Solidity Bytes – How to Check If a Bytes Variable Is Empty

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.

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 { ... }
    }
}

All storage items have a default value equivalent to zero that is identical whether something is stored as that value or the value does not exist.

With a struct, it is as if the struct already exists, but with all its members set to their defaults. There is no pure way to ask "does this struct exist?".

You can can handle this by giving your struct a field called something like .exists, and setting it to true when you create the struct. However, in practice you normally find that there is some field in your struct that always happens to be non-zero if the struct exists, so you can check for that instead of creating a dedicated field for that purpose.