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 purpose strings for revert and require had been launched in v0.4.22. Each strive 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 perform calls with out rolling again the whole transaction (state adjustments within the referred to as perform are nonetheless rolled again, however the ones within the calling perform will not be).

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

Dealing with exterior name failures

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

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

pragma solidity ^0.6.1;

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

There’s a manufacturing facility contract — CharitySplitterFactory which is used to create and handle situations of CharitySplitter. Within the manufacturing facility we are able to wrap the new CharitySplitter(charityOwner) in a strive/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 (deal with => CharitySplitter) public charitySplitters;
    uint public errorCount;
    occasion ErrorHandled(string purpose);
    occasion ErrorNotHandled(bytes purpose);
    perform createCharitySplitter(deal with charityOwner) public {
        strive new CharitySplitter(charityOwner)
            returns (CharitySplitter newCharitySplitter)
        {
            charitySplitters[msg.sender] = newCharitySplitter;
        } catch {
            errorCount++;
        }
    }
}

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

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

Retrieving the error message

We will additional lengthen the strive/catch logic within the createCharitySplitter perform 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 purpose)

perform createCharitySplitter(deal with _charityOwner) public {
    strive new CharitySplitter(_charityOwner) returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    }
    catch Error(string reminiscence purpose)
    {
        errorCount++;
        CharitySplitter newCharitySplitter = new
            CharitySplitter(msg.sender);
        charitySplitters[msg.sender] = newCharitySplitter;
        // Emitting the error in occasion
        emit ErrorHandled(purpose);
    }
    catch
    {
        errorCount++;
    }
}

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

CharitySplitterFactory.ErrorHandled(
    purpose: 'no-owner-provided' (sort: string)
)

2. Utilizing catch (bytes reminiscence purpose)

perform createCharitySplitter(deal with charityOwner) public {
    strive new CharitySplitter(charityOwner)
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    }
    catch (bytes reminiscence purpose) {
        errorCount++;
        emit ErrorNotHandled(purpose);
    }
}

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

CharitySplitterFactory.ErrorNotHandled(
  purpose: hex'08c379a0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000116e6f2d6f776e65722d70726f7669646564000000000000000000000000000000' (sort: bytes)

The above two strategies for retrieving the error string produce the same end result. 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 purpose was supplied.

Future plans

There are plans to launch assist for error sorts that means we will declare errors in the same option to occasions permitting us to catch totally different sort of errors, for instance:

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