what approve and allowance methods are really doing in ERC20 Standard?

Question

The problem is what allowance and approve are really doing?

And what is _spender and what is it doing?

Is there anybody who can explain it to me?

contract Token {
    uint256 public totalSupply;
    function balanceOf(address _owner) constant returns (uint256 balance);
    function transfer(address _to, uint256 value) returns (bool success);
    function transferFrom(address _from, address _to, uint256 value) returns (bool success);
    function approve(address _spender, uint256 _value) returns (bool success);
    function allowance(address _owner, address _spender) constant returns (uint256 remaining);
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

Answer 1

what allowance and approve are doing really?

Let's assume we have user A and user B. A has 1000 tokens and want to give permission to B to spend 100 of them.

• A will call approve(address(B), 100, {"from": address(A)})
• B will check how many tokens A gave him permission to use by calling: allowance(address(A), address(B))
• B will send to his account these tokens by calling: transferFrom(address(A), address(B), 100, {"from": address(B)})