Custom Allocators.md

Examples

Using json with boost stateless fast_pool_allocator

#include <boost/pool/pool_alloc.hpp>
#include "jsoncons/json.hpp"

typedef jsoncons::basic_json<char, boost::fast_pool_allocator<void>> bfp_json;

bfp_json j;

j.insert_or_assign("FirstName","Joe");
j.insert_or_assign("LastName","Smith");

Using json with stateful Boost.Interprocess allocators

#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/containers/vector.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/containers/string.hpp>
#include <cstdlib> //std::system
#include <jsoncons/json.hpp>

using namespace jsoncons;

typedef boost::interprocess::allocator<char,
        boost::interprocess::managed_shared_memory::segment_manager> shmem_allocator;

struct boost_sorted_policy : public sorted_policy
{
    template <class T, class Allocator>
    using sequence_container_type = boost::interprocess::vector<T,Allocator>;

    template <class CharT, class CharTraits, class Allocator>
    using key_storage = boost::interprocess::basic_string<CharT, CharTraits, Allocator>;

    template <class CharT, class CharTraits, class Allocator>
    using string_storage = boost::interprocess::basic_string<CharT, CharTraits, Allocator>;
};

using my_json = basic_json<char,boost_sorted_policy,shmem_allocator>;

int main(int argc, char *argv[])
{
   if(argc == 1){  //Parent process
      //Remove shared memory on construction and destruction
      struct shm_remove
      {
         shm_remove() { boost::interprocess::shared_memory_object::remove("MySharedMemory"); }
         ~shm_remove() noexcept { boost::interprocess::shared_memory_object::remove("MySharedMemory"); }
      } remover;

      //Construct managed shared memory
      boost::interprocess::managed_shared_memory segment(boost::interprocess::create_only, 
                                                         "MySharedMemory", 65536);

      //Initialize shared memory STL-compatible allocator
      const shmem_allocator alloc(segment.get_segment_manager());

      // Create json value with all dynamic allocations in shared memory

      shm_json* j = segment.construct<shm_json>("MyJson")(json_array_arg, semantic_tag::none, alloc);
      j->push_back(10);

      shm_json o(json_object_arg, semantic_tag::none, alloc);
      o.insert_or_assign("category", "reference");
      o.insert_or_assign("author", "Nigel Rees");
      o.insert_or_assign("title", "Sayings of the Century");
      o.insert_or_assign("price", 8.95);

      j->push_back(o);

      shm_json a = shm_json::array(2,shm_json(object_arg_t,semantic_tag::none,alloc),alloc);
      a[0]["first"] = 1;

      j->push_back(a);

      std::pair<shm_json*, boost::interprocess::managed_shared_memory::size_type> res;
      res = segment.find<shm_json>("MyJson");

      std::cout << "Parent:" << std::endl;
      std::cout << pretty_print(*(res.first)) << std::endl;

      //Launch child process
      std::string s(argv[0]); s += " child ";
      if(0 != std::system(s.c_str()))
         return 1;


      //Check child has destroyed all objects
      if(segment.find<my_json>("MyJson").first)
         return 1;
   }
   else{
      //Open managed shared memory
      boost::interprocess::managed_shared_memory segment(boost::interprocess::open_only, 
                                                         "MySharedMemory");

      std::pair<shm_json*, boost::interprocess::managed_shared_memory::size_type> res;
      res = segment.find<shm_json>("MyJson");

      if (res.first != nullptr)
      {
          std::cout << "Child:" << std::endl;
          std::cout << pretty_print(*(res.first)) << std::endl;
      }
      else
      {
          std::cout << "Result is null" << std::endl;
      }

      //We're done, delete all the objects
      segment.destroy<shm_json>("MyJson");
   }
   return 0;
}

Output:

Parent:
[
    10,
    {
        "author": "Nigel Rees",
        "category": "reference",
        "price": 8.95,
        "title": "Sayings of the Century"
    },
    [
        {
            "first": 1
        },
        {}
    ]
]
Child:
[
    10,
    {
        "author": "Nigel Rees",
        "category": "reference",
        "price": 8.95,
        "title": "Sayings of the Century"
    },
    [
        {
            "first": 1
        },
        {}
    ]
]