Reference types behavior in Blockchain / Solidity - Mini Project: Build & Apply

Understanding Reference Types Behavior in Blockchain Smart Contracts

📖 Scenario: Imagine you are building a simple blockchain smart contract to manage a list of users and their balances. You want to understand how reference types like arrays behave when you assign them to new variables or modify them inside functions.

🎯 Goal: You will create a smart contract that stores an array of balances. You will then create a function to demonstrate how modifying a reference type affects the original data. This will help you understand reference types behavior in Solidity.

📋 What You'll Learn

Create a balances array with initial values

Create a tempBalances variable to hold a reference to balances

Write a function to modify tempBalances and observe changes in balances

Print the balances before and after modification

💡 Why This Matters

🌍 Real World

Understanding reference types is important when writing smart contracts that manage data efficiently and correctly on the blockchain.

💼 Career

Blockchain developers must know how data is stored and modified in smart contracts to avoid bugs and security issues.

1

Create the initial balances array

Create a public uint[] array called balances with these exact values: 100, 200, and 300.

Blockchain / Solidity

// Create a public uint[] array called balances with values 100, 200, 300
// Your code here

Hint

Use uint[] public balances = [100, 200, 300]; inside the contract.

2

Create a reference variable to balances

Inside the contract, create a public function called setTempBalances that declares a local uint[] storage variable named tempBalances and assign it to balances.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ReferenceTypes {
    uint[] public balances = [100, 200, 300];

    // Create setTempBalances function and assign tempBalances to balances
    // Your code here
}

Hint

Use uint[] storage tempBalances = balances; inside the function.

3

Modify the reference variable

Inside the setTempBalances function, add code to change the first element of tempBalances to 999.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ReferenceTypes {
    uint[] public balances = [100, 200, 300];

    function setTempBalances() public {
        uint[] storage tempBalances = balances;
        // Change the first element of tempBalances to 999
        // Your code here
    }
}

Hint

Use tempBalances[0] = 999; to modify the first element.

4

Check the updated balances

Add a public view function called getBalances that returns the balances array. This will show the updated balances after calling setTempBalances.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ReferenceTypes {
    uint[] public balances = [100, 200, 300];

    function setTempBalances() public {
        uint[] storage tempBalances = balances;
        tempBalances[0] = 999;
    }

    // Create getBalances function to return balances
    // Your code here
}

Hint

Use function getBalances() public view returns (uint[] memory) { return balances; }.

Practice

1. In blockchain programming, what happens when you assign one reference type variable to another?

easy

A. The variables become independent and unrelated.

B. A new copy of the data is created.

C. The original data is deleted.

D. Both variables point to the same data location.

Solution

1. Step 1: Understand reference assignment

   Assigning one reference type variable to another makes both point to the same data location in memory.

2. Step 2: Effect of changes through references

   Changes made through either variable affect the same original data because they share the reference.

3. Final Answer:

   Both variables point to the same data location. -> Option D

4. Quick Check:

   Reference assignment shares data location = A [OK]

Hint: Reference types share data location, not copy data [OK]

Common Mistakes:

• Thinking assignment copies data
• Assuming variables become independent
• Believing original data is deleted

2. Which of the following is the correct syntax to copy a reference type variable in Solidity (blockchain language)?

easy

A. uint[] memory newArray = oldArray;

B. uint[] newArray = oldArray.copy();

C. uint[] storage newArray = oldArray;

D. uint[] newArray = copy(oldArray);

Solution

1. Step 1: Understand storage vs memory in Solidity

   Storage variables hold data permanently on blockchain, memory variables are temporary copies.

2. Step 2: Correct way to copy reference

   Assigning a storage reference variable to another storage variable copies the reference, not the data.

3. Final Answer:

   uint[] storage newArray = oldArray; -> Option C

4. Quick Check:

   Storage assignment copies reference = B [OK]

Hint: Use 'storage' keyword to copy reference, not 'memory' [OK]

Common Mistakes:

• Using 'memory' copies data, not reference
• Assuming .copy() method exists
• Trying to use undefined copy functions

3. Consider this Solidity code snippet:

uint[] storage arr1 = someArray;
uint[] storage arr2 = arr1;
arr2.push(10);
uint length = arr1.length;

What is the value of length after execution?

medium

A. Length is unchanged from original someArray length.

B. Length increases by 1 due to push on arr2.

C. Length decreases by 1 due to push on arr2.

D. Code causes a compilation error.

Solution

1. Step 1: Understand reference assignment

   arr2 is assigned as a storage reference to arr1, so both point to the same array.

2. Step 2: Effect of push on arr2

   Pushing 10 to arr2 modifies the shared array, increasing its length by 1.

3. Final Answer:

   Length increases by 1 due to push on arr2. -> Option B

4. Quick Check:

   Shared reference means push affects both = C [OK]

Hint: Changes via one reference affect all references [OK]

Common Mistakes:

• Thinking arr1 length stays same
• Assuming arr2 is a copy
• Ignoring reference sharing behavior

4. Identify the error in this Solidity code snippet:

function updateArray(uint[] memory arr) public {
  uint[] storage ref = arr;
  ref.push(5);
}

medium

A. Cannot assign memory array to storage reference directly.

B. Cannot push to a storage array.

C. Function must be declared view.

D. No error; code runs fine.

Solution

1. Step 1: Check assignment between memory and storage

   In Solidity, you cannot assign a memory array directly to a storage reference variable.

2. Step 2: Understand push operation

   Push is allowed on storage arrays, but assignment error occurs before push.

3. Final Answer:

   Cannot assign memory array to storage reference directly. -> Option A

4. Quick Check:

   Memory to storage assignment invalid = A [OK]

Hint: Memory and storage types cannot be assigned directly [OK]

Common Mistakes:

• Assuming memory can be assigned to storage
• Thinking push is disallowed on storage
• Ignoring data location mismatch

5. You want to create a function in Solidity that returns a modified copy of a storage array without changing the original. Which approach correctly uses reference types to achieve this?

hard

A. Create a new memory array, copy elements from storage, modify it, then return.

B. Assign storage array to a storage reference, modify it, then return.

C. Return the storage array directly after modifying it.

D. Use a global variable to store the modified array.

Solution

1. Step 1: Understand data locations for copying

   To avoid changing original storage array, create a new memory array and copy elements.

2. Step 2: Modify and return the memory array

   Modify the memory array safely and return it without affecting storage data.

3. Final Answer:

   Create a new memory array, copy elements from storage, modify it, then return. -> Option A

4. Quick Check:

   Use memory copy to avoid changing storage = D [OK]

Hint: Copy to memory, modify, then return to keep original safe [OK]

Common Mistakes:

• Modifying storage reference directly
• Returning storage array after modification
• Using global variables unnecessarily