Copying a mapping from Contract A to contract B

Question

Is there any way at the moment that I can copy a mapping from one contract to the other?

mapping (address => uint256) public balanceOf;

function importFrom(address _from) onlyOwner {
    Token s = Token(_from);
    balanceOf = s.balanceOf;
}

I want to allow this feature to enable smart contract upgrades. If this is not possible, I know that it is possible to use a delegatecall from one contract to the other; but then the problem is: how do I see the interface of the contract that is being called from the Ethereum wallet? I don't want to call directly the functions in that contract, but rather through the intermediary contract that then makes the call.

Answer 1

There is no 'one-liner' in Solidity to copy/duplicate an entire mapping.

If you maintain a list of the keys of a mapping, it is possible to copy a finite amount of values from one mapping into another.

The amount of copies is finite as there is a limit on the amount of gas which can be consumed in a single block (block.gasLimit). Reading from and writing to contract storage costs gas.

The maximum key => value pairs in a mapping is 2**256. From the Ethereum Yellow Paper, a SSET operation costs 20,000 gas. Multiplying these gives 20000*(2**256) == 2.3e81 gas.

The most recent block.gasLimit when writing was6.7e6. This is dramatically lower than 2.3e81, which was a purposeful underestimation of the true cost.

Duplicating a mapping of even a millionth (1/1,000,000) the maximum mapping size is an unreasonable amount of work for the EVM to do in a single transaction (or block).