Contour Iterators (or Cursors)

Michael Olea <>
21 Aug 2006 20:32:20 -0400

Recently I have been working on problems where I need to iterate over a
cyclic structure; the edges of a "face" in a planar map, for example (a
planar map partitions the plane into faces, edges, and vertices). In this
case the planar map is encoded as a "doubly connected edge list". Each edge
has a start vertex and an end vertex (giving it a direction), a left face
and a right face, and next and previous edges. The idea is to iterate over
the edges forming the outer contour of a face, given the face and some edge
on its outer contour. I would like to use the STL start/end approach
(actually, a cursor/property_map approach, but that is not the issue here).
The difficulty is that the start and end are the same edge. A GOF style
iterator (ignoring error checking, and type parameterization issues) is

class EdgeIterator
    EdgeIterator(const PlanarFace *pF, const PlanarEdge *pE)
      : pFace(pF), pEStart(pE), pENext(0) {}

    const PlanarEdge* First()
        if (! pEStart)
            return pEStart;
        if ( pEStart->LeftFace() == pFace )
            pENext = pEStart->Next();
            pENext = pEStart->Previous();
        return pEStart;

    const PlanarEdge* Next()
        if ( pENext == pEStart )
            return 0;
        const PlanarEdge *pE = pENext;
        if ( pENext->LeftFace() == pFace )
            pENext = pENext->Next();
            pENext = pENext->Previous();
        return pE;

    const PlanarFace *pFace;
    const PlanarEdge *pEStart;
    const PlanarEdge *pENext;

The only idea I have had so far for creating a Start/End pair is to make an
adaptor (ignoring many details):

class EdgeIteratorAdaptor : private EdgeIterator
    EdgeIteratorAdaptor(const PlanarFace *pF, const PlanarEdge *pE)
      : EdgeIterator(pF, pE), pEdge(First()) {}

    const PlanarEdge operator*() const { return pEdge; }

    EdgeIteratorAdaptor& operator++() { pEdge = Next(); return *this; }

    bool operator==(const EdgeIteratorAdaptor& x) const
            { return pEdge == x.pEdge; }

    bool operator!=(const EdgeIteratorAdaptor& x) const
            { return pEdge != x.pEdge; }

    const PlanarEdge *pEdge;

class PlanarFace
    EdgeIteratorAdaptor begin(const PlanarEdge *p) const
                  { return EdgeIteratorAdaptor(this, p); }

     EdgeIteratorAdaptor end() const
                  { return EdgeIteratorAdaptor(this, 0); }

Any better ideas?

-- Michael

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