solidity/docs/examples/modular.rst

.. index:: contract;modular, modular contract

*****************
Modular Contracts
*****************

A modular approach to building your contracts helps you prevent overflow risks
and unexpected tokens. In the example below, the contract uses the ``move`` method
of the ``Balances`` :ref:`library <libraries>` to check that balances sent between
addresses match what you expect.

::

    pragma solidity >=0.4.22 <0.6.0;

    library Balances {
        function move(mapping(address => uint256) storage balances, address from, address to, uint amount) internal {
            require(balances[from] >= amount);
            require(balances[to] + amount >= balances[to]);
            balances[from] -= amount;
            balances[to] += amount;
        }
    }

    contract Token {
        mapping(address => uint256) balances;
        using Balances for *;
        mapping(address => mapping (address => uint256)) allowed;

        event Transfer(address from, address to, uint amount);
        event Approval(address owner, address spender, uint amount);

        function balanceOf(address tokenOwner) public view returns (uint balance) {
            return balances[tokenOwner];
        }
        function transfer(address to, uint amount) public returns (bool success) {
            balances.move(msg.sender, to, amount);
            emit Transfer(msg.sender, to, amount);
            return true;

        }

        function transferFrom(address from, address to, uint amount) public returns (bool success) {
            require(allowed[from][msg.sender] >= amount);
            allowed[from][msg.sender] -= amount;
            balances.move(from, to, amount);
            emit Transfer(from, to, amount);
            return true;
        }

        function approve(address spender, uint tokens) public returns (bool success) {
            require(allowed[msg.sender][spender] == 0, "");
            allowed[msg.sender][spender] = tokens;
            emit Approval(msg.sender, spender, tokens);
            return true;
        }
    }