Solidity Error: Struct containing a (nested) mapping cannot be constructed

Question

My version of solc:

"solc": "^0.7.1",

When I try to construct a struct that contains mapping, I got this error: "Struct containing a (nested) mapping cannot be constructed"

This is my solidity code.

Campaign.sol

pragma solidity >=0.5.0;

contract Campaign {
    struct Request {
        string description;

        // I declared a mapping inside of a struct.
        mapping(address => bool) approvals;

    }
    constructor(uint256 minimum, address creator) {
        ...
    }

    function createRequest(string memory description) public onlyManager {
        Request memory newRequest = Request({ 

            // Here the compiler gives me an error

            description : description

        });
    }
    ...

It says

Struct containing a (nested) mapping cannot be constructed.

Please help me.

Answer 1

The problem is in the construction, i.e., Request({ description: description }) Therefore changing the location of newRequest from memory to storage will not help.

If you really want to make it work, create a state variable, say, mapping (uint => Request) requests. Now, inside your function, you can write Request storage newRequest = requests[index] where index will get incremented later.

For a reference, see: docs

Answer 2

from 0.7.0 do like below:

 struct Request{
            string description;
            uint value;
            address recipient;
            bool complete;
            uint approvalsCount;
            mapping(address => bool) approvals;
        }
        
    uint numRequests;
    mapping (uint => Request) requests;
    
    function createRequest (string memory description, uint value,
            address recipient) public{
                Request storage r = requests[numRequests++];
                r.description = description;
                r.value = value;
                r.recipient = recipient;
                r.complete = false;
                r.approvalsCount = 0;
            
        }

Answer 3

This should be enough:

function createRequest(string memory description) public onlyManager {
    Request storage newRequest = requests.push();
    newRequest.description = description;
}

Answer 4

You cannot create a mapping (or a struct containing a mapping) in memory.

So, convert your code from this:

contract Campaign {
    struct Request {
        string description;
        uint value;
        address recipient;
        bool complete;
        uint approvalCount;
        mapping(address => bool) approvals;
    }

    Request[] public requests;

    function createRequest(string memory description, uint value, address recipient) public restricted {
      Request memory newRequest = Request({
          description: description,
          value: value,
          recipient: recipient,
          complete: false,
          approvalCount: 0
      });
      requests.push(newRequest);
    }
}
    

to this

contract Campaign {
    struct Request {
        string description;
        uint value;
        address recipient;
        bool complete;
        uint approvalCount;
        mapping(address => bool) approvals;
    }
    
    uint numRequests;
    mapping (uint => Request) requests;
    
    function createRequest(string memory description, uint value, address recipient) public restricted {            
        Request storage r = requests[numRequests++];
        r.description = description;
        r.value = value;
        r.recipient = recipient;
        r.complete = false;
        r.approvalCount = 0;
    }
}

Reference

Answer 5

I wish to add some background info, to help those transitioning from a managed-memory environment (like the .NET CLR) to the EVM:

• Usually, you may have instantiated reference types which allocate memory (on the heap). Since solidity EVM memory is more restricted, solidity instead initializes mappings in contract storage instead (instead of in memory). Since instantiating a struct on the stack utilizes memory, this is why this error message pops up when trying to instantiate a solidity struct that contains a mapping.
• Usually, you may have asked a runtime for new memory (e.g.: using the new keyword in C#). In solidity, this step is abstracted away from the developer & instead the solidity EVM automatically does it for us behind the scenes. Specifically, this occurs when the mapping variable is declared. This is why the code examples on this page don't show a step for memory allocation, & instead just jump straight to setting mapping values.

Answer 6

You can't create a struct with a mapping inside. Create a local mapping in your contract at the top level.