Learn Solidity: Complete Example: Voting Ballot Smart Contract

In this post, we will go through the complete example of a Voting Ballot smart contract. This is a fully working smart contract for voting.

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pragma solidity ^0.4.9;

/// @title Voting with delegation.
contract Ballot {
    // This declares a new complex type which will
    // be used for variables later.
    // It will represent a single voter.
    struct Voter {
        uint weight; // weight is accumulated by delegation
        bool voted;  // if true, that person already voted
        address delegate; // person delegated to
        uint vote;   // index of the voted proposal

    // This is a type for a single proposal.
    struct Proposal
        bytes32 name;   // short name (up to 32 bytes)
        uint voteCount; // number of accumulated votes

    address public chairperson;

    // This declares a state variable that
    // stores a `Voter` struct for each possible address.
    mapping(address => Voter) public voters;

    // A dynamically-sized array of `Proposal` structs.
    Proposal[] public proposals;

    /// Create a new ballot to choose one of `proposalNames`.
    function Ballot(bytes32[] proposalNames) {
        chairperson = msg.sender;
        voters[chairperson].weight = 1;

        // For each of the provided proposal names,
        // create a new proposal object and add it
        // to the end of the array.
        for (uint i = 0; i < proposalNames.length; i++) {
            // `Proposal({...})` creates a temporary
            // Proposal object and `proposals.push(...)`
            // appends it to the end of `proposals`.
                name: proposalNames[i],
                voteCount: 0

    // Give `voter` the right to vote on this ballot.
    // May only be called by `chairperson`.
    function giveRightToVote(address voter) {
        if (msg.sender != chairperson || voters[voter].voted) {
            // `throw` terminates and reverts all changes to
            // the state and to Ether balances. It is often
            // a good idea to use this if functions are
            // called incorrectly. But watch out, this
            // will also consume all provided gas.
        voters[voter].weight = 1;

    /// Delegate your vote to the voter `to`.
    function delegate(address to) {
        // assigns reference
        Voter sender = voters[msg.sender];
        if (sender.voted)

        // Forward the delegation as long as
        // `to` also delegated.
        // In general, such loops are very dangerous,
        // because if they run too long, they might
        // need more gas than is available in a block.
        // In this case, the delegation will not be executed,
        // but in other situations, such loops might
        // cause a contract to get "stuck" completely.
        while (
            voters[to].delegate != address(0) &&
            voters[to].delegate != msg.sender
        ) {
            to = voters[to].delegate;

        // We found a loop in the delegation, not allowed.
        if (to == msg.sender) {

        // Since `sender` is a reference, this
        // modifies `voters[msg.sender].voted`
        sender.voted = true;
        sender.delegate = to;
        Voter delegate = voters[to];
        if (delegate.voted) {
            // If the delegate already voted,
            // directly add to the number of votes
            proposals[delegate.vote].voteCount += sender.weight;
        } else {
            // If the delegate did not vote yet,
            // add to her weight.
            delegate.weight += sender.weight;

    /// Give your vote (including votes delegated to you)
    /// to proposal `proposals[proposal].name`.
    function vote(uint proposal) {
        Voter sender = voters[msg.sender];
        if (sender.voted)
        sender.voted = true;
        sender.vote = proposal;

        // If `proposal` is out of the range of the array,
        // this will throw automatically and revert all
        // changes.
        proposals[proposal].voteCount += sender.weight;

    /// @dev Computes the winning proposal taking all
    /// previous votes into account.
    function winningProposal() constant
            returns (uint winningProposal)
        uint winningVoteCount = 0;
        for (uint p = 0; p < proposals.length; p++) {
            if (proposals[p].voteCount > winningVoteCount) {
                winningVoteCount = proposals[p].voteCount;
                winningProposal = p;

    // Calls winningProposal() function to get the index
    // of the winner contained in the proposals array and then
    // returns the name of the winner
    function winnerName() constant
            returns (bytes32 winnerName)
        winnerName = proposals[winningProposal()].name;
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  • Thor Friis

    Hi Toshendra. I’m taking your Learn Solidity course on Blockgeeks and am struggling with the Voting Ballot example. The delegate function was not for the faint-hearted, but as a general observation, there were a lot of “throw” exceptions in the code and as you know, that method is no longer supported in solidity. That makes following the description of the Voting Ballot rather difficult for a neophyte solidity coder. Any chance of you posting an updated version of the Voting Ballot smart contract?