How to create Interface to read Struct in mapping?

Question

There are 2 contracts A and B.

A has a mapping which returns Struct:

mapping (bytes32 => Cup) public cups;

How can we read the cups mapping in contract B?

Following code doesn't work for sure:

interface CupsInteraface {
    function cups(bytes32 cup) external returns (Struct);
}

Answer 1

There is no way to do this in a stable version of solidity, but possible in experimental

pragma experimental ABIEncoderV2;

interface CupInteraface {
    struct S {
        bytes32 s;
    }
    function get(bytes32 cup) external returns (S memory);
}

contract Cup is CupInteraface {
    mapping(bytes32 => S) private _cups;

    constructor(bytes32 cup) public {
        _cups[cup] = S(cup);
    }

    function get(bytes32 cup) public returns (S memory) {
        return _cups[cup];
    }
}

Answer 2

As of right now, it's not possible to return structs in solidity. The most obvious workaround would be to make functions that return individual elements of a specific struct, like, assuming a cup has a title:

function cup_title(bytes32 cup) external returns (string){
   return cups[cup].title
}

And repeat that for every property.

Or, have just one function that returns multiple values from it (ie: all the attributes of the struct).

Or, use the Seriality library.