Solidity Solidity Interfaces, ABIs, and Events

Question 1

What is an interface in Solidity and what restrictions does it have?

Accepted Answer

A contract-like definition with only function signatures and no implementation, state variables, or constructors

Answer

Interfaces declare function signatures (no bodies), cannot have state variables or constructors, and all functions must be external — they define a contract's API without implementation.

Question 2

What is the Solidity ABI (Application Binary Interface) used for?

Accepted Answer

It defines how to encode/decode function calls and return values between contracts and external callers

Answer

The ABI specifies the encoding format for function inputs/outputs (function selectors, parameter types, return types), enabling external tools and contracts to call functions correctly.

Question 3

How is a function selector computed in Solidity?

Accepted Answer

It is the first 4 bytes of the keccak256 hash of the function's signature string (e.g., 'transfer(address,uint256)')

Answer

A function selector is `bytes4(keccak256('functionName(type1,type2,...)'))` — the canonical signature string is hashed and the first 4 bytes are used as a compact identifier.

Question 4

What does `abi.encode` do differently from `abi.encodePacked`?

Accepted Answer

abi.encode pads each value to 32 bytes with standard ABI encoding; abi.encodePacked concatenates values without padding

Answer

`abi.encode` uses standard ABI encoding (32-byte aligned, with dynamic type offsets) suitable for external calls; `abi.encodePacked` produces compact concatenated bytes suitable for hashing.

Question 5

What is the risk of hash collisions when using `abi.encodePacked` with multiple dynamic types?

Accepted Answer

Two different sets of inputs can produce the same packed encoding (e.g., ['aa','b'] and ['a','ab'])

Answer

Because `abi.encodePacked` omits length prefixes for dynamic types, different combinations of strings/bytes can produce identical byte sequences, enabling hash collision attacks.

Solidity Practice Test

Solidity Practice Test

Solidity Solidity Interfaces, ABIs, and Events