I'm trying to use sp_std::collections::btree_map::BTreeMap for offchain worker Local Storage.


let foo = StorageValueRef::persistent(b"bar::foo");
foo.get::<BTreeMap<u32, u32>>();


when a custom struct is used as a key as follows:

use sp_core::{Decode, Encode};

#[derive(Clone, PartialEq, Eq, Encode, Decode, Debug)]
struct Foo(u32);

let foo = StorageValueRef::persistent(b"bar::foo");
foo.get::<BTreeMap<Foo, u32>>();

compilation fails due to:

  18 |     foo.get::<BTreeMap<Foo, u32>>();
     |               ^^^^^^^^^^^^^^^^^^ the trait `sp_core::Decode` is not implemented for `BTreeMap<Foo, u32>`
     = help: the trait `sp_core::Decode` is implemented for `BTreeMap<K, V>`

Curiously, I can use a sp_std::vec::Vec like so

    let foo = StorageValueRef::persistent(b"bar::foo");
    foo.get::<Vec<(Foo, u32)>>();

which compiles just fine.

1 Answer 1



impl_codec_through_iterator! {
    BTreeMap { K: Ord, V } { LikeK, LikeV}
        { K: EncodeLike<LikeK>, LikeK: Encode, V: EncodeLike<LikeV>, LikeV: Encode }
    BTreeSet { T: Ord } { LikeT }
        { T: EncodeLike<LikeT>, LikeT: Encode }
    LinkedList { T } { LikeT }
        { T: EncodeLike<LikeT>, LikeT: Encode }
    BinaryHeap { T: Ord } { LikeT }
        { T: EncodeLike<LikeT>, LikeT: Encode }

It only implements for BTreeMap<K, V: Decode>.

But in your case, your K is a Decodeable thing, which does not satisfy this implementation.

BTreeMap<u32, Foo> will work.

  • oops, my fault, I didn't notice Ord impl was missing, the error message possibly confused me. Thanks ! Commented May 10, 2023 at 23:08

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