solidity – Importance of Return Statements in Solidity Functions

contract-designcontract-developmentsolidity

A value returned by a function in the smart contract cannot be captured as every function returns a hash.

Then, why write return statements in Solidity function?

For example,

event LogNewVoter(address sender, string name, bool isSuccess);


function addVoter(address _voter,string _name, bytes _voterImageHash) payable public returns(bool) {
    require(msg.sender == votingAdmin);
    Voter storage sender = voters[_voter];
    if(sender.isVoter){
        revert("This address is all ready registered as a voter!");
       //throw;
    }
    voters[_voter].name = _name;
    voters[_voter].hasVoted = false;
    voters[_voter].isVoter = true;
    voters[_voter].voterImageHash = _voterImageHash;
    LogNewVoter(msg.sender,_name,true);
    return true;
}

In above question, I can get whether the voter is registered successfully or not by using events. If I can not capture value returned by a function then why to write return statements in function?

Best Answer

In Solidity you can return values from functions to the user if these functions are declared view(/constant).

When they are not declared as view you can still get a return value by using .call() this will execute the call locally and not mine it onto the blockchain however.

Return values do play a big role as they can be returned to other functions even when the function is not view/constant. I'll give a real world example.

The following function creates a new 'bounty' on a bounty contract, it returns a bytes32 value. 

  function createBounty(
    BountyInterface.Bounty storage _bounty,
    string _title,
    string _issueURL,
    string _reference,
    uint _deadline,
    address _issuer,
    uint _reward
  ) external returns (bytes32) {
      bytes32 _index = keccak256(abi.encodePacked(_issueURL));
      _bounty.bounties[_index].title = _title;
      _bounty.bounties[_index].issueURL = _issueURL;
      _bounty.bounties[_index].reference = _reference;
      _bounty.bounties[_index].timestamp = block.timestamp;
      _bounty.bounties[_index].deadline = _deadline;
      _bounty.bounties[_index].status = BountyInterface.statusOptions.ACTIVE;
      _bounty.bounties[_index].issuer = _issuer;
      _bounty.bounties[_index].reward = _reward;
      _bounty.bounty_indices.push(_index);
      emit logCreateBounty(_issuer, msg.sender, _title, _reward);
      return _index;
  }

The return value is then sent to this function on another contract. This will store the return value in an array so that the newly created bounty is associated with a user as well:

  function createBounty(address _group, string _title, string _issueURL, string _reference, uint _deadline, uint _reward) external {
    bytes32 _index = Group(_group).createBounty(_title, _issueURL, _reference, _deadline, msg.sender, _reward);
    People(ContractProvider(CMC).contracts("people-storage")).addBounty(_group, _index, msg.sender);
  }
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