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? – goodvibration Jul 1 '20 at 13:17
  • @goodvibration, i edited the question with the error. currUserID is a uint (1) – André Alçada Padez Jul 1 '20 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. – goodvibration Jul 1 '20 at 13:29
  • You probably wanna take that mapping outside the PoolStruct structure, and redefine it as mapping(uint => PoolStruct) poolUserList. – goodvibration Jul 1 '20 at 13:30
  • could you give me an example? – André Alçada Padez Jul 1 '20 at 13:30

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.' – Sang-hoon Shin Sep 14 '20 at 16:14
  • @Sang-hoon-Shin what does the error say? – hrkrshnn Sep 14 '20 at 16:16
  • It says "Struct containing a (nested) mapping cannot be constructed" – Sang-hoon Shin Sep 14 '20 at 16:23
  • Please check my question: ethereum.stackexchange.com/questions/87451/… – Sang-hoon Shin Sep 14 '20 at 16:24

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