Solidity – Understanding Abi Encoder V2

ethereum-classicsolidity

I came across the following code and found the term "pragma experimental ABIEncoderV2". Can anybody be specific in telling what this actually means?

//pragma solidity ^0.5.2;
pragma experimental ABIEncoderV2;
contract test {

struct document{
   string ipfsHash;
   string documentName;
   bytes32 accessKey;
}

struct grantAccess{
   address owner;
   address single;     
}

This is a part of the code I found over the web.

Best Answer

Solidity v0.4-v0.7

The standard ABI coder does not allow arrays of dynamic types, structs or nested variables between the Solidity contract and the dApp.

The ABI v2 coder; which allows structs, nested and dynamic variables to be passed into functions, returned from functions and emitted by events.

Note: Do not use experimental features on live deployments.

Solidity v0.8 and above

As per the list of v0.8 breaking changes, the ABIEncoderV2 is not experimental anymore - it is actually enabled by default by the compiler.

Related Solutions

Solidity – Handling Unimplemented Functions in Contracts

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

Solidity ABI Encoder – Why the New Experimental ABI Encoder Type is Supported

The standard ABI doesn't support structs. You can return the members of a struct in a fixed-length normally deliminated interface. For example:

function getter(bytes32 key) public returns(byte32 name, uint amount, bool isValid) :

return(myStruct[key].name, myStruct[key].amount, myStruct[key].isValid);

The idea of returning the struct object is experimental, e.g.

return(myStruct[key]);

Also, your snippet doesn't include the whole thing, but I suspect the issuerIndex is a list of keys, not the actual object. Your interface says it returns the object (the experimental feature) but possibly what you meant was returns(bytes32 key) which would be okay.

Hope it helps.

UPDATE

Based on comment below, the issue is the return value is cast as type "Issuer" which would be the experimental struct object. It should be cast as type address.

To return the address of the Issuer from the mapping:

returns(address issuer) { ...

return issuersIndex[index];