Common Patterns

Withdrawal from Contracts

The recommended method of sending funds after an effect is using the withdrawal pattern. Although the most intuitive method of sending Ether, as a result of an effect, is a direct send call, this is not recommended as it introduces a potential security risk. You may read more about this on the Security Considerations page.

This is an example of the withdrawal pattern in practice in a contract where the goal is to send the most money to the contract in order to become the “richest”, inspired by King of the Ether.

In the following contract, if you are usurped as the richest, you will receive the funds of the person who has gone on to become the new richest.

pragma solidity ^0.4.11;

contract WithdrawalContract {
    address public richest;
    uint public mostSent;

    mapping (address => uint) pendingWithdrawals;

    function WithdrawalContract() payable {
        richest = msg.sender;
        mostSent = msg.value;
    }

    function becomeRichest() payable returns (bool) {
        if (msg.value > mostSent) {
            pendingWithdrawals[richest] += msg.value;
            richest = msg.sender;
            mostSent = msg.value;
            return true;
        } else {
            return false;
        }
    }

    function withdraw() {
        uint amount = pendingWithdrawals[msg.sender];
        // Remember to zero the pending refund before
        // sending to prevent re-entrancy attacks
        pendingWithdrawals[msg.sender] = 0;
        msg.sender.transfer(amount);
    }
}

This is as opposed to the more intuitive sending pattern:

pragma solidity ^0.4.11;

contract SendContract {
    address public richest;
    uint public mostSent;

    function SendContract() payable {
        richest = msg.sender;
        mostSent = msg.value;
    }

    function becomeRichest() payable returns (bool) {
        if (msg.value > mostSent) {
            // This line can cause problems (explained below).
            richest.transfer(msg.value);
            richest = msg.sender;
            mostSent = msg.value;
            return true;
        } else {
            return false;
        }
    }
}

Notice that, in this example, an attacker could trap the contract into an unusable state by causing richest to be the address of a contract that has a fallback function which fails (e.g. by using revert() or by just consuming more than the 2300 gas stipend). That way, whenever transfer is called to deliver funds to the “poisoned” contract, it will fail and thus also becomeRichest will fail, with the contract being stuck forever.

In contrast, if you use the “withdraw” pattern from the first example, the attacker can only cause his or her own withdraw to fail and not the rest of the contract’s workings.

Restricting Access

Restricting access is a common pattern for contracts. Note that you can never restrict any human or computer from reading the content of your transactions or your contract’s state. You can make it a bit harder by using encryption, but if your contract is supposed to read the data, so will everyone else.

You can restrict read access to your contract’s state by other contracts. That is actually the default unless you declare make your state variables public.

Furthermore, you can restrict who can make modifications to your contract’s state or call your contract’s functions and this is what this section is about.

The use of function modifiers makes these restrictions highly readable.

pragma solidity ^0.4.11;

contract AccessRestriction {
    // These will be assigned at the construction
    // phase, where `msg.sender` is the account
    // creating this contract.
    address public owner = msg.sender;
    uint public creationTime = now;

    // Modifiers can be used to change
    // the body of a function.
    // If this modifier is used, it will
    // prepend a check that only passes
    // if the function is called from
    // a certain address.
    modifier onlyBy(address _account)
    {
        require(msg.sender == _account);
        // Do not forget the "_;"! It will
        // be replaced by the actual function
        // body when the modifier is used.
        _;
    }

    /// Make `_newOwner` the new owner of this
    /// contract.
    function changeOwner(address _newOwner)
        onlyBy(owner)
    {
        owner = _newOwner;
    }

    modifier onlyAfter(uint _time) {
        require(now >= _time);
        _;
    }

    /// Erase ownership information.
    /// May only be called 6 weeks after
    /// the contract has been created.
    function disown()
        onlyBy(owner)
        onlyAfter(creationTime + 6 weeks)
    {
        delete owner;
    }

    // This modifier requires a certain
    // fee being associated with a function call.
    // If the caller sent too much, he or she is
    // refunded, but only after the function body.
    // This was dangerous before Solidity version 0.4.0,
    // where it was possible to skip the part after `_;`.
    modifier costs(uint _amount) {
        require(msg.value >= _amount);
        _;
        if (msg.value > _amount)
            msg.sender.send(msg.value - _amount);
    }

    function forceOwnerChange(address _newOwner)
        costs(200 ether)
    {
        owner = _newOwner;
        // just some example condition
        if (uint(owner) & 0 == 1)
            // This did not refund for Solidity
            // before version 0.4.0.
            return;
        // refund overpaid fees
    }
}

A more specialised way in which access to function calls can be restricted will be discussed in the next example.

State Machine

Contracts often act as a state machine, which means that they have certain stages in which they behave differently or in which different functions can be called. A function call often ends a stage and transitions the contract into the next stage (especially if the contract models interaction). It is also common that some stages are automatically reached at a certain point in time.

An example for this is a blind auction contract which starts in the stage “accepting blinded bids”, then transitions to “revealing bids” which is ended by “determine auction outcome”.

Function modifiers can be used in this situation to model the states and guard against incorrect usage of the contract.

Example

In the following example, the modifier atStage ensures that the function can only be called at a certain stage.

Automatic timed transitions are handled by the modifier timeTransitions, which should be used for all functions.

Note

Modifier Order Matters. If atStage is combined with timedTransitions, make sure that you mention it after the latter, so that the new stage is taken into account.

Finally, the modifier transitionNext can be used to automatically go to the next stage when the function finishes.

Note

Modifier May be Skipped. This only applies to Solidity before version 0.4.0: Since modifiers are applied by simply replacing code and not by using a function call, the code in the transitionNext modifier can be skipped if the function itself uses return. If you want to do that, make sure to call nextStage manually from those functions. Starting with version 0.4.0, modifier code will run even if the function explicitly returns.

pragma solidity ^0.4.11;

contract StateMachine {
    enum Stages {
        AcceptingBlindedBids,
        RevealBids,
        AnotherStage,
        AreWeDoneYet,
        Finished
    }

    // This is the current stage.
    Stages public stage = Stages.AcceptingBlindedBids;

    uint public creationTime = now;

    modifier atStage(Stages _stage) {
        require(stage == _stage);
        _;
    }

    function nextStage() internal {
        stage = Stages(uint(stage) + 1);
    }

    // Perform timed transitions. Be sure to mention
    // this modifier first, otherwise the guards
    // will not take the new stage into account.
    modifier timedTransitions() {
        if (stage == Stages.AcceptingBlindedBids &&
                    now >= creationTime + 10 days)
            nextStage();
        if (stage == Stages.RevealBids &&
                now >= creationTime + 12 days)
            nextStage();
        // The other stages transition by transaction
        _;
    }

    // Order of the modifiers matters here!
    function bid()
        payable
        timedTransitions
        atStage(Stages.AcceptingBlindedBids)
    {
        // We will not implement that here
    }

    function reveal()
        timedTransitions
        atStage(Stages.RevealBids)
    {
    }

    // This modifier goes to the next stage
    // after the function is done.
    modifier transitionNext()
    {
        _;
        nextStage();
    }

    function g()
        timedTransitions
        atStage(Stages.AnotherStage)
        transitionNext
    {
    }

    function h()
        timedTransitions
        atStage(Stages.AreWeDoneYet)
        transitionNext
    {
    }

    function i()
        timedTransitions
        atStage(Stages.Finished)
    {
    }
}