Storage layout in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Storage slot assignment in Solidity structs

Consider the following Solidity contract snippet. What will be the storage slot of the variable c inside the struct Data?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract StorageTest {
    struct Data {
        uint128 a;
        uint128 b;
        uint256 c;
    }

    Data public data;
}

A Slot 0

B Slot 2

C Slot 1

D Slot 3

💡 Hint

Remember that Solidity packs variables into 32-byte slots when possible.

The struct has two uint128 variables (a and b) which fit together into slot 0. The uint256 variable c requires a full slot and is stored in slot 1.

❓ Predict Output

intermediate

Storage layout of dynamic arrays in Solidity

Given the following Solidity contract, what is the storage slot of the first element of the dynamic array arr?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ArrayStorage {
    uint256[] public arr;
}

A Slot 1

B keccak256(0)

C Slot 0

D keccak256(1)

💡 Hint

Dynamic arrays store their length at the declared slot, elements start at keccak256(slot).

The array length is stored at slot 0. The elements start at the slot computed by keccak256(0).

🔧 Debug

advanced

Identify storage collision in upgradeable contracts

You have two versions of a Solidity contract. Version 1 has variables uint256 x and uint256 y. Version 2 adds a new variable uint256 z before y. What problem arises with this change?

A Storage collision causing <code>y</code> to overwrite <code>z</code>

B No problem, variables are automatically reordered

C Compiler error due to variable reordering

D Storage collision causing <code>z</code> to overwrite <code>y</code>

💡 Hint

Changing variable order in upgradeable contracts can cause storage slot mismatches.

Adding z before y shifts y to a new slot, but existing storage still expects y at the old slot, causing z to overwrite y data.

🧠 Conceptual

advanced

Storage layout of mappings in Solidity

How does Solidity compute the storage slot for a value in a mapping like mapping(address => uint256) balances stored at slot 3?

A keccak256(abi.encodePacked(key, 3))

B Slot 3 + key

C keccak256(abi.encode(3, key))

D Slot 3 * key

💡 Hint

Mapping storage uses keccak256 hash of the key concatenated with the slot.

Solidity stores mapping values at keccak256 of the concatenation of the key and the mapping's slot number.

❓ Predict Output

expert

Storage layout with inheritance and variable shadowing

Consider these Solidity contracts:

pragma solidity ^0.8.0;

contract A {
    uint256 public x = 1;
}

contract B is A {
    uint256 public x = 2;
}

What is the value of B.x() when called?

A 2

B 1

C Compiler error due to variable shadowing

D 0

💡 Hint

Derived contracts can shadow base contract variables, but the derived variable is accessed.

Contract B declares its own x, which shadows A.x. Calling B.x() returns 2.