pragma solidity ^0.4.24;

contract MemoryAndStorage {
    mapping(uint => User) users;
    struct User{
        uint id;
        uint balance;

    function addUser(uint id, uint balance) public {
        users[id] = User(id, balance);   

    function updateBalance(uint id, uint balance) public {
        User storage user = users[id];
        user.balance = balance;

    function getBalance(uint id) public view returns (uint) {
        return users[id].balance;


I have questions about the updateBalance function.

Question 1) As we know , there're pointers (storage pointers and memory pointers) and there're actual datas. First I am interested, if both storage pointer and the data that pointer is actually pointing to are both saved in storage?

Question 2) so in a function updateBalance, where does user variable got created that's pointing to users[id]?

Question 3) if I wrote the User memory user = users[id]; would that variable got created in memory storage and it would watch the data that'is in storage ?

Question 4) If I write the following : User storage user = User(5,10); there's an error. why?

1 Answer 1


I think https://programtheblockchain.com/posts/2018/03/09/understanding-ethereum-smart-contract-storage/ will answer most of your questions.

The key takeaway is that smart contract storage is basically a huge array. So there's no "allocation" or "creation" involved. When you write User storage user = users[id], that just gives you a local reference to that location in storage.

If you wrote User memory user = users[id];, you would instead end up with an in-memory copy of that User.

User storage user = User(5,10); is a problem because User storage user declares a storage reference. (Think of it as a pointer to something in storage.) The right-hand side of the assignment should be something in storage so user can refer to that.

  • So a) if I write User storage user = users[id], where is user variable created? I think you said it got created nowhere. It's just a reference. how can't it be created somewhere? can't storage reference variable point to something that is in memory?
    – Chemistry
    Commented Aug 24, 2018 at 15:18
  • In the compiled bytecode, the location in storage is probably pushed onto the stack somewhere at runtime. No, a storage reference can't point to something in memory. (It can only refer to storage.)
    – user19510
    Commented Aug 24, 2018 at 15:26
  • why do you think User storage user = User(5,10); the right side of this statement User(5,10) gets created in memory and not in storage? because it's in the function ?
    – Chemistry
    Commented Aug 24, 2018 at 15:28
  • What would it mean to "create" it in storage? I think you're assuming some sort of allocation (finding a free spot in storage and writing the data there), which is not how storage works.
    – user19510
    Commented Aug 24, 2018 at 15:30
  • So I finally understand it somehow. The only part missing for me is where reference variables are stored. User storage user where is that user variable? on stack memory? i don't think so. That's what ethereum says : local variables of struct, array or mapping type reference storage by default. so user reference variable is also in storage , but it says no allocation happens on storage . then what do I assume?
    – Chemistry
    Commented Aug 24, 2018 at 15:33

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