Storage layout in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is storage layout in blockchain smart contracts?

Storage layout is how data is organized and stored in the blockchain's persistent storage for a smart contract. It defines where each variable is saved so the contract can access it later.

intermediate

Why is understanding storage layout important when upgrading smart contracts?

Because changing the order or type of variables can corrupt stored data. Proper storage layout ensures new versions read and write data correctly without breaking existing state.

intermediate

How are state variables stored in Ethereum smart contracts?

Each state variable is stored in a 32-byte slot. Variables declared first use lower slot numbers. Complex types like arrays or mappings use hashing to find their storage location.

intermediate

What happens if you add a new variable in the middle of existing variables in a contract?

It shifts the storage slots of variables declared after it, causing data corruption. New variables should be added only at the end to keep the layout consistent.

advanced

Explain how mapping types are stored in storage layout.

Mappings do not store data sequentially. Instead, each key-value pair is stored at a slot calculated by hashing the key with the mapping's slot number. This allows efficient lookups.

In Ethereum, where is the first declared state variable stored?

A Slot 0

B Slot 1

C Slot 32

D Slot 255

The first declared state variable is stored in slot 0, the starting point of storage slots.

What is the risk of changing the order of variables in an upgraded contract?

A Data corruption due to slot mismatch

B No risk, storage adapts automatically

C Faster contract execution

D Lower gas costs

Changing variable order shifts storage slots, causing data corruption.

How are mapping values located in storage?

A They are not stored

B Sequential slots starting from 0

C In a separate database

D By hashing the key with the mapping slot

Mapping values are stored at slots computed by hashing the key with the mapping's slot number.

Where should you add new variables to avoid breaking storage layout?

A At the beginning of the contract

B In the middle of existing variables

C At the end of existing variables

D Anywhere, it doesn't matter

Adding variables at the end preserves existing storage slots and prevents corruption.

What size is each storage slot in Ethereum?

A 16 bytes

B 32 bytes

C 64 bytes

D 128 bytes

Each storage slot in Ethereum is 32 bytes.

Describe how storage layout affects smart contract upgrade safety.

Think about what happens if variables move around in storage.

Explain how mappings are stored differently from simple variables in storage layout.

Consider how keys help find values in storage.