DeclarationError: Uninitialized storage pointer. Struct error when running truffle tests

Question

I've seen other similar questions but they don't explain why this error happens and/or they are case specific questions. So, I have this code:

pragma solidity >=0.4.22 <0.7.0;


contract MagicIdeas {

    struct IdeaCard {
        address owner;
        string title;
        string text;
        string img_url;
        string card_id;
    }

    ....

    function update_card_data(string memory _card_id,
                            string memory _new_title,
                            string memory _new_text,
                            string memory _new_img_url) public {
        IdeaCard storage updatedCard; // ERROR HERE
        updatedCard = IdCardMap[_card_id];
        if(bytes(_new_title).length>0){
            updatedCard.title = _new_title;
        }
        if(bytes(_new_text).length>0){
            updatedCard.text = _new_text;
        }
        if(bytes(_new_img_url).length>0){
            updatedCard.img_url = _new_img_url;
        }
    }

Whe I run truffle test I get the following:

DeclarationError: Uninitialized storage pointer.
        IdeaCard storage updatedCard;
        ^--------------------------^

Why does this happen? How can I solve it?

Extra: The same code compiles correctly on remix.

Answer 1

Storage references will always point to a valid storage slot, even whey they are uninitialized.

So something like this

IdeaCard storage updatedCard;
updatedCard.owner = 0x1234..1234;

worked in solc 0.4 and caused overwriting valid storage slots.

In solc 0.5 it was turned into an error, but in solc 0.6 it is again a warning. Perhaps Truffle is compiling with warnings turned into errors and it make the compilation to fail.

---

In your example it should be easy to solve by combining two lines into one:

IdeaCard storage updatedCard = IdCardMap[_card_id];