How to initialize empty struct array in new struct?

Question

Trying to do this:

contract Foo {
    mapping (bytes32 => Thing) things;

    struct Thing {
        Item[] items;
    }

    struct Item {
        uint number;
    }

    function Foo(bytes32 id) {
        Thing memory thing = Thing(); // What to do here to create empty array of type Item[]?
        things[id] = thing;
    }
}

and I get the error message:

Untitled:13:30: Error: Wrong argument count for function call: 0 arguments given but expected 1.
   Thing memory thing = Thing();

Answer 1

Some issues:

1. The compiler does not yet support copying memory struct arrays to storage, so things[id] = thing; will fail.

2. Thing thing; will initialize thing to a Thing with thing.items set to an empty array, so there's no need to use the constructor at all.

3. Since all elements of mappings come pre-initialized to their "zero" values, there's no need to create a Thing in memory, then copy it to the mapping. Just assign values to the mapping's properties.

---

For example, this is the code I would use, with some added fields to better illustrate the concept:

contract Foo {
    mapping (bytes32 => Thing) things;

    struct Thing {
        Item[] items;
        uint _bar;
    }

    struct Item {
        uint number;
    }

    function Foo(bytes32 id, uint bar) {
        things[id]._bar = bar;
    }
}

This demonstrates how mapping elements are pre-initialized. things[id].items will return an empty Item[].

In addition, you can create an uninitialized Thing, and all of it's fields will default to zero (ore empty, false, etc.)