Returning dynamic arrays: out of gas

Question

I'm trying to return a dynamic array of structures from a Solidity function to a client. Since I need to build the result, I'm declaring a storage variable and add elements to it:

function conversations(address user) public view returns (Conversation[]){
    Conversation[] userConversations;
    userConversations.push(conversations[0]);
    return userConversations;
}

However, I keep getting an out of gas exception in my Truffle tests. How can it run out of gas when it's a view function? I understand that userConversations is a storage variable (or I won't be able to execute push on it), but does it really modify the state of the contract? If it does, how do you rewrite it so that you can get a dynamic array?

I've asked a similar question before but I've no definitive answer https://ethereum.stackexchange.com/questions/28040/can-i-eth-call-a-non-constant-function-from-javascript. — Ismael, Jul 23 '18 at 17:17

score 4 · Answer 1 · answered Jul 23 '18 at 13:38

You get an out of gas because your Conversation[] userConversations; that you believe is in memory is actually on storage.

Try to change it to Conversation[] memory userConversations; and you will see that it no longer compiles because .push is not available on memory arrays.

How about you try that:


function getConversations() public view returns (Conversation [] userConversations) {
    uint length = conversations.length;
    userConversations = new Conversation[](length);
    for (uint i = 0; i < length; i++) {
        userConversations[i] = conversations[i];
    }
}

score 0 · Accepted Answer · answered Jul 23 '18 at 11:14

0

userConversations.push(conversations[0]);

definitely modifies the state, so this cannot be used in a view function. You basically need to write a getter function to this setter function or make userConversations public, in that case it will get a getter function automatically.

answered Jul 23 '18 at 11:14

n1cK

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userConversations is a temp variable created just to build up an array to return.
Also, a getter for an array cannot return this array -- it's a function that requires an index and returns an element of this array. Am I right?
– ulu Jul 23 '18 at 11:22
When I try to return a field which is a dynamic array, it says Error: invalid solidity type!: tuple[] – ulu Jul 23 '18 at 11:27
1

You can't return struct as of now, it's available only in experimental version. I'm wondering why compiler won't throw error.
Edit: also i think userConversations actually is being written to storage, as it doesn't have memory keyword. – Maxpeinas Jul 23 '18 at 11:37
Other than that, it's of course not possible yet to return dynamically sized data yet. – n1cK Jul 23 '18 at 22:31

Returning dynamic arrays: out of gas

2 Answers2