ERC2o is a problem that has already been solved and you don't have to create a contract from scratch every time you need a token. There are well-tested and widely used libraries such as openzeppelin that include contracts for the ERC20 standard. Since these are used by many ICOs and products, they can have zero vulnerabilities.

The library is very modularized and you can use the components as per your requirements. It even includes interfaces that you can use with other contracts.

You can get the complete ERC20 implementation from https://github.com/OpenZeppelin/openzeppelin-solidity/tree/master/contracts/token/ERC20 or install it via npm with the npm install -E openzeppelin-solidity command.