Copying an array of structs to memory

Question

I have some code that attempts to copy an array to mapping:

// Mapping of user addresses to their payment configuration
mapping(address => PaymentConfiguration[]) internal configurations;

// Event to signal that a user's payment configuration has been updated
event ConfigurationUpdated(address user, PaymentConfiguration[] paymentConfigurations);

function updateConfiguration(PaymentConfiguration[] calldata paymentConfigurations) onlyOwner public {
    //TODO: we should probably add validations to assure that payment configuration total percentage doesn't exceed the denominator
    configurations[_msgSender()] = paymentConfigurations;
    emit ConfigurationUpdated(_msgSender(), paymentConfigurations);
}

I believe this issue is caused by the fact that you can't copy an array of structs to memory. Is there a simple way I can replicate this behavior

Answer 1

Array in storage would have a length field to keep track of the total length of the array. If you're adding entry to storage array, you need to use .push, which will also update the length.

// append only
for (uint i; i < paymentConfigurations.length; ) {
  configurations[_msgSender()].push(paymentConfigurations[i])

  unchecked {
    ++i;
  }
}

You would need to add some logic if you're replacing the old array with the new one: basically if i < length, you overwrite, and if i> length, you push.

---

If you don't want to keep track of the total length, (if you don't need to iterate through them), it's better to make it a mapping:

// msg.sender => index => config
mapping(address => mapping(uint => PaymentConfiguration)) internal configurations;

So that it would be cheaper to update the storage this way

for (uint i; i < paymentConfigurations.length; ) {

  configurations[_msgSender()][i] = paymentConfigurations[i];

  unchecked {
    ++i;
  }
}