Solidity

Question 1

How to create a mapping

Solidity

Answer 1

mapping(_KeyType => _ValueType) myMapping;

Question 2

What are modifiers

Answer 2

Modifiers can automatically check a condition prior to executing the function.

function close() onlyOwner {
    selfdestruct(owner);
}

Question 3

Types of functions

Answer 3

View or Constant Functions
- Functions can be declared view or constant in which case they promise not to modify the state, but can read from them.

Pure Functions
- Functions can be declared pure in which case they promise not to read from or modify the state.
- Operate on argument only.

Payable Functions
- Functions that receive Ether are marked as payable function.

Question 4

Variable storage options

Answer 4

storage
- variable is a state variable (store on blockchain).

memory
- variable is in memory and it exists while a function is being called.
- Allocated by the callee and their value can be modified inside the function (they’re mutable).
- You can declare a variable inside a function memory located as well as it’s parameters.

calldata
- special data location that contains function arguments, only available for external functions.
- Value is allocated by the caller.
- No variables declared inside the function and no parameters of a function which is not external.

Question 5

What are events

Answer 5

• EVM logging facilities
• Can be used to “call” JavaScript callbacks in the user interface of a dapp, which listen for these events.

Question 6

Describe error handling

Answer 6

• assert(bool condition): throws if the condition is not met - to be used for internal errors.
• require(bool condition): throws if the condition is not met - to be used for errors in inputs or external components.
• revert(): abort execution and revert state changes