Solidity 0.6.x features: try/catch statement

The try/catch syntax introduced in 0.6.0 is arguably the most important leap in error dealing with capabilities in Solidity, since motive strings for revert and require had been launched in v0.4.22. Each attempt and catch have been reserved key phrases since v0.5.9 and now we are able to use them to deal with failures in exterior operate calls with out rolling again the whole transaction (state modifications within the referred to as operate are nonetheless rolled again, however the ones within the calling operate aren’t).

We’re shifting one step away from the purist “all-or-nothing” method in a transaction lifecycle, which falls in need of sensible behaviour we frequently need.

Dealing with exterior name failures

The attempt/catch assertion means that you can react on failed exterior calls and contract creation calls, so you can’t use it for inner operate calls. Word that to wrap a public operate name throughout the identical contract with attempt/catch, it may be made exterior by calling the operate with this..

The instance under demonstrates how attempt/catch is utilized in a manufacturing unit sample the place contract creation may fail. The next CharitySplitter contract requires a compulsory handle property _owner in its constructor.

pragma solidity ^0.6.1;

contract CharitySplitter {
    handle public proprietor;
    constructor (handle _owner) public {
        require(_owner != handle(0), "no-owner-provided");
        proprietor = _owner;
    }
}

There’s a manufacturing unit contract — CharitySplitterFactory which is used to create and handle cases of CharitySplitter. Within the manufacturing unit we are able to wrap the new CharitySplitter(charityOwner) in a attempt/catch as a failsafe for when that constructor may fail due to an empty charityOwner being handed.

pragma solidity ^0.6.1;
import "./CharitySplitter.sol";
contract CharitySplitterFactory {
    mapping (handle => CharitySplitter) public charitySplitters;
    uint public errorCount;
    occasion ErrorHandled(string motive);
    occasion ErrorNotHandled(bytes motive);
    operate createCharitySplitter(handle charityOwner) public {
        attempt new CharitySplitter(charityOwner)
            returns (CharitySplitter newCharitySplitter)
        {
            charitySplitters[msg.sender] = newCharitySplitter;
        } catch {
            errorCount++;
        }
    }
}

Word that with attempt/catch, solely exceptions occurring contained in the exterior name itself are caught. Errors contained in the expression aren’t caught, for instance if the enter parameter for the new CharitySplitter is itself a part of an inner name, any errors it raises is not going to be caught. Pattern demonstrating this behaviour is the modified createCharitySplitter operate. Right here the CharitySplitter constructor enter parameter is retrieved dynamically from one other operate — getCharityOwner. If that operate reverts, on this instance with “revert-required-for-testing”, that won’t be caught within the attempt/catch assertion.

operate createCharitySplitter(handle _charityOwner) public {
    attempt new CharitySplitter(getCharityOwner(_charityOwner, false))
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    } catch (bytes reminiscence motive) {
        ...
    }
}
operate getCharityOwner(handle _charityOwner, bool _toPass)
        inner returns (handle) {
    require(_toPass, "revert-required-for-testing");
    return _charityOwner;
}

Retrieving the error message

We are able to additional prolong the attempt/catch logic within the createCharitySplitter operate to retrieve the error message if one was emitted by a failing revert or require and emit it in an occasion. There are two methods to attain this:

1. Utilizing catch Error(string reminiscence motive)

operate createCharitySplitter(handle _charityOwner) public {
    attempt new CharitySplitter(_charityOwner) returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    }
    catch Error(string reminiscence motive)
    {
        errorCount++;
        CharitySplitter newCharitySplitter = new
            CharitySplitter(msg.sender);
        charitySplitters[msg.sender] = newCharitySplitter;
        // Emitting the error in occasion
        emit ErrorHandled(motive);
    }
    catch
    {
        errorCount++;
    }
}

Which emits the next occasion on a failed constructor require error:

CharitySplitterFactory.ErrorHandled(
    motive: 'no-owner-provided' (kind: string)
)

2. Utilizing catch (bytes reminiscence motive)

operate createCharitySplitter(handle charityOwner) public {
    attempt new CharitySplitter(charityOwner)
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    }
    catch (bytes reminiscence motive) {
        errorCount++;
        emit ErrorNotHandled(motive);
    }
}

Which emits the next occasion on a failed constructor require error:

CharitySplitterFactory.ErrorNotHandled(
  motive: hex'08c379a0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000116e6f2d6f776e65722d70726f7669646564000000000000000000000000000000' (kind: bytes)

The above two strategies for retrieving the error string produce an identical consequence. The distinction is that the second methodology doesn’t ABI-decode the error string. The benefit of the second methodology is that additionally it is executed if ABI decoding the error string fails or if no motive was offered.

Future plans

There are plans to launch help for error varieties which means we will declare errors in an identical approach to occasions permitting us to catch totally different kind of errors, for instance:

catch CustomErrorA(uint data1) { … }
catch CustomErrorB(uint[] reminiscence data2) { … }
catch {}