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Data location · easy

7. Storage, memory and calldata

The three data locations and their costs.

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`storage` is the contract's persistent state and by far the most expensive. `memory` is a scratch area wiped after the call. `calldata` is the read-only, non-modifiable input of the transaction and the cheapest place to read arguments from.

solidity
function sum(uint256[] calldata xs) external pure returns (uint256 t) {
    for (uint256 i; i < xs.length; ++i) t += xs[i];
}

function touch() external {
    Stake storage s = stakes[msg.sender]; // reference — writes persist
    Stake memory copy = stakes[msg.sender]; // copy — writes are discarded
}

A `storage` reference writes through to state; a `memory` copy does not.

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Check your understanding

  1. 1. Which location is cheapest for external function arguments?

  2. 2. What happens to writes made to a `memory` struct copy?

  3. 3. Which location persists between transactions?

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