Why does a mapping update itself when I am not doing anything?

Question

The title is a little bit vague but I've seen this contract and tried it on remix:

pragma solidity ^0.4.24;

contract Tester {

  struct Account {
    uint256 balance; 
  }

  mapping (address => Account) public accounts;

  function addBalance() public payable {
    Account storage a = accounts[msg.sender];
    a.balance = a.balance + msg.value;

  }


}

What happens is the following: 1. I check the value in the mapping and it's 0 2. I execute the addBalance function sending 10 wei 3. I check the value in the mapping and I see it's 10

How is that possible ? I am not updating the mapping.

Answer 1

This line:

Account storage a = accounts[msg.sender];

...declares a storage pointer, which points into the mapping. Therefore, any change you make to a is reflected in the mapping.

This is different to a normal "storage variable", which is just a variable that persists in the contract's state (as opposed to a local/memory variable, which does not).

---

Edit:

Can't immediately find anything useful in the documentation, but there's this, written by Rob: Storage Pointers in Solidity