New features: Class BaseCollection

There is a common abstract base class for all collections for a given item type (e.g., gp_Pnt). Developer X can arbitrarily name this base class like MyPackage_BaseCollPnt in the examples above. This name is further used in the declarations of any (non-abstract) collection class to designate the C++ inheritance.

This base class has the public API:

- abstract class Iterator as the base class for all Iterators descried above;

- Iterator& CreateIterator () const - creates and returns the Iterator on this collection;

- Standard_Integer Size () const - returns the number of items in this collection;

- void Assign (const NCollection_BaseCollection& theOther) - copies the contents of the Other to this collection object;

These members enable accessing any collection without knowing its exact type. In particular, it makes possible to implement methods receiving objects of the abstract collection type:

#include <NColection_Map.hxx>

typedef NCollection_Map<gp_Pnt> MyPackage_MapOfPnt;

typedef NCollection_BaseCollection<gp_Pnt> MyPackage_BaseCollPnt;

MyPackage_MapOfPnt aMapPnt;

....

gp_Pnt aResult = COG (aMapPnt);

....

gp_Pnt COG(const MyPackage_BaseCollPnt& theColl)

{

gp_XYZ aCentreOfGravity(0., 0., 0.);

// create type-independent iterator (it is abstract type instance)

MyPackage_BaseCollString::Iterator& anIter = theColl.CreateIterator();

for (; anIter.More(); anIter.Next()) {

aCentreOfGravity += anIter.Value().XYZ();

}

return aCentreOfGravity / theColl.Size();

}

Note that there are fundamental differences between the shown type-independent iterator and the iterator belonging to a particular non-abstract collection:

- Type-independent iterator can only be obtained via the call CreateIterator(); the typed iterator - only via the explicit construction.

- Type-independent iterator is an abstract class, so it is impossible to copy it or to assign it to another collection object; the typed iterators can be copied and reassigned using the method Init() .

- Type-independent iterator is actually destroyed when its collection object is destroyed; the typed iterator is destroyed as any other C++ object in the corresponding C++ scope.

The common point between them is that it is possible to create any number of both types of iterators on the same collection object.