[CONNECT C++] ElementAgenda, IElementSet

> IElementSet and ElementAgenda appear to serve the same purpose

They don't.
ElementAgenda gives you EditElementHandles; IElementSet gives you ElementHandles.
IElementSet is purposely opaque (user need neither know nor care how the elements are supplied); ElementAgenda is not.
In fact ElementAgenda's *iterator* is an implementation of IElementSet.
You build an IElementSet by implementing the interface on some collection of element(s).

Unknown said:

Build an IElementSet by implementing the interface on some collection of element(s)

Something like this?

struct MyCollection : IElementSet
{
   size_t index;
   bvector<MyClassInheritsFromElementHandle> data;
   MyCollection () : index (0) {}
   virtual bool GetFirst (ElementHandleR eh) override { index= 0; eh = data [index]; }
   virtual bool GetNext  (ElementHandleR eh) override { eh = data [++index]; }
};

Unknown said:

Build an IElementSet by implementing the interface on some collection of element(s)

Something like this?

struct MyCollection : IElementSet
{
   size_t index;
   bvector<MyClassInheritsFromElementHandle> data;
   MyCollection () : index (0) {}
   virtual bool GetFirst (ElementHandleR eh) override { index= 0; eh = data [index]; }
   virtual bool GetNext  (ElementHandleR eh) override { eh = data [++index]; }
};

sure, assuming some bounds-checking and return statements.

or simpler example:

struct OneElement : IElementSet
{
    ElementHandle m_eh;
    size_t GetCount() override { return 1; }
    bool GetFirst(ElementHandleR eh) override { eh=m_eh; return true; }
    bool GetNext(ElementHandleR) override { return false; }
};