Mastering Blockchain Programming with Solidity

The allowed variable stores approved balances

The allowed state variable is defined in the contract as follows:

mapping (address => mapping (address => uint256)) internal allowed;

allowed is a nested mapping of two Solidity mapping data types. In the ERC20 standard specification, it is possible for a token holder, X, to assign some allowances to another account, Y, so that Y is allowed to take the approved number of tokens from the token balance of X:

Sample approval flow

As in the preceding diagram, Owner has 100 TKN ERC20 tokens, out of which he has given approval of 50 TKN to Approved person. At any time the Approved person can withdraw ...