[Ethereum] using mappings inside structs


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?

Best Answer

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.

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