What is the zero, empty or null value of a struct?

Question

If I have a mapping of string => custom defined struct, how do I check if a value is defined in the mapping?

The docs state that "every possible key exists and is mapped to a value whose byte-representation is all zeros".

What is the zero-value of a custom defined struct such that one can conditionally check for its existence?

Answer 1

You check that a value is defined in the mapping by checking it is not zero.

If an explicit setting of zero has meaning for your application, you need auxiliary data (or structure) to track when a value of zero has been explicitly set.

A lightweight approach would be to add a bool property to the struct (say named initialized), and to set it to true when the zero is explicitly set. As all bools are false (0) by default, you can check against the value true.

Alternatively, check that each member of the struct is zero. If a member is a string, cast it to bytes and then check its length is zero.

For an example see How to test if a struct state variable is set

Another data structure or mapping may be needed depending on the application.

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Here is a related example on using a pair to check the meaning of zero:

contract C {
    uint[] counters;
    function getCounter(uint index)
        returns (uint counter, bool error) {
            if (index >= counters.length) return (0, true);
            else return (counters[index], false);
        }
    function checkCounter(uint index) {
        var (counter, error) = getCounter(index);
        if (error) { ... }
        else { ... }
    }
}

Answer 2

There's really no such thing as "empty". An uninitialized index of the mapping is simply equal to the " zero" value of the proper type.

To check if a value has been assigned, just check if balances[msg.sender]== 0. If a user accesses the contract, but the balance should be 0, you can use a address => int256 mapping, and use -1 for 0 balances

Answer 3

function checkArray(bytes a) returns (bool){
    if ((a) && (a.length > 0))
        return true;
    return false;
}

Answer 4

For an unsigned index value of an array of struct (say Customer)- to check if the entry exists in given array of structs, you can do something like below:

 struct Customer {
    string userName;
    string dataHash;
    address bank;
    int256 upVotes;
}

Customer[] public customers; 

...
//assuming we have some customers added (customers.length>0)
...

function getCustomerIndex(string memory userName) internal view returns(bool, uint256) {
    for(uint i=0; i<customers.length; i++) {
        if (stringEquals(customers[i].userName, userName)){
            return (true, i);
        }
    }
    return (false, 0);
}

The explicit (false, 0) suggests that the customer with given username is not found in the customers struct array.

Now in my other code I can use the above function as follow:

// return 1 implies customer found and removed from struct Customer array
// return 0 implies no customer found in the struct Customer array
function removeCustomer(string calldata userName) external returns (uint8) {
        (bool result, uint256 index) = getCustomerIndex(userName);
        if (result){
            for (uint i=index+1; i<customers.length-1; i++){
                customers[i-1] = customers[i];
            }
            customers.length--;
            return 1;
        }
        return 0;
    }