ResetVariable

In Solidity, using the delete keyword to remove a state variable resets it to the default value for its type. The default values differ according to the data type:

• boolean: The default value is false.
• string: The default value is an empty string "".
• int: The default value is 0.
• uint: The default value is 0.
• enum: The default value is the first element of the enumeration.
• address: The default value is 0x0000000000000000000000000000000000000000 (or address(0)).
• function: For internal functions, the default is an empty internal function, and for external functions, it is an empty external function.
• mapping: All elements are set to their default values.
• struct: All members are set to their default values.
• Dynamic arrays: The length is reset to 0, effectively making it an empty array [].
• Static arrays: All elements are set to their default values.

Compared to directly updating the state variable to its default value, the gas costs differ. When a state variable is removed using the delete keyword, Solidity generates a specific opcode, SSTORE, to set the variable's value to its type's default value. The delete operation is a high-level action recognized and optimized by the compiler, which can clear all related storage at once, whereas manually resetting might require more operations and higher gas costs.

DemoCode

The smart contract ResetVariable below defines a state variable balance and two different methods to delete the balance state variable data. These methods consume different amounts of gas:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract ResetVariable {
    uint balance;

    // Normal update of balance: 22238 gas
    function normalUpdate() external {
        balance = 1;
    }

    // Update and delete: 2316 gas
    function deleteAfterUpdate() external {
        balance = 1;
        delete balance;
    }

    // Update and assign to default value: 2360 gas
    function resetToDefault() external {
        balance = 1;
        balance = 0;
    }
}

Recommendations for gas optimization:

🌟 Compared to directly updating a state variable to its default value, using the delete keyword to remove the variable consumes less gas.