Problems indexing MongoDB array in c++ -

Problems indexing MongoDB array in c++ -

hello wonder i'm doing wrong here:

auto o = m_mongodbcc.findone( "db.items", query( "mystring" << s), &bson("arr" << bson("$slice"<< bson_array(index<<1) )) );

i want index arr array int index, code returns total array of ints.

thanks.

c++ mongodb