Mapping is a reference type as arrays and structs. Following is the syntax to declare a mapping type.

mapping(_KeyType => _ValueType)

Where

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

• _ValueType − can be any type.

Considerations

• Mapping can only have type of storage and are generally used for state variables.

• Mapping can be marked public. Solidity automatically create getter for it.

Example

Try the following code to understand how the mapping type works in Solidity.

pragma solidity ^0.5.0;

contract LedgerBalance {
   mapping(address => uint) public balances;

   function updateBalance(uint newBalance) public {
      balances[msg.sender] = newBalance;
   }
}
contract Updater {
   function updateBalance() public returns (uint) {
      LedgerBalance ledgerBalance = new LedgerBalance();
      ledgerBalance.updateBalance(10);
      return ledgerBalance.balances(address(this));
   }
}

Run the above program using steps provided in Solidity First Application chapter.

First Click updateBalance Button to set the value as 10 then look into the logs which will show the decoded output as −

Output

{
   "0": "uint256: 10"
}