What is Ethereum's ERC20 standard token

In the final analysis, everyone agrees on the names of variables and functions of a piece of code and what they represent.

The core of this standard is the following parts

1. totalSupply [represents the total amount of tokens]
2. balanceOf [This is an array, the key of the array is the address, and the value is the number of tokens held by this address]
3. transfer(address _to, uint256 _value) returns (bool success) [Determine whether the number to send is correct]
4. transferFrom(address _from, address _to, uint256 _value) returns (bool success)[Initiate transaction]

event:

1. Transfer(address indexed _from, address indexed _to, uint256 _value). [Called when a transaction is initiated]

 

The following is the code of XXXChain, referred to as XC, with a total amount of 100 million

pragma solidity ^0.4.8;
contract tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData); }

contract XXXChain {
    /* Public variables of the token */
    string public standard = 'XXXChain 0.1';
    string public name;
    string public symbol;
    uint8 public decimals;
    uint256 public totalSupply;

    /* This creates an array with all balances */
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    /* This generates a public event on the blockchain that will notify clients */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /* This notifies clients about the amount burnt */
    event Burn(address indexed from, uint256 value);

    /* Initializes contract with initial supply tokens to the creator of the contract */
    function GlobalBusinessChain() {
        balanceOf[msg.sender] =  100000000 * 1000000000000000000;              // Give the creator all initial tokens
        totalSupply =  100000000 * 1000000000000000000;                        // Update total supply
        name = "XXXChain";                                   // Set the name for display purposes
        symbol = "XC";                               // Set the symbol for display purposes
        decimals = 18;                            // Amount of decimals for display purposes
    }

    /* Send coins */
    function transfer(address _to, uint256 _value) {
        if (_to == 0x0) throw;                               // Prevent transfer to 0x0 address. Use burn() instead
        if (balanceOf[msg.sender] < _value) throw;           // Check if the sender has enough
        if (balanceOf[_to] + _value < balanceOf[_to]) throw; // Check for overflows
        balanceOf[msg.sender] -= _value;                     // Subtract from the sender
        balanceOf[_to] += _value;                            // Add the same to the recipient
        Transfer(msg.sender, _to, _value);                   // Notify anyone listening that this transfer took place
    }

    /* Allow another contract to spend some tokens in your behalf */
    function approve(address _spender, uint256 _value)
        returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }

    /* Approve and then communicate the approved contract in a single tx */
    function approveAndCall(address _spender, uint256 _value, bytes _extraData)
        returns (bool success) {
        tokenRecipient spender = tokenRecipient(_spender);
        if (approve(_spender, _value)) {
            spender.receiveApproval(msg.sender, _value, this, _extraData);
            return true;
        }
    }

    /* A contract attempts to get the coins */
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
        if (_to == 0x0) throw;                                // Prevent transfer to 0x0 address. Use burn() instead
        if (balanceOf[_from] < _value) throw;                 // Check if the sender has enough
        if (balanceOf[_to] + _value < balanceOf[_to]) throw;  // Check for overflows
        if (_value > allowance[_from][msg.sender]) throw;     // Check allowance
        balanceOf[_from] -= _value;                           // Subtract from the sender
        balanceOf[_to] += _value;                             // Add the same to the recipient
        allowance[_from][msg.sender] -= _value;
        Transfer(_from, _to, _value);
        return true;
    }

    function burn(uint256 _value) returns (bool success) {
        if (balanceOf[msg.sender] < _value) throw;            // Check if the sender has enough
        balanceOf[msg.sender] -= _value;                      // Subtract from the sender
        totalSupply -= _value;                                // Updates totalSupply
        Burn(msg.sender, _value);
        return true;
    }

    function burnFrom(address _from, uint256 _value) returns (bool success) {
        if (balanceOf[_from] < _value) throw;                // Check if the sender has enough
        if (_value > allowance[_from][msg.sender]) throw;    // Check allowance
        balanceOf[_from] -= _value;                          // Subtract from the sender
        totalSupply -= _value;                               // Updates totalSupply
        Burn(_from, _value);
        return true;
    }
}