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How to use Unlock in other contracts

Smart contract developers can easily integrate with Unlock Protocol on-chain, allowing them to limit access to certain features or change the behavior of functions based on membership status!
In this tutorial, we will focus on integrating a Lock itself, rather than the Unlock contract.
The lock contract has multiple functions you could use to assess the membership status of a given address. The most obvious one is called balanceOf. It will either return 1 if the address owns a valid membership and 0 otherwise (whether the membership has expired, or the address never had one).
Here is an example on how to easily create a contract that has a "members only" feature:
pragma solidity ^0.5.0;
​
import 'contracts/interfaces/IPublicLock.sol';
​
contract PaidOnlyFeature
{
IPublicLock public lock;
​
constructor(IPublicLock _lockAddress) public
{
lock = _lockAddress;
}
​
function paidOnlyFeature() public
{
require(lock.balanceOf(msg.sender) > 0, 'Purchase a membership first!');
// ...and then implement your feature as normal. If needed you could even make that a modifier
}
​
// You can also have free features of course
}