How Do I Update A Struct Property in Solidity

Question

I am trying to update the value of one of the props in a struct in a solidity contract.

updating a variable works fine if it is not in a struct. but when i try to update a value in a struct like in the contract below, gas is used to update the contracts as i would expect, but when i read the updated value, it has not been changed.

Can anyone see how my contracts definition is incorrect?

My contract is defined as:

Users.sol

pragma solidity ^0.4.17;

contract Users {
    struct User {
        string firstName;
        string lastName;
    }
    mapping(address => User) public users;

    function getFirstName() public view returns (string) {
        User memory user = users[msg.sender];
        return user.firstName;
    }

    function setFirstName(newName) public {
        User memory user = users[msg.sender];
        user.firstName = newName;
    }
}

i'm new to working with solidity so any help is appreciated.

Answer 1

This line:

User memory user = users[msg.sender];

Should use storage instead:

User storage user = users[msg.sender];

If you use memory, then you're making a copy of the struct in memory and then updating that. If you use storage, you'll get a reference to the struct in storage, so your modifications will be persisted.

Answer 2

i rephrased the questing with a better example. i don't think i should delete this thread because there are many ways to phrase a question. instead, it may be more useful to provide a link to the solution:

Updating a Value in a Struct