Solidity 0.6.x features: try/catch statement

The try/catch syntax introduced in 0.6.0 is arguably the largest leap in error dealing with capabilities in Solidity, since motive 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 are usually not).

We’re transferring 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 permits you to react on failed exterior calls and contract creation calls, so you can’t use it for inside perform calls. Be aware that to wrap a public perform name throughout the identical contract with strive/catch, it may be made exterior by calling the perform with this..

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

pragma solidity ^0.6.1;

contract CharitySplitter {
    tackle public proprietor;
    constructor (tackle _owner) public {
        require(_owner != tackle(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 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 (tackle => CharitySplitter) public charitySplitters;
    uint public errorCount;
    occasion ErrorHandled(string motive);
    occasion ErrorNotHandled(bytes motive);
    perform createCharitySplitter(tackle charityOwner) public {
        strive new CharitySplitter(charityOwner)
            returns (CharitySplitter newCharitySplitter)
        {
            charitySplitters[msg.sender] = newCharitySplitter;
        } catch {
            errorCount++;
        }
    }
}

Be aware that with strive/catch, solely exceptions taking place contained in the exterior name itself are caught. Errors contained in the expression are usually not caught, for instance if the enter parameter for the new CharitySplitter is itself a part of an inside name, any errors it raises is not going to 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(tackle _charityOwner) public {
    strive new CharitySplitter(getCharityOwner(_charityOwner, false))
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    } catch (bytes reminiscence motive) {
        ...
    }
}
perform getCharityOwner(tackle _charityOwner, bool _toPass)
        inside returns (tackle) {
    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 realize this:

1. Utilizing catch Error(string reminiscence motive)

perform createCharitySplitter(tackle _charityOwner) public {
    strive 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)

perform createCharitySplitter(tackle charityOwner) public {
    strive 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 analogous consequence. The distinction is that the second technique doesn’t ABI-decode the error string. The benefit of the second technique is that it is usually executed if ABI decoding the error string fails or if no motive was offered.

Future plans

There are plans to launch assist for error sorts that means we will declare errors in an analogous method to occasions permitting us to catch totally different kind of errors, for instance:

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