The below code gets compiled but when foo is called invalid opcode error is thrown

Question

pragma solidity >=0.5.0 <0.7.0;
contract Mycontract{
    uint256 a;
    struct User
    {  string name; }
    User[] users;
    function foo() public{
      User storage user=users[0];
      user.name="hey";
    }
    function get() public view returns(string memory)
    {   return users[0].name; }
}

Answer 1

It is caused by this : User storage user=users[0];

You try to assign to user the first element of the users array but you can't access an empty array.

If you want to push your object into the array you can use array.push().