How to use bounded vector for storage

Question

I'm trying to use BoundedVec from frame_support for one of my storage items like this,

pub(super) type Payloads<T: Config> = StorageMap<_, Blake2_128Concat, T::BlockNumber, Vec<Payload>, ValueQuery>;

However I'm confused how to implement BoundedVec here instead of Vec as I tried something like this which failed

pub(super) type Payloads<T: Config> = StorageMap<_, Blake2_128Concat, T::BlockNumber, BoundedVec<Payload, 100>, ValueQuery>;

Giving me the error, error[E0747]: constant provided when a type was expected

Can someone who has already worked on this, give me an example of how to use BoundedVec?

Answer 1

Having a look at the pallet-preimage shows how it can be done, the Substrate repo contains examples almost anything that you would want to do with it 😊.

The storage item:

/// The preimages stored by this pallet.
#[pallet::storage]
pub(super) type PreimageFor<T: Config> =
    StorageMap<_, Identity, T::Hash, BoundedVec<u8, T::MaxSize>>;

The config which defines the MaxSize type:

#[pallet::config]
pub trait Config: frame_system::Config {
    /// Max size allowed for a preimage.
    type MaxSize: Get<u32>;
}

And the runtime where the concrete value is injected:

parameter_types! {
    pub const PreimageMaxSize: u32 = 4096 * 1024;
}

impl pallet_preimage::Config for Runtime {
    type MaxSize = PreimageMaxSize;
}

Answer 2

BoundedVec is defined here in the Substrate code base as below:

#[derive(Encode, scale_info::TypeInfo)]
#[scale_info(skip_type_params(S))]
pub struct BoundedVec<T, S>(Vec<T>, PhantomData<S>);

As the compiler message shows, it's expecting a type for the second generic parameter. You can use it as BoundedVec<Payload, ConstU32<100>> as you can also find in the BoundedVec tests, which would make your type declaration:

pub(super) type Payloads<T: Config> = StorageMap<_, Blake2_128Concat, T::BlockNumber, BoundedVec<Payload, ConstU32<100>>, ValueQuery>;