why a parameter must be "storage" or "memory"?

Question

I am creating a token like this:

pragma solidity ^0.5.0;

import "../utils/math/SafeMath.sol";


contract ExampleDivisibleNFTs {
    using SafeMath for uint256;

    // Percentage of ownership over a token  
    mapping(address => mapping(string => uint)) ownerToTokenShare;

    // If a token has been created
    mapping(string => bool) mintedToken;

    modifier onlyNonExistentToken(string _tokenId) {
        require(mintedToken[_tokenId] == false, "the NFT already exists");
        _;
    }

    modifier onlyExistentToken(string _tokenId) {
        require(mintedToken[_tokenId] == true, "the NFT doesn't exists");
        _;
    }

}

and I am testing with remix but I have the following problem

Data location must be "storage" or "memory" for parameter in function, but none was given.

modifier onlyNonExistentToken(string _tokenId) {
                              ^-------------^

why a parameter may to be a "storage", It is not always "memory" ?

Answer 1

The solidity compiler requires an explicit declaration of the data location for persistency and semantic reasonings.

With that being said, in your particular smart contract, the _tokenID string is passed by reference and because of the fact that the reference --> points to your mintedToken mapping, then it’s a reference to storage which is why the compiler begs you to explicitly declare it as such.

Answer 2

memory is to be used with an array, struct, string, and mapping, I guess it's the desired default expected by solidity. You can use the storage at the expense of your gas.