require, assert, revert and error - Solidity Part 2.5

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Error handling is a crucial aspect of any programming language, and Solidity, the language used for developing smart contracts on the Ethereum blockchain, is no exception. In this post, we’ll explore the various mechanisms available in Solidity for managing errors, ensuring your smart contracts are robust, secure, and user-friendly.

Source Code: https://github.com/scaihai/enkwadore-blog-blockchain-demos/tree/main/solidity/contracts/2.5

1. The Importance of Error Handling

Error handling in smart contracts is vital for several reasons:

  • Security: Prevents unexpected behavior and vulnerabilities.
  • User Experience: Provides clear feedback to users, helping them understand what went wrong.
  • Reliability: Ensures that contracts behave as expected under different conditions.

2. Basic Error Handling Constructs

Solidity offers several constructs for handling errors, including:

  • require
  • assert
  • revert
  • Custom Errors (Solidity 0.8.4 and above)

Let’s delve into each of these.

require

The require statement is used to validate conditions before executing further code. If the condition is not met, the transaction is reverted, and an optional error message is returned.

SimpleWallet.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract SimpleWallet {
    mapping(address => uint) public balances;

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        balances[msg.sender] -= amount;
        payable(msg.sender).transfer(amount);
    }
}

In this example, require ensures that the user has enough balance before proceeding with the withdrawal.

assert

The assert statement is used to check for conditions that should never be false. It is typically used for internal error checking and invariant testing. If an assert condition fails, it indicates a bug in the code, and the transaction is reverted.

Ownership.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Ownership {
    address public owner;

    constructor() {
        owner = msg.sender;
    }

    function setOwner(address newOwner) public {
        owner = newOwner;
        assert(owner == newOwner); // This should always be true
    }
}

revert

The revert function is explicitly used to revert a transaction and provide a reason for the failure. It is useful for more complex error handling logic.

Token.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Token {
    mapping(address => uint) public balance;

    function transfer(address to, uint amount) public {
        if (balance[msg.sender] < amount) {
            revert("Insufficient balance for transfer");
        }
        balance[msg.sender] -= amount;
        balance[to] += amount;
    }
}

Custom Errors

Introduced in Solidity 0.8.4, custom errors are a more gas-efficient way to handle errors compared to revert strings.

SimpleWallet2.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

contract SimpleWallet2 {
    mapping(address => uint) public balance;

    error InsufficientBalance(uint requested, uint available);

    function deposit() public payable {
        balance[msg.sender] += msg.value;
    }

    function withdraw(uint amount) public {
        if (amount > balance[msg.sender]) {
            revert InsufficientBalance(amount, balance[msg.sender]);
        }
        balance[msg.sender] -= amount;
        payable(msg.sender).transfer(amount);
    }
}

Custom errors are defined using the error keyword and can provide more detailed information about the error.

Best Practices for Error Handling

  1. Use require for User Inputs: Validate user inputs and conditions at the start of functions using require. This helps catch errors early and provide useful feedback to users.
  2. Reserve assert for Internal Checks: Use assert for conditions that should never fail. These are typically checks for internal invariants or critical state assumptions.
  3. Provide Clear Error Messages: When using require and revert, include descriptive error messages to help users understand what went wrong.
  4. Utilize Custom Errors for Efficiency: When gas efficiency is a concern, consider using custom errors instead of revert strings, especially in frequently called functions.
  5. Fail Early and Cleanly: Ensure that functions fail as early as possible if an error condition is detected, minimizing unnecessary computation and gas usage.

Conclusion

Effective error handling in Solidity is essential for creating secure and reliable smart contracts. By using require, assert, revert, and custom errors appropriately, you can ensure that your contracts handle errors gracefully and provide clear feedback to users.

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