Storage vs memory vs calldata in Blockchain / Solidity

Introduction

In blockchain programming, especially in smart contracts, you need to know where data is kept. Storage, memory, and calldata are places to store data, each with different uses and costs.

When you want to save data permanently on the blockchain.

When you need temporary data during a function call.

When you want to pass data to a function without changing it.

Syntax

Blockchain / Solidity

function example(uint[] memory tempArray, uint[] calldata inputArray) public {
    uint[] storage savedArray;
    // Use tempArray, inputArray, and savedArray here
}

storage means data is saved permanently on the blockchain.

memory means data is temporary and only exists during the function call.

calldata is a special read-only area for function inputs, cheaper than memory.

Examples

This declares a storage array that keeps data permanently.

Blockchain / Solidity

uint[] storage myStorageArray; // Permanent data stored on blockchain

This creates a temporary array that exists only during the function call.

Blockchain / Solidity

function tempData() public {
    uint[] memory tempArray = new uint[](5); // Temporary data
}

This function takes input data that cannot be changed and is cheaper to access.

Blockchain / Solidity

function readOnly(uint[] calldata input) external {
    // input is read-only and cheaper to use
}

Sample Program

This smart contract shows how to use storage, memory, and calldata. The saveData function takes input from calldata and saves it permanently in storage. The getTempSum function copies storage data to memory to calculate the sum without changing the stored data.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract DataExample {
    uint[] public storedData; // storage

    function saveData(uint[] calldata inputData) external {
        // Clear old data
        delete storedData;
        // Copy inputData from calldata to storage
        for (uint i = 0; i < inputData.length; i++) {
            storedData.push(inputData[i]);
        }
    }

    function getTempSum() public view returns (uint) {
        uint[] memory tempArray = storedData; // copy storage to memory
        uint sum = 0;
        for (uint i = 0; i < tempArray.length; i++) {
            sum += tempArray[i];
        }
        return sum;
    }
}

Important Notes

Storage is expensive because it writes data permanently on the blockchain.

Memory is cheaper and temporary, good for calculations inside functions.

Calldata is the cheapest for input data but is read-only.

Summary

Storage keeps data permanently on the blockchain.

Memory is temporary and used inside functions.

Calldata is read-only input data, cheaper than memory.

Practice

1. Which of the following correctly describes storage in Solidity?

easy

A. It stores data permanently on the blockchain.

B. It holds temporary data only during function execution.

C. It is used for read-only input data in external functions.

D. It is a type of event logging mechanism.

Solution

1. Step 1: Understand storage role

   storage variables are saved permanently on the blockchain and persist between function calls.

2. Step 2: Compare with memory and calldata

   memory is temporary during function execution, and calldata is read-only input data for external functions.

3. Final Answer:

   It stores data permanently on the blockchain. -> Option A

4. Quick Check:

   Storage = permanent data [OK]

Hint: Storage is permanent, memory is temporary, calldata is input [OK]

Common Mistakes:

• Confusing storage with memory as temporary
• Thinking calldata can be modified
• Assuming storage is read-only

2. Which keyword is used to declare a function parameter that is read-only and passed from an external caller?

easy

A. calldata

B. storage

C. memory

D. event

Solution

1. Step 1: Identify parameter data location for external input

   Function parameters from external callers that should not be modified use calldata.

2. Step 2: Recall other data locations

   memory is for temporary data inside functions, storage is persistent, and event is unrelated to data location.

3. Final Answer:

   calldata -> Option A

4. Quick Check:

   External input = calldata [OK]

Hint: External read-only params use calldata [OK]

Common Mistakes:

• Using memory for external parameters
• Confusing event with data location
• Using storage for function parameters

3. What will be the output of this Solidity function when called externally with _data = [1, 2, 3]?

function process(uint[] calldata _data) external pure returns (uint) {
    uint sum = 0;
    for (uint i = 0; i < _data.length; i++) {
        sum += _data[i];
    }
    return sum;
}

medium

A. Runtime error

B. 6

C. Compilation error

D. 0

Solution

1. Step 1: Understand calldata usage

   The function receives an array _data as calldata, which is read-only but accessible for reading elements.

2. Step 2: Calculate sum of array elements

   Sum of [1, 2, 3] is 1 + 2 + 3 = 6, which is returned.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   Sum of input array = 6 [OK]

Hint: Sum array elements from calldata works like memory [OK]

Common Mistakes:

• Thinking calldata cannot be read
• Expecting compilation error due to calldata
• Confusing sum calculation logic

4. Identify the error in this Solidity function:

function updateData(uint[] calldata _input) external {
    _input[0] = 10;
}

medium

A. Calldata arrays cannot be passed as parameters.

B. Function must be marked pure.

C. Missing return statement.

D. Cannot assign to calldata array elements.

Solution

1. Step 1: Check mutability of calldata

   calldata is read-only, so assigning to _input[0] causes an error.

2. Step 2: Validate other options

   The function can be non-pure, no return needed, and calldata arrays are allowed as parameters.

3. Final Answer:

   Cannot assign to calldata array elements. -> Option D

4. Quick Check:

   Calldata is read-only [OK]

Hint: Calldata arrays are read-only, no assignment allowed [OK]

Common Mistakes:

• Trying to modify calldata elements
• Thinking pure is required here
• Assuming return is mandatory

5. You want to write a Solidity function that accepts a large array from an external caller, processes it temporarily, and then stores the result permanently. Which combination of data locations should you use for the parameter and the storage variable?

hard

A. Parameter as memory, storage variable as calldata

B. Parameter as storage, storage variable as memory

C. Parameter as calldata, storage variable as storage

D. Parameter as calldata, storage variable as memory

Solution

1. Step 1: Choose parameter data location for large external input

   calldata is efficient and read-only for large external arrays, avoiding unnecessary copying.

2. Step 2: Choose storage variable for permanent data

   To keep data permanently on blockchain, use storage for the variable that stores the processed result.

3. Final Answer:

   Parameter as calldata, storage variable as storage -> Option C

4. Quick Check:

   External input = calldata, permanent store = storage [OK]

Hint: Use calldata for input, storage for permanent save [OK]

Common Mistakes:

• Using memory for large external input (inefficient)
• Trying to store permanent data in memory or calldata
• Confusing storage and memory roles