#include <iostream>
#include <boost/interprocess/managed_mapped_file.hpp>
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/index/rtree.hpp>
namespace bi = boost::interprocess;
namespace bg = boost::geometry;
namespace bgm = bg::model;
namespace bgi = bg::index;
int main()
{
    typedef bgm::point<float, 2, bg::cs::cartesian> point_t;
    typedef point_t value_t;
    typedef bgi::linear<32, 8> params_t;
    typedef bgi::indexable<value_t> indexable_t;
    typedef bgi::equal_to<value_t> equal_to_t;
    typedef bi::allocator<value_t, bi::managed_mapped_file::segment_manager> allocator_t;
    typedef bgi::rtree<value_t, params_t, indexable_t, equal_to_t, allocator_t> rtree_t;
    {
        bi::managed_mapped_file file(bi::open_or_create, "data.bin", 1024*1024);
        allocator_t alloc(file.get_segment_manager());
        rtree_t * rtree_ptr = file.find_or_construct<rtree_t>("rtree")(params_t(), indexable_t(), equal_to_t(), alloc);
        std::cout << rtree_ptr->size() << std::endl;
        rtree_ptr->insert(point_t(1.0, 1.0));
        rtree_ptr->insert(point_t(2.0, 2.0));
        std::cout << rtree_ptr->size() << std::endl;
    }
    {
        bi::managed_mapped_file file(bi::open_or_create, "data.bin", 1024*1024);
        allocator_t alloc(file.get_segment_manager());
        rtree_t * rtree_ptr = file.find_or_construct<rtree_t>("rtree")(params_t(), indexable_t(), equal_to_t(), alloc);
        std::cout << rtree_ptr->size() << std::endl;
        rtree_ptr->insert(point_t(3.0, 3.0));
        rtree_ptr->insert(point_t(4.0, 4.0));
        std::cout << rtree_ptr->size() << std::endl;
    }
    return 0;
}

Expected results

0
2
2
4