1. Introduction

1.1 Brief Explanation of the Tutorial's Goal

This tutorial aims to guide you through the best practices for writing secure Solidity code. Solidity is a statically-typed programming language designed for developing smart contracts that run on Ethereum. The security of these smart contracts is paramount as they handle and store value. Any vulnerability can lead to significant monetary loss.

1.2 What The User Will Learn

By following this tutorial, you will learn about common vulnerabilities in Solidity and how to avoid them. You will also understand how to use certain language constructs safely and effectively.

1.3 Prerequisites

A basic understanding of Solidity language is required. Familiarity with blockchain technology and Ethereum would also be beneficial.

2. Step-by-Step Guide

2.1 Detailed Explanation of Concepts

To write secure Solidity code, you need to understand common vulnerabilities and how to mitigate them. Here are some key concepts:

1. Reentrancy Attacks: This attack can occur when you call an external contract while the current contract’s state is not finalized. To prevent it, always finalize the state before calling external contracts.

2. Arithmetic Overflows and Underflows: Solidity's arithmetic operations are vulnerable to overflows and underflows. This can be mitigated by using the SafeMath library for arithmetic operations.

3. Uninitialized Storage Pointers: An uninitialized storage pointer can overwrite data in a contract. Always initialize storage pointers.

2.2 Clear Examples with Comments

// SafeMath library to prevent overflows and underflows
library SafeMath {
  function add(uint a, uint b) internal pure returns (uint) {
    uint c = a + b;
    require(c >= a, "SafeMath: addition overflow");
    return c;
  }
  // other safe operations...
}

2.3 Best Practices and Tips

• Avoid using low-level calls as they can introduce security risks.
• Limit the amount of Ether a contract can hold.
• Use the latest version of Solidity.
• Regularly perform security audits.

3. Code Examples

3.1 Code Snippet

contract SecureContract {
  using SafeMath for uint256;
  uint256 public contractBalance;

  function deposit() public payable {
    // Using SafeMath's add function to prevent overflow
    contractBalance = contractBalance.add(msg.value);
  }
}

3.2 Detailed Comments

This contract uses the SafeMath library to ensure safe addition, preventing overflows. It also keeps track of the contract's balance.

3.3 Expected Output or Result

The expected result would be the secure addition of values without the risk of overflow.

4. Summary

In this tutorial, we have looked at common vulnerabilities in Solidity and how to prevent them. We have also covered using SafeMath for safe arithmetic operations and other best practices for writing secure Solidity code.

5. Practice Exercises

Exercise 1: Write a contract that securely transfers Ether from one account to another.
Solution: Use the transfer function for secure Ether transfer.

contract TransferContract {
  function transfer(address payable _to, uint256 _amount) public {
    // Securely transferring ethers
    _to.transfer(_amount);
  }
}

Exercise 2: Write a contract that securely increments a counter.
Solution: Use SafeMath to prevent overflows.

contract Counter {
  using SafeMath for uint256;
  uint256 public counter;

  function increment() public {
    // using SafeMath's increment function to prevent overflow
    counter = counter.add(1);
  }
}

Tips for further practice: Keep practicing with more complex contracts, try to implement a simple ERC20 token using SafeMath.

Additional resources:
- Solidity Documentation
- Ethereum Smart Contract Best Practices