not yet created account

Question

In https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/Address.sol the function detecting contract

function isContract(address account) internal view returns (bool) {
    // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
    // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
    // for accounts without code, i.e. `keccak256('')`
    bytes32 codehash;
    bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
    // solhint-disable-next-line no-inline-assembly
    assembly { codehash := extcodehash(account) }
    return (codehash != accountHash && codehash != 0x0);
}

The comment says that there is a distinction between not-yet created accounts and account without code. I don't understand it, I thought all the possible accounts are already technically created just their balance and code are 0, resp. empty in the beginning.

Answer 1

Although from a theoretical point there are similarities between non-existent accounts and an account without bytecode, both behave similarly when they are the recipient of a transfer. From a practical point of view there are important differences.

A non-existent accounts cost nothing, it doesn't exist yet. On the other hand a non-empty account without bytecode requires allocation in the Ethereum World State and it has the be kept around forever.

Another place where it is useful to distinguish between a non-existent account and an empty account is during a contract's constructor execution. Contract exists and can make calls to others but its bytecode will not be assigned until the constructor finishes.

Answer 2

The answer is in the specification of eip-1052:

In case the account does not exist or is empty (as defined by EIP-161)...

And if we follow to the of specification of eip-161, then we find out that:

• The nonce of a non-existent account is set to the "normal" value (typically 0)
• The nonce of an existent (possibly empty) account is higher than the "normal" value

So in short, my understanding is that once you make the very first transaction to or from an account, it changes from non-existent to empty.