I am working on creating a basic voting smart contract. But when assigning Question obj in the map, I'm getting this error:

pragma solidity ^0.8.0;

contract Voting {
    struct Question {
        uint256 questionId;
        Option[] options;

    struct Option {
        uint256 optionId;
        string optionValue;

    // map(questionId => Question)
    mapping(uint256 => Question) public mapQuestions;

    function addQuestion(string memory _option1) public {
        Question memory quest;
        quest.questionId = 1;
        quest.options[0] = Option(1, _option1);

        mapQuestions[quest.questionId] = quest;

When compiling, I'm getting this error:

copying of type struct Voting.Option memory[] memory to storage not yet supported

1 Answer 1


Arrays and mappings are only allocated to storage right now, and when you create Question in memory you try to allocate the array inside to memory. You would usually get a reference onto the struct from a mapping and work with it like that:

function addQuestion(string calldata _option1) public {
    uint _id = 1;
    Question storage quest = mapQuestions[_id];
    quest.questionId = _id;
    quest.options.push(Option(_id, _option1));
  • 1
    This worked like a charm! Thanks @Kostiantyn for providing a simplistic yet elegant solution.
    – VikasGS
    Dec 30, 2021 at 21:00

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