What is approve and allowance method

Question

I have this following code that I saw in an example and I am trying to understanding what it does. I came across two methods approve and allowance and I am not understanding what it does. Can you explain?

The code is as follows:

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

approve(): This method is used by one user to give permission to use some of its tokens by another user.

Suppose there are two users A and B. A has 100 tokens with it and wants to let B use 50 tokens from its balance(balance of A:100). So A uses the approve method to approve B to use 50 tokens from it.

When A calls approve(address(B), 50) , B is allowed to use 50 tokens from 100 tokens of user A.

allowance(): This method is used by an user to check how many tokens some other user has allowed it to use.

B calls allowance(address(A), address(B)) to know how many tokens has A allowed B to use (from A’s tokens). In this case, the result will be 50.