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 purpose strings for revert and require have 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 operate calls with out rolling again the entire transaction (state adjustments within the known as operate are nonetheless rolled again, however the ones within the calling operate should not).

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 frequently need.

Dealing with exterior name failures

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

The instance beneath demonstrates how strive/catch is utilized in a manufacturing unit sample the place contract creation would possibly 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 strive/catch as a failsafe for when that constructor would possibly 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 purpose);
    occasion ErrorNotHandled(bytes purpose);
    operate createCharitySplitter(handle charityOwner) public {
        strive new CharitySplitter(charityOwner)
            returns (CharitySplitter newCharitySplitter)
        {
            charitySplitters[msg.sender] = newCharitySplitter;
        } catch {
            errorCount++;
        }
    }
}

Notice that with strive/catch, solely exceptions taking place contained in the exterior name itself are caught. Errors contained in the expression should not caught, for instance if the enter parameter for the new CharitySplitter is itself a part of an inside name, any errors it raises won’t 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 strive/catch assertion.

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

Retrieving the error message

We will additional lengthen the strive/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 realize this:

1. Utilizing catch Error(string reminiscence purpose)

operate createCharitySplitter(handle _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)

operate createCharitySplitter(handle 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 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’s also executed if ABI decoding the error string fails or if no purpose was supplied.

Future plans

There are plans to launch assist for error varieties which means we will declare errors in an analogous technique to occasions permitting us to catch totally different sort of errors, for instance:

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