TGeoManager* geom;                                                                                                                                      
TGeoVolume* topNode;                                                                                                                                    
std::map<std::string,TGeoMedium*> MediumMap;   


geom = new TGeoManager("myGeomanager","Production of Root Geometry Objects");                                          
  
TGeoMaterial *matVacuum = new TGeoMaterial("vacuum",0,0,0);                                                                                             
MediumMap["Vacuum"] = new TGeoMedium("vacuum", 1, matVacuum);                                                                                       

topNode = geom->MakeBox("TOP",MediumMap["Vacuum"],1000,1000,1000);
geom->SetTopVolume(topNode);


[.....]
topNode->AddNode(....);
[.....]


geom->CloseGeometry();                    
geom->Export("myGeo.root");