I am studying popular Ethereum smartcontract's source code and there is something i do not understand on ERC20 standard.
Let's took this example:
Here is a portion of this smartcontract's solidity source code:
mapping (address => uint256) private _balances; uint256 private _totalSupply;
As you can see, the total supply is stored on a separate variable. This total supply is the sum of all the balances.
What i've learned about Solidity optimization is we should give priority to storage rather than calculation.
If i had to write an ERC20 contract, i would write a view function which sums _balances in order to provide total supply. Why ? A view function can sum balances for free gas because it does not write anything on blockchain. If we have a _totalSupply variable, we have to update it each time one balance change. So it will cost some gas to write this variable.
My question is: Why everybody puts a _totalSupply variable on ERC20 contracts, rather than a view function which sums balances ?