I'm trying to use a mapping inside a struct:

struct PoolStruct {
    uint currentUserID;
    uint activeUserID;
    uint price;
    uint minimalReferrals;
    mapping(uint => address) poolUserList;

the code compiles, so i guess it's somehow permitted. The problem starts when i try to assign to that property:

PoolStruct memory pool;

pool = PoolStruct({
  currentUserID: 1,
  activeUserID: 1,
  price: POOL_PRICES[i],
  minimalReferrals: POOL_MINIMAL_REFERRALS[i]

pool.poolUserList[1] = msg.sender;

get an error on the last line...

TypeError: Member "poolUserList" is not available in struct Definitions.PoolStruct memory outside of storage.
       pool.poolUserList[currUserID] = msg.sender;

Is there any way to make this work?

  • 1
    What error are you getting? What is currUserID? Jul 1, 2020 at 13:17
  • @goodvibration, i edited the question with the error. currUserID is a uint (1) Jul 1, 2020 at 13:24
  • Yeah, well, you can use mapping only in storage (i.e., non-local) variables. So the same goes for a struct which holds a mapping. Jul 1, 2020 at 13:29
  • 1
    You probably wanna take that mapping outside the PoolStruct structure, and redefine it as mapping(uint => PoolStruct) poolUserList. Jul 1, 2020 at 13:30
  • could you give me an example? Jul 1, 2020 at 13:30

3 Answers 3


The problem is that mappings can only live in storage. When you define PoolStruct memory pool;, the mapping member cannot be created in memory, and therefore the memory struct should be treated as if the mapping member never existed (for solidity < 0.7.0).

Starting from solidity 0.7.0, the line PoolStruct memory pool will produce an error saying that structs containing (nested) mappings must have storage as data location.

  • @hrkshnn, Then, how should I declare? 'PoolStruct storage pool' also seems to throw an error. Saying 'Struct containing a (nested) mapping cannot be constructed.' Sep 14, 2020 at 16:14
  • @Sang-hoon-Shin what does the error say?
    – hrkrshnn
    Sep 14, 2020 at 16:16
  • It says "Struct containing a (nested) mapping cannot be constructed" Sep 14, 2020 at 16:23
  • Please check my question: ethereum.stackexchange.com/questions/87451/… Sep 14, 2020 at 16:24

Just keep the mapping outside the struct. you do not have to keep inside the struct

     mapping(uint => address) poolUserList;

A mapping can only be returned in a struct in a function, if the function is internal or when using a library.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.