[Ethereum] How to return struct when function is called

selfdestructsoliditystruct

I'd like to return struct type User. However, when I tried the following getUser function, it returns an error. Is there any way that struct data would be returned from a function?

Contract

struct User{
    uint256 user_id;
    bytes32 name;
    bytes32 address;
    bytes32 birth_day;
}
mapping (uint256 => User) public users;

function getUser(uint256 user_id) constant returns (User) {
        return users[user_id];
}

Error

client/lib/contracts/User.sol: Solidity errors: :125:58: Error: Expected type name

function getUser(uint256 user_id) constant returns (struct User) {

Best Answer

You can not return a struct because Solidity implements them only as a loose bag of variables, they are not real objects.

You can use a solution from this answer: https://ethereum.stackexchange.com/a/3614/264

Update

Since 0.4.17 you can use pragma experimental ABIEncoderV2 to return structs. Of course, until the experimental keyword is removed, it's not safe to use it in production apps.

Related Solutions

[Ethereum] call mapping as arguments of function

The second error you get tells you that external functions cannot have internal types as arguments (i.e. structs and mappings). You can solve that one by specifying internal or private for your function.

For the first error, im not sure if it is a bug or not, but you cannot pass mappings as arguments. What you could do is wrap them in a struct and pass the struct as a parameter.

One final notice is that if you declare your mapping as a state variable then you have access to it directly in your function. So in you example table could just be a state variable...especially since you only use it for storage purposes.

[Ethereum] How to return a mapping type

You need to break the interface down into smaller chunks. In practice, you'll want to extend the internal storage pattern a little, I think.

Iterable mapping comes to mind. You keep lists of keys to mappings in arrays. Something like this:

  struct VoterStruct {
    address voterAddress;
    uint256 tokensBought;
    mapping (bytes32 => uint256) tokensUsed;
    bytes32[] tokenIndex; // a list of mapping keys that exist for the voter
  }

  mapping (address => VoterStruct) public voterStructs;

Then you can expose functions that allow iterating over lists while the contract returns one set of values at a time. Very sketchy to inspire some ideas:

// client can discover how may tokens are in the list for one voter
function getVoterTokenCount(address voter) returns(uint tokenCount) {
    return voterStructs[voter].tokenIndex.length;}

// client can discover the tokenId for a voter in a given row
function getVoterTokenAtIndex(address voter, uint index) returns(bytes32 tokenId) {
    return voterStructs[voter].tokenIndex[index];}

// client can discover token used by voter and tokenId
function getVoterTokenUsed(address voter, bytes32 tokenId) returns(uint tokensUsed) {
    return voterStructs[voter].tokensUsed[tokenId];}

// append keys to the list as you go
function insertVoterToken(address voter, bytes32 tokenId) returns(bool success) {
    voterStructs[voter].tokenIndex.push(tokenId);
    return true;}

Hope I didn't flub the syntax. It might be possible to sketch out the contract a little more. Would need a general description of what it's going to do. When I use this pattern, I always add some rules to ensure there are no duplicate keys, and so on.

Anyway, possibly the hints will inspire some ideas.

Hope it helps.

Best Answer

Update

Related Solutions

[Ethereum] call mapping as arguments of function

[Ethereum] How to return a mapping type

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