crear token ERC-20 a partir de una función de contrato

Me gustaría hacer una función para crear un contrato con la interfaz ERC-20. Pero tengo algunos problemas.

Si alguien me ayuda, se lo agradeceré.

Aquí está el código:

pragma solidity ^0.5.0;
import 'https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/math/SafeMath.sol';
/*
library SafeMath {
    function add(uint a, uint b) internal pure returns (uint c) {
        c = a + b;
        require(c >= a);
    }
    function sub(uint a, uint b) internal pure returns (uint c) {
        require(b <= a);
        c = a - b;
    }
    function mul(uint a, uint b) internal pure returns (uint c) {
        c = a * b;
        require(a == 0 || c / a == b);
    }
    function div(uint a, uint b) internal pure returns (uint c) {
        require(b > 0);
        c = a / b;
    }
}
*/
contract SubToken{
    function totalSupply() public view returns (uint);
    function balanceOf(address tokenOwner) public view returns (uint balance);
    //function allowance(address tokenOwner, address spender) public view returns (uint remaining);
    function transfer(address to, uint tokens) public returns (bool success) ;
    function approve(address spender, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);

    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);

}
contract MainContrat is SubToken {
    mapping(address => uint) balances;

    mapping(address => mapping(address => uint)) allowed;
    mapping(address => address[]) public created;

    mapping(address => SubToken) public tokenlist;

    address[] public contracts;
    address owner = msg.sender;

    using SafeMath for uint;

    string public symbol;
    string public  name;
    uint8 public decimals;
    uint _totalSupply;

    constructor(string memory _name,string memory _symbol,uint8 _decimals,uint256 _totalSupply)  public{
        name = _name;
        symbol = _symbol;
        decimals=_decimals;
        balances[msg.sender]=_totalSupply;
    }    

    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address _owner) public view  returns (uint256 balance) {
        return balances[_owner];
    }


    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        emit Transfer(_from, _to, _value);
        return true;
    }
/*
    function createNew(string memory _name,string memory _symbol,uint8 _decimals,uint256 _totalSupply) public returns(address newToken){
        SubToken newSubToken = (new SubToken(_name,_symbol,_decimals,_totalSupply));
        created[msg.sender].push(address(newSubToken));
        newSubToken.transfer(msg.sender, _totalSupply);
        return address(newSubToken);

    }
    */
    /*
    function createNewContract() public returns(address){
    SubToken st = new SubToken();
    contracts.push(st);
    address(st);
    }
    */
    event Transfer(address indexed _from, address indexed _to, uint256 _value);

    event Approval(address indexed _owner, address indexed _spender, uint256 _value);


}

contract factory{
    address[] public contracts;
    mapping(address => address[]) public created;

    function createNewContract(string memory _name,string memory _symbol,uint8 _decimals,uint256 _totalSupply) public returns(address){
    MainContrat st = new MainContrat(_name,_symbol,_decimals,_totalSupply);
    contracts.push(address(st));
    return address(st);
    }
}

Tengo errores en mis funciones createContract. errores es:

TypeError: tratando de crear una instancia de un contrato abstracto. Subtoken st = nuevo Subtoken(); ^----------^ browser/crtcont2.sol:29:5: Falta implementación: función de aprobación (dirección de gasto, uint tokens) devoluciones públicas (bool éxito); ^------------------------------------------------- --------------------------^ browser/crtcont2.sol:26:5: Falta la implementación: function balanceOf(address tokenOwner) vista pública devuelve ( saldo de unidad); ^------------------------------------------------- --------------^ browser/crtcont2.sol:25:5: Falta implementación: función totalSupply() vista pública devuelve (uint); ^------------------------------------------------^ browser/crtcont2.sol:28:5: Falta implementación: transferencia de función (dirección a, tokens uint) devoluciones públicas (bool éxito); ^------------------------------------------------- ----------------------^ browser/crtcont2.sol:30:5: Falta implementación: función transferFrom(dirección de, dirección a, uint tokens) retornos públicos (éxito bool); ^------------------------------------------------- ----------------------------------------^