Fastest way to create smart contract erc20

Today, I will share with you how to create smart contract erc20.

So, what is the erc20? 
An ERC20 token is a blockchain-based asset with similar functionality to bitcoin, ether, and bitcoin cash: it can hold value and be sent and received. ERC20 is an official protocol for proposing improvements to the Ethereum (ETH) network. ERC stands for Ethereum Request for Comment, and 20 is the proposal identifier. This is a common standard for creating tokens on the Ethereum blockchain.

Let's start.



* The programing: solidity (.sol)
The Remix: open source tool that helps you write Solidity contracts straight from the browser. 
* Some base functions of contract:

These are very short and basic
function totalSupply() public view returns (uint256);
function balanceOf(address tokenOwner) public view returns (uint);
function allowance(address tokenOwner, address spender)
public view returns (uint);
function transfer(address to, uint tokens) public returns (bool);
function approve(address spender, uint tokens)  public returns (bool);
function transferFrom(address from, address to, uint tokens) public returns (bool);

These two specically events will be invoked or emitted when a user is granted rights to withdraw tokens  and then transferred token.
event Approval(address indexed tokenOwner, address indexed spender,
 uint tokens);
event Transfer(address indexed from, address indexed to,
 uint tokens);

A few points regarding ERC20 and Solidity nomenclature:
- A public function can be accessed outside of the contract itself
- view basically means constant, i.e. the contract’s internal state will not be changed by the function
- An event is Solidity’s way of allowing clients e.g. your application frontend to be notified on specific occurrences within the contract

Model

erc20 model


Set the total of the tokens:
uint256 totalSupply_;
constructor(uint256 total) public {
   totalSupply_ = total;
   balances[msg.sender] = _totalSupply;
}

Get total token supply
function totalSupply() public view returns (uint256) {
  return totalSupply_;
}

Get token balance of owner
function balanceOf(address tokenOwner) public view returns (uint) {
  return balances[tokenOwner];
}

Transfer token to other account
function transfer(address receiver,
                 uint numTokens) public returns (bool) {
  require(numTokens <= balances[msg.sender]);
  balances[msg.sender] = balances[msg.sender] — numTokens;
  balances[receiver] = balances[receiver] + numTokens;
  emit Transfer(msg.sender, receiver, numTokens);
  return true;
}

Approve to withdraw token
function approve(address delegate,
                uint numTokens) public returns (bool) {
  allowed[msg.sender][delegate] = numTokens;
  emit Approval(msg.sender, delegate, numTokens);
  return true;
}

Get Number of Tokens Approved for Withdrawal
function allowance(address owner,
                  address delegate) public view returns (uint) {
  return allowed[owner][delegate];
}

Transfer Tokens by Delegate
function transferFrom(address owner, address buyer,
                     uint numTokens) public returns (bool) {
  require(numTokens <= balances[owner]);
  require(numTokens <= allowed[owner][msg.sender]);
  balances[owner] = balances[owner] — numTokens;
  allowed[owner][msg.sender] =
        allowed[from][msg.sender] — numTokens;
  balances[buyer] = balances[buyer] + numTokens;
  Transfer(owner, buyer, numTokens);
  return true;
}

SafeMath Solidity Library: use SafeMath for uint256; verify the correctness of the passed parameters. Should assert fail, the function execution will be immediately stopped and all blockchain changes shall be rolled back.
library SafeMath { // Only relevant functions
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
  assert(b <= a);
  return a — b;
}
function add(uint256 a, uint256 b) internal pure returns (uint256)   {
  uint256 c = a + b;
  assert(c >= a);
  return c;
}
}


Deployment: 

It's easy to deploy it. You can use metamask extension to add  and login to owner account. Use remix tool online to call "Complile" and click "Deploy".

Simple Smart Contract erc20 code :
pragma solidity >=0.4.22 <0.6.0;

contract owned {
    address public owner;

    constructor() public {
        owner = msg.sender;
    }

    modifier onlyOwner {
        require(msg.sender == owner);
        _;
    }

    function transferOwnership(address newOwner) onlyOwner public {
        owner = newOwner;
    }
}

interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes calldata _extraData) external; }

contract TokenERC20 {
    // Public variables of the token
    string public name;
    string public symbol;
    uint8 public decimals = 4;
    // 18 decimals is the strongly suggested default, avoid changing it
    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 generates a public event on the blockchain that will notify clients
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

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

    /**
     * Constrctor function
     *
     * Initializes contract with initial supply tokens to the creator of the contract
     */
    constructor(
        uint256 initialSupply,
        string memory tokenName,
        string memory tokenSymbol
    ) public {
        totalSupply = initialSupply * 10 ** uint256(decimals);  // Update total supply with the decimal amount
        balanceOf[msg.sender] = totalSupply;                    // Give the creator all initial tokens
        name = tokenName;                                       // Set the name for display purposes
        symbol = tokenSymbol;                                   // Set the symbol for display purposes
    }

    /**
     * Internal transfer, only can be called by this contract
     */
    function _transfer(address _from, address _to, uint _value) internal {
        // Prevent transfer to 0x0 address. Use burn() instead
        require(_to != address(0x0));
        // Check if the sender has enough
        require(balanceOf[_from] >= _value);
        // Check for overflows
        require(balanceOf[_to] + _value > balanceOf[_to]);
        // Save this for an assertion in the future
        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        // Subtract from the sender
        balanceOf[_from] -= _value;
        // Add the same to the recipient
        balanceOf[_to] += _value;
        emit Transfer(_from, _to, _value);
        // Asserts are used to use static analysis to find bugs in your code. They should never fail
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }

    /**
     * Transfer tokens
     *
     * Send `_value` tokens to `_to` from your account
     *
     * @param _to The address of the recipient
     * @param _value the amount to send
     */
    function transfer(address _to, uint256 _value) public returns (bool success) {
        _transfer(msg.sender, _to, _value);
        return true;
    }

    /**
     * Transfer tokens from other address
     *
     * Send `_value` tokens to `_to` in behalf of `_from`
     *
     * @param _from The address of the sender
     * @param _to The address of the recipient
     * @param _value the amount to send
     */
    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    /**
     * Set allowance for other address
     *
     * Allows `_spender` to spend no more than `_value` tokens in your behalf
     *
     * @param _spender The address authorized to spend
     * @param _value the max amount they can spend
     */
    function approve(address _spender, uint256 _value) public
        returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    /**
     * Set allowance for other address and notify
     *
     * Allows `_spender` to spend no more than `_value` tokens in your behalf, and then ping the contract about it
     *
     * @param _spender The address authorized to spend
     * @param _value the max amount they can spend
     * @param _extraData some extra information to send to the approved contract
     */
    function approveAndCall(address _spender, uint256 _value, bytes memory _extraData)
        public
        returns (bool success) {
        tokenRecipient spender = tokenRecipient(_spender);
        if (approve(_spender, _value)) {
            spender.receiveApproval(msg.sender, _value, address(this), _extraData);
            return true;
        }
    }

    /**
     * Destroy tokens
     *
     * Remove `_value` tokens from the system irreversibly
     *
     * @param _value the amount of money to burn
     */
    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);   // Check if the sender has enough
        balanceOf[msg.sender] -= _value;            // Subtract from the sender
        totalSupply -= _value;                      // Updates totalSupply
        emit Burn(msg.sender, _value);
        return true;
    }

    /**
     * Destroy tokens from other account
     *
     * Remove `_value` tokens from the system irreversibly on behalf of `_from`.
     *
     * @param _from the address of the sender
     * @param _value the amount of money to burn
     */
    function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(balanceOf[_from] >= _value);                // Check if the targeted balance is enough
        require(_value <= allowance[_from][msg.sender]);    // Check allowance
        balanceOf[_from] -= _value;                         // Subtract from the targeted balance
        allowance[_from][msg.sender] -= _value;             // Subtract from the sender's allowance
        totalSupply -= _value;                              // Update totalSupply
        emit Burn(_from, _value);
        return true;
    }
}

/******************************************/
/*       ADVANCED TOKEN STARTS HERE       */
/******************************************/

contract TokenCoin is owned, TokenERC20 {

    uint256 public sellPrice;
    uint256 public buyPrice;

    mapping (address => bool) public frozenAccount;

    /* This generates a public event on the blockchain that will notify clients */
    event FrozenFunds(address target, bool frozen);
    event Buy();

    /* Initializes contract with initial supply tokens to the creator of the contract */
    constructor(
        uint256 initialSupply,
        string memory tokenName,
        string memory tokenSymbol
    ) TokenERC20(500000000, "TokenName", "TKN") public {}

    /* Internal transfer, only can be called by this contract */
    function _transfer(address _from, address _to, uint _value) internal {
        require (_to != address(0x0));                          // Prevent transfer to 0x0 address. Use burn() instead
        require (balanceOf[_from] >= _value);                   // Check if the sender has enough
        require (balanceOf[_to] + _value >= balanceOf[_to]);    // Check for overflows
        require(!frozenAccount[_from]);                         // Check if sender is frozen
        require(!frozenAccount[_to]);                           // Check if recipient is frozen
        balanceOf[_from] -= _value;                             // Subtract from the sender
        balanceOf[_to] += _value;                               // Add the same to the recipient
        emit Transfer(_from, _to, _value);
    }

    /// @notice Create `mintedAmount` tokens and send it to `target`
    /// @param target Address to receive the tokens
    /// @param mintedAmount the amount of tokens it will receive
    function mintToken(address target, uint256 mintedAmount) onlyOwner public {
        balanceOf[target] += mintedAmount;
        totalSupply += mintedAmount;
        emit Transfer(address(0), address(this), mintedAmount);
        emit Transfer(address(this), target, mintedAmount);
    }

    /// @notice `freeze? Prevent | Allow` `target` from sending & receiving tokens
    /// @param target Address to be frozen
    /// @param freeze either to freeze it or not
    function freezeAccount(address target, bool freeze) onlyOwner public {
        frozenAccount[target] = freeze;
        emit FrozenFunds(target, freeze);
    }

    /// @notice Allow users to buy tokens for `newBuyPrice` eth and sell tokens for `newSellPrice` eth
    /// @param newSellPrice Price the users can sell to the contract
    /// @param newBuyPrice Price users can buy from the contract
    function setPrices(uint256 newSellPrice, uint256 newBuyPrice) onlyOwner public {
        sellPrice = newSellPrice;
        buyPrice = newBuyPrice;
    }

    /// @notice Buy tokens from contract by sending ether
    function buy() payable public {
        uint amount = msg.value / buyPrice;                 // calculates the amount
        _transfer(address(this), msg.sender, amount);       // makes the transfers
    }

    /// @notice Sell `amount` tokens to contract
    /// @param amount amount of tokens to be sold
    function sell(uint256 amount) public {
        address myAddress = address(this);
        require(myAddress.balance >= amount * sellPrice);   // checks if the contract has enough ether to buy
        _transfer(msg.sender, address(this), amount);       // makes the transfers
        msg.sender.transfer(amount * sellPrice);            // sends ether to the seller. It's important to do this last to avoid recursion attacks
    }
}

It's simple and fast :D

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