Holding a large number of SEXPs in C++

gistfile1.cpp

#include <vector>
#include <utility>
#include <iterator>
#include <R.h>
#include <Rdefines.h>

template<typename Iter>
SEXP makePolygon(Iter b, Iter e) {
    SEXP
        coords;

    double
        *xi,
        *yi;
    size_t
        n(static_cast<size_t>(std::distance(b, e)));

    PROTECT(coords = allocMatrix(REALSXP, n, 2));

    xi = REAL(coords);
    yi = xi + n;

    for(; b!=e; ++b, ++xi, ++yi) {
        *xi = b->first;
        *yi = b->second;
    }

    // make call to Polygon_c to make the polygon here in practice,
    // but just return the matrix for this little example
    UNPROTECT(1);
    return coords;
}



SEXP makePolygonsFromPairList(SEXP pairList) {
    // make call to Polygons_c to make the polygons here in practice,
    // but just return the list of 'polygons' for this little example.
    return coerceVector(pairList, VECSXP);
}



struct PolyHolder {
    PolyHolder(void) {
        PROTECT_WITH_INDEX(currentRegion = R_NilValue, &icr);
        PROTECT_WITH_INDEX(regions = R_NilValue, &ir);
    }

    ~PolyHolder(void) {
        UNPROTECT(2);
    }

    void notifyEndRegion(void) {
        REPROTECT(regions = CONS(makePolygonsFromPairList(currentRegion), regions), ir);
        REPROTECT(currentRegion = R_NilValue, icr);
    }

    template<typename Iter>
    void addSubPolygon(Iter b, Iter e) {
        REPROTECT(currentRegion = CONS(makePolygon(b, e), currentRegion), icr);
    }

    SEXP getPolygons(void) {
        return regions;
    }

private:
    PROTECT_INDEX
        ir,
        icr;

    SEXP
        currentRegion,
        regions;
};



// The function to be used by .Call
// A silly little example that creates two 'regions', each
// of which contains two copies of the same polygon.
extern "C" SEXP testPolyHolder() {
    typedef std::pair<double, double> Point;
    typedef std::vector<Point> Polygon;

    Polygon
        p(5);

    PolyHolder
        ph;

    // a unit square with lower left at origin
    p[0] = p[4] = Point(0.0, 0.0);
    p[1] = Point(0.0, 1.0);
    p[2] = Point(1.0, 1.0);
    p[3] = Point(1.0, 0.0);

    for(int i(0); i<2; ++i) {
        for(int j(0); j<2; ++j) {
            ph.addSubPolygon(p.begin(), p.end());
        }
        ph.notifyEndRegion();
    }

    return ph.getPolygons();
}