How to verify that a string, or a struct with only string properties, is initialised or not, inside of a mapping?

According to the docs, it means checking that the element of the mapping is at its 0-value, which is unclear to me when talking about strings.

contract C {
    mapping (address => string) m1;
    mapping (address => StringStruct) m2;

    struct StringStruct {
        string someString; // Always defined when initialising this struct
        // There could be other string properties here...
    function amIInBothMappings() returns (bool) {
         // Check that both m1[msg.sender] and m2[msg.sender].someString are non-0

In the example: what is the best way to implement amIInBothMappings that checks that msg.sender is un-initialised in both mappings m1 and m2?


2 Answers 2


One way is to check for the length of a String:

if (bytes(m1[msg.sender]).length != 0 && bytes(m2[msg.sender].someString).length != 0)
    // do your thing

See the answer I posted here

  • 1
    indeed, the bytes().length trick seems to work well!
    – lajarre
    Jun 6, 2016 at 12:55

Another way is to compare the sha3 hashes of the strings

if (sha3(myVariable) != sha3("")) {
 // is not empty string

However, this method is probably less efficient.

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