Solidity Solidity Interfaces, ABIs, and Events 2

Question 1

What is the purpose of `indexed` parameters in Solidity events?

Accepted Answer

Indexed parameters are stored in log topics, enabling fast filtering of events by that value using eth_getLogs

Answer

Up to 3 parameters per event can be `indexed`; they are stored as log topics (not in the data field) and can be efficiently filtered by Ethereum nodes.

Question 2

How many `indexed` parameters can a non-anonymous Solidity event have?

Accepted Answer

3 (since the first topic is reserved for the event signature hash)

Answer

EVM logs have 4 topic slots; one is used for the event signature keccak256 hash, leaving 3 for indexed parameters in non-anonymous events.

Question 3

What is the topic0 in an Ethereum event log?

Accepted Answer

The keccak256 hash of the event signature (e.g., keccak256('Transfer(address,address,uint256)'))

Answer

Topic0 is always the keccak256 hash of the event's canonical signature string, used to identify which event type was emitted regardless of which contract emitted it.

Question 4

Can Solidity events be read by other smart contracts during execution?

Accepted Answer

No, events are write-only from the EVM's perspective and can only be read off-chain via node APIs

Answer

Event logs are stored in the transaction receipt, not in contract storage or memory accessible to the EVM — they are only readable off-chain via JSON-RPC or indexers like The Graph.

Question 5

What does the `emit` keyword do in Solidity and when was it made mandatory?

Accepted Answer

It triggers event emission; the keyword was made mandatory in Solidity 0.4.21 to distinguish events from function calls

Answer

The `emit` keyword explicitly marks event emission statements, added in 0.4.21 to improve code readability by visually distinguishing events from function calls.

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Solidity Solidity Interfaces, ABIs, and Events 2