Solidity – Handling abi.decode() and Unused Variables

abisolcsolidity

I am decoding bytes as the following:

      // Decode Solidity tightly packed arguments
      (uint8 _, uint128 stakeId, address behalf) = abi.decode(userData, (uint8, uint128, address));  // solhint-disable-line

I get a compiler warning:

Variable '_' is declared but never used.

What would be the right way to suppress this warning beautifully? I know how to do this for function arguments, but not sure how what is a good practice with abi.decode.

The obvious way does not work:

/Users/moo/code/dawn-erc20/contracts/Staking.sol:255:23: ParserError: Expected ',' but got identifier
      (uint8, uint128 stakeId, address behalf) = abi.decode(userData, (uint8, uint128, address));  // solhint-disable-line

Best Answer

You could get rid of it in the function-call as well, i.e.:

(uint128 stakeId, address behalf) = abi.decode(userData, (uint128, address));

But I suppose that would just beat your purpose to begin with.

So the quickest workaround which comes to mind is to just add a dummy reference:

(uint8 _, uint128 stakeId, address behalf) = abi.decode(userData, (uint8, uint128, address));
_; // a dummy reference to an unused local variable

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[Ethereum] The solc is not generatig *.abi file with the option –abi and -o build

It seems like a command syntax issue. This works for me:

solc --abi greeter.sol -o build

Gas Cost – Do Intermediate Memory Variables Cost Gas in Solidity?

Here is more information about storing data in solidity https://solidity.readthedocs.io/en/latest/introduction-to-smart-contracts.html#storage-memory-and-the-stack

Those variables, which you defined, will be stored in stack(if there is a space and you keep them in one function). in case if compile will optimize it properly it will be for free, if not, each variable will cost you PUST and POP operations (3+2 gas)

UPDATE

Optimized bytecode costs same amount of gas for both versions. Checked on pragma solidity 0.4.24

Ropsten test (Remix)

pragma solidity ^0.4.24;
contract A {
    uint t;

    function run() public returns(uint){
        uint startTime = 9;
        uint allowedTime = 7;

        uint v = startTime + allowedTime; // second test without v
        t = now + v;
        return t;
    }
}

Results:

with v
  deploy 95237 gas
  func 41470 gas

without v
  deploy 95237 gas
  func 41470 gas

Best Answer

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