Constant and Non-Constant Block Attributes

By Balaji Ramamoorthy

When iterating attributes of a specific block reference, you may not find all of them as expected. The reason for this is because an AcDbBlockTableRecord owns all attribute definitions, and an AcDbBlockReference (the insert of a block) owns all the non-constant attributes. If a block definition defines some constant attributes, AutoCAD will not create attributes in the AcDbBlockReference for them. By iterating the AcDbBlockReference object, you can find only the non-constant attributes, if you also want the constant attributes, you must iterate the AcDbBlockTableRecord as well.

The following sample code demonstrates this:

ads_name ename ;
ads_point pt ;
acedEntSel (L"Select a Block Reference: ", ename, pt) ;
 
AcDbObjectId id ;
acdbGetObjectId (id, ename) ;
 
AcDbBlockReference *pRef ;
acdbOpenAcDbObject ((AcDbObject *&)pRef, id, AcDb::kForRead) ;
 
//----- List Non-Const attributes 
AcDbObjectIterator  *pIter ;
pIter = pRef->attributeIterator () ; 
int cnt =0 ; 
while ( !pIter->done () )
{ 
    cnt++ ;
    AcDbObjectId ida =pIter->objectId () ;
    acutPrintf (L"nAttribute %d - ObjectId %ld", cnt, ida) ;
    pIter->step();
} 
delete pIter ;
acutPrintf (L"n%d Non-Const Attribute(s) Found!", cnt) ;
 
//----- List Constant attributes 
cnt =0 ; 
id =pRef->blockTableRecord () ;
AcDbBlockTableRecord *pRec ; 
acdbOpenAcDbObject ((AcDbObject *&)pRec, id, AcDb::kForRead) ;
if ( pRec->hasAttributeDefinitions () == Adesk::kTrue ) 
{
    AcDbBlockTableRecordIterator *pIter2 ; 
    pRec->newIterator (pIter2) ;
    while ( !pIter2->done () )
    { 
        AcDbEntity *pEnt ; 
        pIter2->getEntity (pEnt, AcDb::kForRead) ; 
        if ( pEnt->isKindOf (AcDbAttributeDefinition::desc ()) )
        {
            AcDbAttributeDefinition *pDef=(AcDbAttributeDefinition *)pEnt ; 
            if ( pDef->isConstant () == Adesk::kTrue ) 
            {
                cnt++; 
                acutPrintf (L"nConstant Attribute %d - ObjectId %ld)", cnt, pDef->objectId()) ;
            } 
        } 
        pEnt->close () ; 
        pIter2->step () ;
    } 
    delete pIter2 ;
} 
acutPrintf (L"n%d Constant Attribute(s) Found!", cnt) ;
 
pRec->close () ;
pRef->close () ;