How do I initialise empty arrays and structs in nested structs data structures?

contract NewBook {

        struct ReviewerData {
            string name,
            uint industryXP
            // foobar...

        struct ChapterData {
            // string chapterID;
            string title;
            string color;
            uint pages;
            //How do I initialise the two elements below with empty values?
            ReviewerData[] reviewer;
            address[] reviewers;

        struct BookData {
            // string bookID;
            ChapterData[] chapters;

        mapping(bytes32 => BookData) books; // bookId => BookData

        function addChapter(bytes32 _bookID, string memory title, string color, uint pages) public {
            ChapterData memory c = ChapterData({
                title: title,
                color: color,
                pages: pages,
                //how to initialise empty reviewer struct?
                //how to initialise empty reviewers array?

From the Solidity documentation on dynamic arrays:

Arrays have a length member that contains their number of elements. The length of memory arrays is fixed (but dynamic, i.e. it can depend on runtime parameters) once they are created. For dynamically-sized arrays (only available for storage), this member can be assigned to resize the array.

and on the lack of null values:

The concept of “undefined” or “null” values does not exist in Solidity, but newly declared variables always have a default value dependent on its type. To handle any unexpected values, you should use the revert function to revert the whole transaction, or return a tuple with a second bool value denoting success.

In your example - instead of creating the new struct in memory and then using push to append it to the storage array, you can increase the length of books[_bookID].chapters which will initialize a new, empty ChapterData struct. Then edit this new storage struct in place.

    function addChapter(bytes32 _bookID, string memory title, string color, uint pages) public {
        uint _length = books[_bookID].chapters.length;

        ChapterData storage c = books[_bookID].chapters[_length];
        c.title = title;
        c.color = color;
        c.pages = pages;

| improve this answer | |
  • I understand that these types will be initialised with a 0 value but you cannot just ignore elements of a struct, they must be strictly declared and initialised with a value when you attempt to add a struct to another struct. – NowsyMe Jul 14 '19 at 14:41
  • sorry, i misunderstood the question a bit. I've edited my answer, I hope it's more helpful. – iamdefinitelyahuman Jul 14 '19 at 14:56

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