Unfamiliar data types

Value types

Mapping

• Key-value map implemented as a hash table
  • so it is NOT sorted → cannot iterate through it unlike a map in C++
  • else use an array

mapping(_KeyType => _ValueType)

• _KeyType − can be any built-in types plus bytes and string. No reference type or complex objects are allowed.
• _ValueType − can be any type.

Address

• Holds the 20 byte value representing public key of an Ethereum address
• address.balance(), address.transfer() functions
• address payable : Same as address, but with the additional members transfer  and send
  • address payable is an address you can send Ether to, while you are not supposed to send Ether to a plain address, for example because it might be a smart contract that was not built to accept Ether

address payable x = 0x212...;
address myAddress = this;
if (x.balance < 10 && myAddress.balance >= 10) x.transfer(10);

Bytes

• byte = 8 bits of data
• can use bytes1 - bytes32 in Solidity
• often used as alternatives to string in Solidity to save gas (but both exist)