How do I send back ethers to the sender of the tokens in a smart contract

Question

I am creating an ERC20 token using solidity. I wish my program to automatically send equivalent ethers to the address of someone who sends, say 20 ETH to a smart contract address. How to do that? Also, I want to set the price of the token in ETH. Please provide me with a sample code for that. 

Answer 1

Whenever a smart contract receives ether via a transfer, it executes the fallback function, here you can access the msg.value and know how much ether in 'Wei'  the msg.sender sent. If you have a token rate you can issue your tokens depending on the amount of Wei sent.

Follwing fallback function might be of help-

//fallback function can be used to buy tokens
  function () external payable {
    buyTokens(msg.sender);
  }

  // low level token purchase function
  function buyTokens(address beneficiary) public payable {
    require(beneficiary != address(0));
    require(validPurchase());

    uint256 weiAmount = msg.value;

    // calculate token amount to be created
    uint256 tokens = weiAmount.mul(rate);

    // update state
    weiRaised = weiRaised.add(weiAmount);

    token.mint(beneficiary, tokens);
    TokenPurchase(msg.sender, beneficiary, weiAmount, tokens);

    forwardFunds();
  }

Or you can also create a payable function that allows your contract to get paid and send them to the people. 

A simple code is given below:

function() public payable {
    uint toMint = msg.value/price;
    totalSupply += toMint;
    balances[msg.sender] += toMint;

    emit Transfer(0, msg.sender, toMint);
}