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

Indexed parameters are stored in contract storage for permanent access

Answer

Indexed parameters reduce the gas cost of emitting the event

Answer

Indexed parameters allow events to be called like functions

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

4 (all four EVM log topics)

Answer

1 (only the first parameter can be indexed)

Answer

Unlimited (all parameters can be indexed)

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

The contract address that emitted the event

Answer

The block number when the event was emitted

Answer

The first indexed parameter of the event

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

Yes, using the eth_getLogs opcode inside a contract

Answer

Yes, but only events from the same contract

Answer

Yes, using the LOGREAD opcode in assembly

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

It sends ETH to the event listener addresses

Answer

It writes event data to contract storage

Answer

It broadcasts the event to all connected nodes immediately

Question 6

What is the gas cost structure for emitting a Solidity event?

Accepted Answer

375 base gas + 375 per topic + 8 gas per byte of non-indexed data

Answer

5,000 gas per event regardless of size

Answer

21,000 gas base + 200 gas per parameter

Answer

Events are free and do not consume gas