
    – Kalanamith
    Commented Dec 20, 2022 at 2:21

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