I wanted to make a Table class that is supposed to work like a table with columns and rows, and values in the cells. Obviously I need a function that would allow to get the value from a particular cell. I wanted this function (operator() in my case) to return a reference, so that it would be easy to change the cell’s contents.

Now, I wanted to check in this function whether or not such a cell exists (i.e. are there enough columns and rows in the table). The problem is, I can’t just return something like a nullptr if col/row index is out of bounds in a function that returns a reference.
I’ve read some other topics on SO and people suggested using optional<reference_wrapper>. Now, the problem is, I can’t assign anything to the returned value. Here’s the code:

class Table {
public:
    class Column {
    public:
        std::string Header;
        int Index = 0;
        std::vector<std::string> colCells;
        Column(std::string colHeader) { Header = colHeader; }
    };

    std::vector<Column> Columns;
    
    std::optional<std::reference_wrapper<std::string>> operator()(int col, int row) {
        return Columns[col].colCells[row];
    }

    void AddColumn(std::string colHeader) {
        Column col(colHeader);
        col.Index = Columns.size();
        Columns.push_back(col);
        for (auto& row : Columns[0].colCells) { //for each row in the first column (we created a column first, so there's at least this one)
            col.colCells.push_back("");         //create a cell with an empty string    
        }
    }
    void AddRow() {
        for (auto& col : Columns) {
            col.colCells.push_back("");     //add empty string at the end of each column
        }
    }
};

AddColumn and AddRow methods aren’t important here, I guess, but I just pasted them here for the code to be complete.
Now, I tried this:

data::tbl(4, 0) = "Col: 2, Row: 0";

And that’s the errors I get. It seems as if this wasn’t an actual reference to the cell’s data. I should add that everything works fine the same way if I don’t use optional and have the operator

What is happening and why?

PS. I know that another way to do all that, would be to change the return value to a pointer. But then I would have to dereference the returned pointer everywhere in the code like this:

*data::tbl(4, 0) = "Col: 2, Row: 0";

Which is something I’d rather avoid if possible.