How to give struct members default values

Question

I am working on some complex Relational databases like structure in solidity and need to have some values initialized to a custom value not the usual 0.

I want to do something like this : (Status is an enum)

  struct AttributeStats {
    bytes32 ValidationDate = now;
    Status status = 1;
    bytes32 ValidationDuration = 356;
    bytes32 Value;
  }

I don't think this is supported by solidity, but i need it to happen.

Answer 1

No, this can't be done in Solidity. The reason is that there's no explicit allocation, and all slots in storage are implicitly zero until set to something else.

A pattern to use here would be to use a boolean value indicating whether a struct had been initialized (false by default because that's the zero value of a boolean), and everywhere you read a struct, check that flag and initialize if needed first. Something like this:

struct AttributeStats {
    bool initialized;
    Status status;
    bytes32 validationDuration;
    bytes32 value;
}
mapping (uint256 => AttributeStats) attributeStats;

function initializeIfNeeded(uint256 id) internal {
    AttributeStats stats = attributeStats[id];

    if (!stats.initialized) {
        stats.initialized = true;
        stats.validationDate = now;
        stats.status = 1;
        stats.validationDuration = 356;
    }        
}

function doSomething(uint256 id) public {
    initializeIfNeeded(id);

    AttributeStats storage stats = attributeStats[id];

    // ...
}

(It's unclear what now you meant to use in your pseudocode. I assumed you want to use the time when you initialize the struct, but if you wanted to use something like the time of contract creation, just store that in a variable in the constructor and use that instead.)

Answer 2

There is no built-in support for this in Solidity.

A workaround is to create a factory method where you can initialize the fields:

function createAttributeStats() public returns (AttributeStats memory attrStats) {
    attrStats = AttributeStats(now, 1, 356, 0);
}