GeomNode combine_with

hi,I use GeomNode combine_with function to make a new GeomNode,and when i do not need it, i use remove_all_geoms to remove the geom,but it looks do not work

code:
make new:
PT(GeomNode) geom_node_guandao=new GeomNode(“allguandao”);
for (…)
{
geom_node_guandao->combine_with(_pnode);
}
m_mainGaundaonode=geom_node_guandao;

clear:
m_mainGaundaonode->remove_all_geoms();
m_mainGaundaonode=NULL;

when it runs many times, Memory is growing, it looks like remove_all_geoms do not release memory