Hello,
I have come across a piece of code of a real-world project I’m looking at in more detail for deepening my understanding of the best way how to manage information on the ETH blockchain.
The below code is a part of a contract which can be found here
https://etherscan.io/address/0x2467aa6b5a2351416fd4c3def8462d841feeecec?a=0x2467aa6b5a2351416fd4c3def8462d841feeecec#code
Code excerpt:
contract VestedToken is StandardToken, LimitedTransferToken {
uint256 MAX_GRANTS_PER_ADDRESS = 20;
struct TokenGrant {
address granter; // 20 bytes
uint256 value; // 32 bytes
uint64 cliff;
uint64 vesting;
uint64 start; // 3 * 8 = 24 bytes
bool revokable;
bool burnsOnRevoke; // 2 * 1 = 2 bits? or 2 bytes?
} // total 78 bytes = 3 sstore per operation (32 per sstore)
mapping (address => TokenGrant[]) public grants;
event NewTokenGrant(address indexed from, address indexed to, uint256 value, uint256 grantId);
/**
* @dev Grant tokens to a specified address
* @param _to address The address which the tokens will be granted to.
* @param _value uint256 The amount of tokens to be granted.
* @param _start uint64 Time of the beginning of the grant.
* @param _cliff uint64 Time of the cliff period.
* @param _vesting uint64 The vesting period.
*/
function grantVestedTokens(
address _to,
uint256 _value,
uint64 _start,
uint64 _cliff,
uint64 _vesting,
bool _revokable,
bool _burnsOnRevoke
) public {
// Check for date inconsistencies that may cause unexpected behavior
require(_cliff >= _start && _vesting >= _cliff);
require(tokenGrantsCount(_to) < MAX_GRANTS_PER_ADDRESS); // To prevent a user being spammed and have his balance locked (out of gas attack when calculating vesting).
uint256 count = grants[_to].push(
TokenGrant(
_revokable ? msg.sender : 0, // avoid storing an extra 20 bytes when it is non-revokable
_value,
_cliff,
_vesting,
_start,
_revokable,
_burnsOnRevoke
)
);
transfer(_to, _value);
NewTokenGrant(msg.sender, _to, _value, count - 1);
}
/**
* @dev Revoke the grant of tokens of a specifed address.
* @param _holder The address which will have its tokens revoked.
* @param _grantId The id of the token grant.
*/
function revokeTokenGrant(address _holder, uint256 _grantId) public {
TokenGrant storage grant = grants[_holder][_grantId];
require(grant.revokable);
require(grant.granter == msg.sender); // Only granter can revoke it
address receiver = grant.burnsOnRevoke ? 0xdead : msg.sender;
uint256 nonVested = nonVestedTokens(grant, uint64(now));
// remove grant from array
delete grants[_holder][_grantId];
grants[_holder][_grantId] = grants[_holder][grants[_holder].length.sub(1)];
grants[_holder].length -= 1;
balances[receiver] = balances[receiver].add(nonVested);
balances[_holder] = balances[_holder].sub(nonVested);
Transfer(_holder, receiver, nonVested);
}
The part that triggered my curiosity is towards the end of the code where it deletes a specific grantId of the array. In the course, we learned that this was a problem as there is now an empty / 0 space in the array and no easy way to close that gap. But it seems the code handles this challenge with the 2 lines:
delete grants[_holder][_grantId];
grants[_holder][_grantId] = grants[_holder][grants[_holder].length.sub(1)];
grants[_holder].length -= 1;
I don’t quite understand how this works, as I fail to grasp
a) what the “grants[_holder][grants[_holder].length.sub(1)];” achieves other than deleting the right-most position of the array. But then there is this nested approach to the (un-nested) mapping which is unclear to me.
b) also, the next line seems like an additional reduction of the length of the array - why the double reduction of “length”?
Would greatly appreciate any insights anyone could offer to better understand how an empty position in an array can be effectively removed (assuming that this code is a good example for achieving just that).
Thanks.
Yestome