How to set default value for array attributes

We were asked some array questions recently, and I found that there isn't a good sample in Maya devkit. This article will demonstrate how to initialize an array with MArrayDataBuilder.  

MPxNode::Compute isn't a good choice to give your array a default value. The reasons are that the compute will be called after a plug is dirty, or it will be connected on demand. The proper way to give your array a default value is to use the MPxNode::postConstructor method. For example, I can create a default value for the weightListNode sample from the Maya devkit with following code:

void weightList::postConstructor()
{
    MStatus status = MStatus::kSuccess;
    // Get the datablock outside the compute with MPxNode::forceCache
    MDataBlock block = this->forceCache();

    // The remaining part is the same as compute
    unsigned i, j;
    MObject thisNode = thisMObject();
    MPlug wPlug(thisNode, aWeights); 

    // Write into aWeightList
    for( i = 0; i < 3; i++) {
        status = wPlug.selectAncestorLogicalIndex( i, aWeightsList );
        MDataHandle wHandle = wPlug.constructHandle(block);
        MArrayDataHandle arrayHandle(wHandle, &status);
        //McheckErr(status, "arrayHandle construction failed\n");
        MArrayDataBuilder arrayBuilder = arrayHandle.builder(&status);
        
        for( j = 0; j < i+2; j++) {
            MDataHandle handle = arrayBuilder.addElement(j,&status);        
            float val = (float)(1.0f*(i+j)); 
            handle.set(val);
        }
        status = arrayHandle.set(arrayBuilder);
        
        wPlug.setValue(wHandle);
        wPlug.destructHandle(wHandle);
    }
}

This will be called after the node is created and will insert three elements into the weightList attribute. That's it :)