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Collection and Traits

In this part, we will be using the structs built previously and write public functions to add and fetch data from our contract.


For this contract, we are going to store our voters in a HashMap with AccountId as a key and Voter instance as value. The HashMap can be imported from the ink_storage crate by:

use ink_storage::collections::HashMap;

The proposals will be stored in a Vec collection that can be imported from the ink_prelude crate. ink_prelude is a collection of data structures that operate on contract memory during contract execution. The vector can be imported by:

    use ink_prelude::vec::Vec;

Vectors can be instantiated in the same way as a HashMap. New objects can be added or referenced to from a vector using:

    let proposals: Vec<proposals> = Vec::new();
// adding a new proposal
name:String::from("Proposal # 1"),
vote_count: 0,
// returns the proposal at index 0 if exists else returns None
let proposal = self.proposals.get(0).unwrap();

Remember that the vector.get returns an Option not the actual object!


A trait tells the Rust compiler about the functionality a particular type has, and can be shared with other types. You can read more about them here. Before using the custom built structures inside the Ballot storage, certain traits are required to be implemented for Voter and Proposal structs. These traits include:

  • Debug: Allows debug formatting in format strings
  • Clone : This trait allows you to create a deep copy of object
  • Copy : The copy traits allows you to copy a value of a field
  • PackedLayout: Types that can be stored to and loaded from a single contract storage cell
  • SpreadLayout: Types that can be stored to and loaded from the contract storage.

You can learn more about these traits over here and here. These traits are implemented using the derive attribute:

    #[derive(Clone, Copy, Debug, scale::Encode, scale::Decode, SpreadLayout, PackedLayout, scale_info::TypeInfo)]
struct XYZ {

Your Turn!

You need to:

  • Create a proposal Vec and voters HashMap in Ballot struct.
  • Update the constructor, so that it initializes a Vec of proposals and a HashMap of voters. Also update the voters HashMap to include the chair person as a voter.
  • Create getters for both storage items.
  • Write add_voter function to create a voter by the given AccountId and insert it in the HashMap of voters.
  • Write add_proposal function that creates a Proposal object and inserts it tot he vector of proposals.

Remember to run cargo +nightly test to test your work.

#![cfg_attr(not(feature = "std"), no_std)]

use ink_lang as ink;

mod ballot {
use ink_storage::collections::HashMap;
use ink_prelude::vec::Vec;
use ink_storage::traits::{PackedLayout, SpreadLayout};

// Structure to store Proposal information
#[derive(Clone, Debug, scale::Encode, scale::Decode, SpreadLayout, PackedLayout,scale_info::TypeInfo)]
struct Proposal {
name: String,
vote_count: u32,

// Structure to store Proposal information
#[derive(Clone, Debug, scale::Encode, scale::Decode, SpreadLayout, PackedLayout,scale_info::TypeInfo)]
pub struct Voter {
weight: u32,
voted: bool,
delegate: Option<AccountId>,
vote: Option<i32>,

/// Defines the storage of your contract.
/// Add new fields the struct in order
/// to add new static storage fields to your contract.
pub struct Ballot {
chair_person: AccountId,
// ACTION: create a voters hash map with account id as key and Voter as value

// ACTION: create a proposals vector

impl Ballot {
pub fn new() -> Self {
// get chair person address
let chair_person = Self::env().caller();

// ACTION: create empty proposals and voters variables
// * let proposals =
// * let mut voters =

// ACTION: add chair persons voter object in voters hash map
// HINT: Use hashmap.insert(key,value)

Self {

// return chair person id
pub fn get_chairperson(&self) -> AccountId {

// return the provided voter object
pub fn get_voter(&self, voter_id: AccountId) -> Option<&Voter>{

// return the count of voters
pub fn get_voter_count(&self) -> usize{

/// the function adds the provided voter id into possible
/// list of voters. By default the voter has no voting right,
/// the contract owner must approve the voter before he can cast a vote
pub fn add_voter(&mut self, voter_id: AccountId) -> bool{
// ACTION: check if voter already exits, if yes return false
// * if not exists, create an entry in hash map
// * with default weight set to 0 and voted to false
// * and return true

// HINT: use hashmap.get() to get voter
// and use options.some() to check if voter exists

/// given an index returns the name of the proposal at that index
pub fn get_proposal_name_at_index(&self, index:usize) -> &String {

/// returns the number of proposals in ballet
pub fn get_proposal_count(&self) -> usize {

/// adds the given proposal name in ballet
pub fn add_proposal(&mut self, proposal_name: String){

/// Unit tests in Rust are normally defined within such a `#[cfg(test)]`
/// module and test functions are marked with a `#[test]` attribute.
/// The below code is technically just normal Rust code.
mod tests {
/// Imports all the definitions from the outer scope so we can use them here.
use super::*;

// Alias `ink_lang` so we can use `ink::test`.
use ink_lang as ink;

fn new_works() {
let mut proposal_names: Vec<String> = Vec::new();
proposal_names.push(String::from("Proposal # 1"));
let ballot = Ballot::new();

fn adding_proposals_works() {
let mut ballot = Ballot::new();
ballot.add_proposal(String::from("Proposal #1"));

fn adding_voters_work() {
let mut ballot = Ballot::new();
let account_id = AccountId::from([0x0; 32]);