Checks-Effects-Interactions pattern in Blockchain / Solidity - Mini Project: Build & Apply

Checks-Effects-Interactions Pattern in Solidity

📖 Scenario: You are building a simple Ethereum smart contract for a wallet that allows users to withdraw their funds safely.In blockchain programming, it is important to follow the Checks-Effects-Interactions pattern to avoid security issues like reentrancy attacks.

🎯 Goal: Learn how to implement the Checks-Effects-Interactions pattern in Solidity by creating a withdraw function that first checks conditions, then updates state, and finally interacts with external contracts.

📋 What You'll Learn

Create a mapping called balances to store user balances

Add a function deposit to allow users to add funds

Add a function withdraw that uses the Checks-Effects-Interactions pattern

Print the remaining balance after withdrawal

💡 Why This Matters

🌍 Real World

Smart contracts on Ethereum must be secure to protect users' funds. The Checks-Effects-Interactions pattern is a key security practice.

💼 Career

Blockchain developers use this pattern daily to write safe contracts that handle money and interact with other contracts.

1

Create the balances mapping and deposit function

Create a public mapping called balances that maps address to uint. Then write a public deposit function that increases the sender's balance by msg.value.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    // Create balances mapping
    // Write deposit function
    
    // Your code here
}

Need a hint?

Use mapping(address => uint) public balances; to store balances.

The deposit function should be payable and add msg.value to balances[msg.sender].

2

Add a withdraw function with a check for sufficient balance

Add a public function called withdraw that takes a uint amount parameter. Inside, first check that balances[msg.sender] is at least amount using require.

Blockchain / Solidity

pragma solidity ^0.8.0;

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

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

    // Add withdraw function with balance check
    
    // Your code here
}

Need a hint?

Use require to check if the sender has enough balance before proceeding.

3

Apply the Effects step by updating the balance before interaction

Inside the withdraw function, after the require check, subtract amount from balances[msg.sender]. This is the Effects step.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    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");
        // Subtract amount from balance
        
        // Your code here
    }
}

Need a hint?

Update the user's balance by subtracting the withdrawal amount before sending funds.

4

Complete the withdraw function with the interaction and print remaining balance

In the withdraw function, after updating the balance, send amount to msg.sender using payable(msg.sender).transfer(amount). Then add a print statement to output the remaining balance of msg.sender.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    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;
        // Send amount to sender
        // Print remaining balance
        
        // Your code here
    }
}

Need a hint?

Use payable(msg.sender).transfer(amount) to send Ether.

Solidity cannot print to console, so use an event to show the remaining balance.