Solidity – Checking Zero Value of Structure

solidity

I have the following statements:

Voter sender = voters[msg.sender];
if (!sender || sender.voted) throw;

I'm new to solidity, so I might not be aware of how (or if) coercion works.

As I found out in this question,

[…] every possible key exists and is mapped to a value whose byte-representation is all zeros […]

Knowing that, shouldn't the !sender statement above return true?

Best Answer

If Voter is a struct, the code !voter[sender] will probably result in an error. In your case, you'll want to check the voted or another field in the Voter struct.

For now, checking guards via voters[msg.sender].voted == 0 is probably the most reliable approach.

As a side note, strings can be a bit tricker: some_string == 0 will not work. In this case I use:

function(string value) { 
   if (bytes(value).length == 0)
     ...don't do it...
}

Related Solutions

Solidity Structs – What is the Zero, Empty, or Null Value?

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.

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 { ... }
    }
}

Solidity Validations – Checking for Valid Enum Values in Solidity

Possibly here is what you can do.

Declare array to store the enum choices

In constructor call push all the enum choices in the array

In the method to match the enum choice just loop the array and match it against the passed enum choice

Best Answer

Related Solutions

Solidity Structs – What is the Zero, Empty, or Null Value?

Solidity Validations – Checking for Valid Enum Values in Solidity

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