Solidity Interface – Understanding Contracts Calling Unimplemented Functions with Chainlink

chainlinkinterfacessoliditysolidity-0.6.x

I'm new to solidity and OOP. I'm trying to understand the code in this simple lottery game example:

https://github.com/alphachainio/chainlink-lottery/blob/master/ethereum/contracts/

I am confused when they call governance.randomness() in line 57 in Lottery.sol. but I don't see the randomness() function implemented any where in any of the contract files.

The answer below explains that the unimplemented function will be implemented in the contract provided in CEth cToken = CEth(_cEtherContract);

https://stackoverflow.com/a/64734548/4488843

But in the GitHub lottery example, I see the function is no where declared in any of the contracts (lottery.sol, randomness.sol and governance.sol).

is the function initiated when governance contract's init() function is called and it sets the randomness state variable? or is there something I'm missing in the code?

Best Answer

For your randomness example it is important to know that Solidity generates getters for all public variables. In the governance contract there is a public variable randomness which generates the mentioned function as a getter: https://github.com/alphachainio/chainlink-lottery/blob/master/ethereum/contracts/Governance.sol#L6.

In general Solidity does not perform any check if a contract implements a specific interface when casting it to that interface. More info on this can be found here: How is an interface initialised? (there are quite some other questions also explaining this ;) )

Related Solutions

Solidity – How to Unit Test Struct Values

Unfortunately structures were not part of the original solidity ABI specification so they will return instead a tuple of values.

Another problem is that javascript doesn't support integer with sufficient precision. To avoid problems like losing precision or rounding errors web3 will wrap integers to objects BigNumber, see What are *C*, *E* and *S* properties in message call return object? for other details.

To compare BigNumbers you can convert them to ordinary javascripts numbers with .toNumber() (this will only work for small integers https://stackoverflow.com/questions/307179/what-is-javascripts-highest-integer-value-that-a-number-can-go-to-without-losin), for large numbers probably is best to convert to string or use some of the methods from BigNumber library.

return Adoption.deployed()
  .then(function(instance) {
    instance.createDog(5, "John");
    return instance.dogs(0);
  })
  .then(function(res) {
    // dog age should equal 5
    assert.equal(res[0].toNumber(), 5);
    // dog name should equal John
    assert.equal(res[1], "John");
  });

How to Properly Define an Interface in Solidity 0.6 and Above

Answer

There are two new rules to be aware of:

All interface functions are implicitly virtual. If you want to be explicit about it, the compiler throws a warning.
All functions inheriting from the interface must set the override modifier on every function that overrides an interface function, lest the compiler throws an error.

Example

Try to see what happens if you add virtual or remove override from this:

interface MyInterface {

    function getBlockNumber() external view returns (uint256);
}

contract MyContract is MyInterface {

    function getBlockNumber() public view override returns (uint256) {
        return block.number;
    }
}

Further Documentation

Note that the virtual and override modifiers don't apply only to interfaces. I recommend reading more about them in this other thread and the solidity 0.6 breaking changes docs.