Sending extra data beside or inside a new smart contract

Question

I'm doing a research about Ethereum smart contracts security and I need to send some extra data (for example some json data) beside or inside a new smart contract when I'm deploying that contract on the network. Is it possible and how can I do it?

Answer 1

There are different ways to to this. First, you may just embed the data into smart contract's code like this:

contract Foo {
  string public constant bar = "Hello, World!";
}

Second, you may pass the data as a constructor parameter:

contract Foo {
  constructor (string memory _bar) public {
  }
}

In this example, constructor doesn't do anything with the data. However, off-chain applications will still be able to access the data. To make such accsss more convenient, you may log the data in event:

contract Foo {
  event Bar (string bar);

  constructor (string memory _bar) public {
    emit Bar (_bar);
  }
}

If you want the data to be accessible by other functions of this smart contract, you may save it into a storage variable:

contract Foo {
  string private bar;

  constructor (string memory _bar) public {
    bar = _bar;
  }
}

If you need the data to be accessible by other smart contracts, make the storage variable public:

contract Foo {
  string public bar;

  constructor (string memory _bar) public {
    bar = _bar;
  }
}

If you don't need the ability to read the data, but only to verify it, consider storing, only the hash of the data:

contract Foo {
  bytes32 public barHash;

  constructor (string memory _bar) public {
    barHash = keccak256 (bytes (_bar));
  }
}

or even pass only the hash to save gas on deployment:

contract Foo {
  bytes32 public barHash;

  constructor (bytes32 _barHash) public {
    barHash = _barHash;
  }
}

Hope this would help.

Answer 2

You can put in the data field any arbitrary data, but the transaction has a size limit, is about 780kB AFAIK.