Use OGR to parse a Shapefile (here, we want to parse an OSM Shapefile to get the main roads)

ConvertFromGEOStoOGR.cpp

OGRLineString  * convertFromGEOStoOGR(LineString * ls) {
    OGRLineString * lineString = (OGRLineString*) OGRGeometryFactory::createGeometry(wkbLineString);
    for(int i = 0; i < ls->getNumPoints(); ++i) {
        Point * pt = ls->getPointN(i);
        lineString->addPoint(pt->getX(), pt->getY());
    }
    return lineString;
}

ConvertFromOGRToGEOS.cpp

using namespace geos;
using namespace geos::geom;

LineString * convertFromOGRToGEOS(OGRLineString * ls) {
    CoordinateSequence * coordinates = new CoordinateArraySequence();
    for(int i = 0; i < ls->getNumPoints(); ++i) {
        OGRPoint pt;
        ls->getPoint(i, &pt);
        Coordinate coord(pt.getX(), pt.getY());
        coordinates->add(coord);
    }
    GeometryFactory globalFactory;
    return globalFactory.createLineString(coordinates);
}

Point * convertFromOGRToGEOS(OGRPoint * pt) {
    GeometryFactory globalFactory;
    return globalFactory.createPoint(Coordinate(pt->getX(), pt->getY()));
}

ShapefileParser.cpp

OGRRegisterAll();
OGRDataSource *poDS;

poDS = OGRSFDriverRegistrar::Open(filename.toStdString().c_str(), FALSE);
if( poDS == NULL )
{
   qWarning() << "Open failed for " << filename;
   return;
}

QList<QPair<OGRGeometry*,GeometryAttribute*> > _shapestoDraw;
QSet<QString> acceptedHWFieldsSet = QSet<QString>();
acceptedHWFieldsSet << "primary_link" << "tertiary_link" << "trunk_link" << "motorway" << "road" <<  "secondary_link" << "tertiary" << "motorway_link" << "secondary" << "trunk" << "primary";

for(int i = 0; i < poDS->GetLayerCount(); ++i) {
   OGRLayer  *poLayer = poDS->GetLayer(i);
   
   qDebug() << "Loading layer" << QString::fromStdString(poLayer->GetName()) << "...";
   
   OGRFeature *poFeature;
   OGRSpatialReference *poTarget = new OGRSpatialReference();
   poTarget->importFromProj4(_project->loader()->getOutputProj());

   poLayer->ResetReading();
   OGRFeatureDefn *poFDefn = poLayer->GetLayerDefn();
   
   int hwIdx = poFDefn->GetFieldIndex("highway");
   
   while( (poFeature = poLayer->GetNextFeature()) != NULL )
   {
       QString HWFieldStr = QString::fromStdString(poFeature->GetFieldAsString(hwIdx));
       if(acceptedHWFieldsSet.contains(HWFieldStr)) {
           OGRGeometry *poGeometry = poFeature->GetGeometryRef();
           if( poGeometry != NULL
               && wkbFlatten(poGeometry->getGeometryType()) == wkbPoint )
           {
               poGeometry->transformTo(poTarget);
               OGRPoint * pt = (OGRPoint *) poGeometry;
               _shapestoDraw.append(qMakePair(pt, new GeometryAttribute(50,QColor(0,0,255))));

           }
           else if (poGeometry != NULL
                    && wkbFlatten(poGeometry->getGeometryType()) == wkbLineString)
           {
               poGeometry->transformTo(poTarget);
               OGRLineString * ls = (OGRLineString *) poGeometry;
               _shapestoDraw.append(qMakePair(ls, new GeometryAttribute(1,QColor(255,0,0))));

           }
       }
   }
}

// Do not delete any structure as they will be used later
qDebug() << "loaded shapefile with" << _shapestoDraw.count() << "features";