Token Standards and Creation Flashcards
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Read the first 20 Token Standards and Creation flashcards as text
Which ERC-20 function returns the number of tokens a spender is still allowed to withdraw from an owner's account?
Answer: allowance(owner, spender)
allowance(owner, spender) returns the remaining amount the spender is permitted to withdraw.
In ERC-20, what is the well-known race-condition risk associated with the approve() function?
Answer: A spender can front-run a changed approval and spend both the old and new amounts
Changing an existing non-zero allowance lets a spender front-run and use both old and new amounts.
What does the ERC-20 decimals() function actually control?
Answer: Only the display formatting of token amounts off-chain
decimals() is purely informational for UI display; the contract stores integer values regardless.
Which event must an ERC-20 token emit when tokens move between addresses?
Answer: Transfer(from, to, value)
The Transfer event is required on every token movement, including minting (from the zero address).
Why is it best practice to emit a Transfer event from the zero address when minting ERC-20 tokens?
Answer: So off-chain indexers can track supply changes consistently
Emitting Transfer from address(0) lets explorers and indexers treat minting as a standard balance change.
What problem does the ERC-2612 'permit' extension solve for ERC-20 tokens?
Answer: It allows gasless approvals via off-chain signed messages
ERC-2612 permit lets users approve spenders with a signature, removing the need for a separate approve transaction.
If an ERC-20 transfer() returns false instead of reverting, what is the recommended way to handle it safely in a calling contract?
Answer: Use a SafeERC20 wrapper that checks the return value and reverts on failure
SafeERC20 wrappers verify the boolean return and revert, handling non-compliant tokens correctly.
What is the key difference between ERC-721 and ERC-1155 token standards?
Answer: ERC-1155 supports both fungible and non-fungible tokens in one contract, ERC-721 only non-fungible
ERC-1155 is a multi-token standard handling fungible, non-fungible, and semi-fungible tokens together.
Which ERC-721 function returns the owner of a specific token ID?
Answer: ownerOf(tokenId)
ownerOf(tokenId) returns the address that currently owns the given NFT.
What is the primary purpose of the ERC-721 tokenURI() function?
Answer: Return a URI pointing to the token's JSON metadata
tokenURI() returns a link to metadata describing the NFT, often including name, image, and attributes.
Why does ERC-1155 enable significant gas savings for transferring many tokens?
Answer: It supports batch operations like safeBatchTransferFrom in a single transaction
Batch transfer functions let multiple token types and amounts move in one transaction, reducing overhead.
What safety check distinguishes safeTransferFrom from transferFrom in ERC-721?
Answer: It verifies the recipient contract implements onERC721Received
safeTransferFrom calls onERC721Received to ensure contract recipients can handle NFTs, preventing locked tokens.
In ERC-1155 metadata, what does the {id} substitution placeholder in a URI represent?
Answer: The lowercase hex token ID padded to 64 characters
Clients replace {id} with the 64-character zero-padded lowercase hex token ID to fetch per-token metadata.
Which interface must a contract implement to be recognized as ERC-721 compliant via ERC-165?
Answer: supportsInterface returning true for the ERC-721 interface ID
ERC-165's supportsInterface lets contracts advertise that they implement the ERC-721 interface ID.
When deploying a token contract, why is using OpenZeppelin's audited contracts generally preferred over writing from scratch?
Answer: They reduce the risk of security bugs through community-reviewed, tested code
Audited libraries lower the chance of introducing vulnerabilities compared to custom implementations.
What is a 'mintable' token, and what access-control risk does it introduce?
Answer: A token whose supply can be increased; the minter role must be tightly controlled to prevent inflation abuse
Mintable tokens allow new supply creation, so unrestricted minting permissions can be exploited to dilute holders.
What does making a token 'burnable' allow?
Answer: Holders or authorized roles to permanently destroy tokens, reducing total supply
Burning sends tokens out of circulation (often to address zero), decreasing totalSupply.
Why might a token contract include a 'pausable' feature?
Answer: To halt transfers during an emergency or exploit, buying time to respond
Pausable lets an admin freeze token activity temporarily if a vulnerability or attack is detected.
What is the main trade-off of deploying an upgradeable (proxy-based) token contract?
Answer: It adds flexibility but introduces centralization and proxy-related security risks
Upgradeable proxies allow logic changes but rely on an admin and add storage-layout and trust risks.
During token creation, what does setting an initial supply in the constructor and minting it to the deployer accomplish?
Answer: It allocates the entire starting supply to the deployer's address at deployment
Constructor minting credits the deployer with the initial supply, which can then be distributed.
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