How to return "Null" or an "Empty" object in Solidity?

Question

I am currently writing a Smart Contract in Solidity. The smart contract, amongst other information, stores an array of properties object at the general level. The property object Looks like this:

    struct PropertyObj {
    string id;
    uint weiPrice;
    address owner;
}

Now there is a specific function that iterates over the array, finds the property and returns it (code below)

    function getPropertyByid(string memory _propertyId)private view returns(PropertyObj memory){
    for(uint i = 0; i<PropertyArray.length; i++){
        if (keccak256(bytes((PropertyArray[i].id))) == keccak256(bytes((_propertyId)))) {
            return PropertyArray[i];
        }
        return null;
    }
}

The "Problem" is that, unlike other programming languages, Solidity does not allow to return null (as far as I am concerned).

In other words, if throughout the iteration we do not find the property, then what we shall return if we specified that we need to return PropertyObj memory in the function signature?

Petr Hejda Petr Hejda · Accepted Answer · 2021-05-11T20:27:14

Solidity does not have null value, as you're correctly stating.

Your function can throw an exception using the revert() function.

It also seems that your implementation has a logical error. Your example would "return null" if the hash was not found during the first iteration. Instead, you may want to throw the exception after the loop has ended.

for(uint i = 0; i<PropertyArray.length; i++){
    if (keccak256(bytes((PropertyArray[i].id))) == keccak256(bytes((_propertyId)))) {
        return PropertyArray[i];
    }
}

revert('Not found');

Other option would be to return the empty object (with default values, i.e. zeros), if it fits your use case.

for(uint i = 0; i<PropertyArray.length; i++) {
    // ...
}

// not found, return empty `PropertyObj`
PropertyObj memory emptyPropertyObj;
return emptyPropertyObj;

How to return "Null" or an "Empty" object in Solidity?

1 Answers