openzeppelin-contracts/contracts/drafts/Counter.sol

pragma solidity ^0.5.0;

/**
 * @title Counter
 * @author Matt Condon (@shrugs)
 * @dev Provides an incrementing uint256 id acquired by the `Counter#next` getter.
 * Use this for issuing ERC721 ids or keeping track of request ids, anything you want, really.
 *
 * Include with `using Counter for Counter.Counter;`
 * @notice Does not allow an Id of 0, which is popularly used to signify a null state in solidity.
 * Does not protect from overflows, but if you have 2^256 ids, you have other problems.
 * (But actually, it's generally impossible to increment a counter this many times, energy wise
 * so it's not something you have to worry about.)
 */
library Counter {
    struct Counter {
        uint256 current; // default: 0
    }

    function next(Counter storage index) internal returns (uint256) {
        index.current += 1;
        return index.current;
    }
}