Crowdfunding example contract in Solidity

This is an example contract that models the serpent crowdfunding example in Solidity. Note that Solidity is still far from being specified, this is only a proposal and comments are welcome.

contract Crowdfunding {
    struct CampaignData {
        address recipient;
        uint contributed;
        uint goal;
        uint deadline;
        uint num_contributions;
        mapping(uint => Contribution) contributions;
    }

    struct Contribution {
        address contributor;
        uint amount;
    }

    uint nextCampaignId;
    mapping(uint256 => CampaignData) campaigns;

    // Start a new campaign.
    function start(address recipient, uint256 goal, uint256 deadline) returns (uint id) {
        var campaign = campaigns[nextCampaignId];
        campaign.recipient = recipient;
        campaign.goal = goal;
        campaign.deadline = deadline;
        nextCampaignId ++;
        id = nextCampaignId;
    }

    // Contribute to the campaign with id $(campaignId).
    function contribute(uint256 campaignId) {
        var campaign = campaigns[campaignId];
        if (campaign.deadline == 0) // check for non-existing campaign
            return;
        campaign.contributed += msg.value;
        var contribution = campaign.contributions[campaign.num_contributions];
        contribution.contributor = msg.sender;
        contribution.amount = msg.value;
        campaign.num_contributions++;
    }

    // Check whether the funding goal of the campaign with id $(campaignId)
    // has been reached and transfer the money.
    function checkGoalReached(uint256 campaignId) returns (bool reached) {
        var campaign = campaigns[campaignId];
        if (campaign.deadline > 0 && campaign.contributed >= campaign.goal) {
            campaign.recipient.send(campaign.contributed);
            // clear storage, we have to do it explicitly for the mapping since
            // it is not possible to enumerate all set keys.
            for (uint i = 0; i < campaign.num_contributions; ++i)
                delete campaign.contributions[i]; // zero out its members
            delete campaign;
            reached = true;
        }
    }

    // Check whether the deadline of the campaign with id $(campaignId) has
    // passed. In that case, return the contributed money and delete the
    // campaign.
    function checkExpired(uint campaignId) returns (bool expired) {
        expired = false;
        var campaign = campaigns[campaignId];
        if (campaign.deadline > 0 && block.timestamp > campaign.deadline) {
            // pay out the contributors
            for (uint i = 0; i < campaign.num_contributions; ++i) {
                campaign.contributions[i].contributor.send(
                    campaign.contributions[i].amount);
                delete campaign.contributions[i];
            }
            delete campaign;
            expired = true;
        }
    }

    // Return the amount contributed to the campaign with id $(campaignId) by
    // the sender of the transaction.
    function getContributedAmount(uint campaignId) returns (uint amount) {
        amount = campaigns[campaignId].contributed;
    }
}

A note on "objects" in solidity: Just as in many other languages, elementary types (integers, hashes, reals, ...) are always copied with every assignment and function call. Complex objects (structs, perhaps strings and texts) are always assigned and passed to other functions "by reference", which means that modifications will be carried back to the original object. If such a complex object is assigned to or retrieved from the state, modifications will be carried over in both ways.

Mappings can be seen as hash tables which are completely pre-filled with zeros. Neither mappings themselves nor structs (recursively) containing mappings can be passed to other functions, also it is not possible to iterate over all keys in a mapping.