General Solidity I

Question 1

What is a mapping variable

Answer 1

A mapping is a data structure in solidity that associates a key with a value. Mappings can be used to store and retrieve data efficiently in smart contracts on the blockchain.

Question 2

What is a struct

Answer 2

A struct is a user-defined data type that groups together variables of different data under the same name.
Struct Guns {
Bool Ak47;
Uint shots fired;
}

Question 3

What is the difference between a uint and an int?

Answer 3

Unassigned intergers can only store positive values while signed intergers can store both a negative and positive value.

Question 4

What is the visibility of public functions/variables in solidity?

Answer 4

Public functions/variables can be accessed externally or internally.

Question 5

What is the visibility of internal functions/variables in solidity?

Answer 5

Internal functions/variables can be accessed internally or by derived/inherited contracts.

Question 6

In solidity, what is a transaction? (writing)

Answer 6

A transaction is a message sent from one contract or account to another that modifies the state of the blockchain.

Question 7

In solidity, what is a call? (Reading)

Answer 7

A call is a message sent from one smart contract to another that does not modify the state.

Question 8

What is a “receive” function?

Answer 8

A recieve function is a payable function that is used to send value with no data attached.
recieve() external payable {

Question 9

What is “unchecked {“ in solidity

Answer 9

It is used to disable run time checks for mathematical operations (over/underflow). Less relevant in new versions where patched.

Question 10

What is over/under flow in solidity

Answer 10

Over/under flow was a feature in older versions of solidity that allowed numbers to wrap around after breaching their maximum/minimum value. It has uses, such as saving on gas in certain cases, but can also open contracts up to malicious attacks. In versions 0.8+, you can activate this feature by entering in “unchecked {“.

Question 11

What is a “require” statement inside of solidity

Answer 11

A conditional statement that requires something to be true or it will revert, throw an error, and can add a string to let the user understand why the transaction failed. If triggered, the user will only lose the gas used up to the requirement.

Question 12

What is an invariant violation in solidity?

Answer 12

An invariant violation occurs when a condition that should always be true is not true. For example, if a contract assumes that a certain variable will always have a value within a certain range, if it ends up somehow outside of that range, this would be an invariant violation.

Question 13

What is assert in solidity, and when should it be used?

Answer 13

Assert checks for a condition to be true. If it is false, it will revert all changes made to the state of the contract. Unlike require, it will not return any gas. Assert would best be used to check for internal errors or invariant violations. It should not be used in any situation where it can be accidentally triggered. If we are setting parameters to validate user inputs or set function preconditions, we should use require instead.

Question 14

When does Solidity automatically generate a getter function for a mapping variable?

Answer 14

When a mapping is declared to be public. If our mapping is private or internal, we must make our own. The automatically generated getter function will have the same name as the original mapping variable.
Ex: mapping(address => unit) public balances;
•The getter function generated would be called “balances” and take an address argument.

Question 15

How can we access specific variables from a struct?

Answer 15

As an example:
struct Funder {
address addr;
uint amount;
}
We could then say:
Funder public _funder;
_funder.addr = 0x123
_funder.amount= 50000
(_funder.var inside of function tho)

Question 16

What are instances in solidity

Answer 16

Instances are new instances of a contract with their own state and behavior/address added to the blockchain and interacted by, with other contracts or external users.
Typically utilizing the “new” keyword.

Question 17

What does changing the state mean?

Answer 17

In general, it refers to modifying the data stored on the blockchain. It costs more gas than reading from the chain.

Question 18

What is the difference between revert and require?

Answer 18

A require statement will return all gas not used after the require statement. A revert statement will consume all gas before and after.

Question 19

What are the differences between EOA’s and contract accounts?

Answer 19

•EOA’s are regular externally owned accounts controlled by a private key. EOA’s have a balance of eth and can initiate transactions on their own.
•Contract accounts are special accounts associated with a smart contract. They could be the owner of a token, for example, who might use this for the obvious added functionality. Or normal people who want to trade on a DEX.

Question 20

How can we tell the difference between an EOA and a smart contract address?

Answer 20

•EOA addresses will always start with a non-zero character. •Smart contract addresses may start with a 0 but do not have to. •In practice, when interacting with the eth network, most wallets/tools can automatically distinguish between the two. Some wallets and blockchain explorers may also provide additional relevant details to help identify the address type.

Question 21

What is the gas price in solidity? How does this affect a transaction?

Answer 21

The gas price is how much ether you are willing to spend to process a transaction. The more transactions being processed and the number of available miners at any given time will also affect the gas price. The more gas you are willing to pay to process a transaction, the quicker it will go through.

Question 22

Why do writing transactions not return a value? How can we still include a return value?

Answer 22

Writing transactions do not return a value because their transactions are not instantaneous. Considering confirmation time on a transaction may vary on network activity and other factors, it is difficult to know exactly when a transaction will be confirmed, or if it will simpily fail. We can still get a return value externally by using and emitting events. This will log the data where we can access the values elsewhere.

Question 23

What are the differences between ERCs and EIPs regarding Ethereum?

Answer 23

Ethereum Request for Comments is a formalized process for suggesting token standards to ensure interoperabillity/compatabillity with other Eth contracts/wallets.
Ethereum Improvement Proposals are proposals for Improving Ethereum protocal itself. Outlining changes/improvements/features at the protocal level to improve aspects of the Eth network itself.

Question 24

What is the difference between an “ownable” token contract and a token contract with “roles”?

Answer 24

Ownable contracts allow us to specify a single owner address with special privileges and access to certain restricted operations (IE: Transferring/Renouncing ownership)
Token contracts with roles, allow us to specify multiple different addresses to specific tasks/roles. We could declare a “minter” role to an address for instance, who has the abillity to mint new tokens.

Question 25

What are Open Zeppelin audited smart contracts? What are their purpose?

Answer 25

Open Zeppelin audited smart contracts essentially amount to reuseable smart contracts that have undergone an extensive security audit by independent experts. These reuseable smart contracts offer bug/exploit free code developers can use while developing their own project.

Question 26

What are ERC721 tokens?

Answer 26

The token standard for Non-Fungible Tokens (NFTs) where unlike an ERC20 token each ERC721 token is unique. Allowing each token in the contract to represent something specific.

Question 27

What are ERC20 tokens

Answer 27

Technical standard for implementing tokens on the Eth blockchain formalized in 2015. The ERC20 token was the 20th Ethereum Request for Comments introduced by a team of devs to encourage token interoperabillity/compatibillity across eth contracts/wallets. Each token is identical and the token is treated like it’s own entity running off the Ethereum network.

Question 28

What is a “library” for in solidity?

Answer 28

A seperate type of contract inside of a contract, that is internal in visibillity and stateless in nature. The functions inside of a library (Ex: SafeMath) .add + .sub, act as reusable bits of code, that can be reused in inherited contracts in the same/seperate file. While internal functions can do this also, Libraries save on gas in that, they are deployed seperately from the “parent contract” and are only ever deployed once. The code is never duplicated even when inherited in a new file.

Question 29

What are named imports, and how are they written?

Answer 29

Named imports allow us to import specific smart contracts from a file.
* import {contract1} from “src/groupOfContracts.sol”;

Question 30

What are ERC1155 contracts?

Answer 30

A token structure allowing developers to mint a theoretically unlimited amount of fungible and NFTs within the same contract.