Quick Start - Step 3

Step 3 : Edit Smart Contract

In this tutorial, we have two sample smart contracts available to experiment with.

  • Coin.sol: A minimal token contract
  • EIP20.sol: An implementation of EIP20 tokens provided by ConsenSys

Locate the Smart Contract files for an EIP20 Coin

Navigate to truffle/contracts/eip20, and observe that there are two Solidity files in this directory: EIP20Interface.sol and EIP20.sol

cd truffle/contracts/eip20
ls

Understand the Smart Contract Files

EIP20Interface.sol defines the required functions for the ERC20 base standard.

/// @param _owner The address from which the balance will be retrieved
/// @return The balance
function balanceOf(address _owner) public view returns (uint256 balance);
/// @notice send `_value` token to `_to` from `msg.sender`
/// @param _to The address of the recipient
/// @param _value The amount of token to be transferred
/// @return Whether the transfer was successful or not
function transfer(address _to, uint256 _value) public returns (bool success);

EIP20.sol provides implementations of the functions declared in EIP20Interface.sol. Here are the implementation of the two functions that we looked at in the interface.

function balanceOf(address _owner) public view returns (uint256 balance) {
    return balances[_owner];
}
function transfer(address _to, uint256 _value) public       returns (bool success) {
    require(balances[msg.sender] >= _value);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    emit Transfer(msg.sender, _to, _value); //solhint-disable-line indent, no-unused-vars
    return true;
}

Edit the EIP20 Token Name

At the top of the EIP20.sol token contract, the constructor function defined inputs for: Initial amount, token name, decimal unit, and token symbol. You can customize these parameters to create your own token.

function EIP20(
    uint256 _initialAmount,
    string _tokenName,
    uint8 _decimalUnits,
    string _tokenSymbol
) public {
    balances[msg.sender] = _initialAmount;               // Give the creator all initial tokens
    totalSupply = _initialAmount;                        // Update total supply
    name = _tokenName;                                   // Set the name for display purposes
    decimals = _decimalUnits;                            // Amount of decimals for display purposes
    symbol = _tokenSymbol;                               // Set the symbol for display purposes
    emit Transfer(msg.sender, msg.sender, 0); 
    emit Approval(msg.sender, msg.sender, 0);
}

To set those values, open 3_deploy_tokens.js in folder truffle/migrations/. This is a migration script that controls the deployment of smart contracts. We will talk more about migrations in next step. The contents of the file should look like this.

const EIP20 = artifacts.require('./EIP20.sol');

module.exports = (deployer) => {
  deployer.deploy(EIP20, 10000, 'Simon Bucks', 1, 'SBX');
};

Now let’s change the name to something you like, e.g. Flower Token. We are now ready ready to deploy this contract, which will generate our own 10000 Flower Tokens!

module.exports = (deployer) => {
  deployer.deploy(EIP20, 10000, 'Flower Token', 1, 'FLT');
};

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