First of all, we have to admit that the resulting graph is a SINGLE object. I assume you want it to be of the same type Graph as your original two graphs g1 and g2. It means you have a choice to do one of the following:
Graph g = g1 + g2 or g1 += g2 (both choices are in pseudo-code, of course).
In any case, the result will contain a copy of the second graph, not the "original" object of the graph g2.
BGL actually provides you with a function to exactly do that, namely, the function "copy_graph"
You can do something like the following
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/copy.hpp>
typedef boost::adjacency_list<> Graph;
typedef Graph::vertex_descriptor vertex_t;
void merging(Graph & g1, vertex_t v_in_g1, const Graph & g2, vertex_t u_in_g2)
{
typedef boost::property_map<Graph, boost::vertex_index_t>::type index_map_t;
//for simple adjacency_list<> this type would be more efficient:
typedef boost::iterator_property_map<typename std::vector<vertex_t>::iterator,
index_map_t,vertex_t,vertex_t&> IsoMap;
//for more generic graphs, one can try //typedef std::map<vertex_t, vertex_t> IsoMap;
IsoMap mapV;
boost::copy_graph( g2, g1, boost::orig_to_copy(mapV) ); //means g1 += g2
vertex_t u_in_g1 = mapV[u_in_g2];
boost::add_edge(v_in_g1, u_in_g1, g1);
}
See also copy a graph (adjacency_list) to another one