boost::interprocess Containers of containers NOT i

Question

I have the example demo program with a boost::interprocess Containers of containers type. But I like to use the class also a normal class within my process memory. Can someone help me to write a constructor which takes no arguments to have the class initialized in my current process memory.

#include <boost/interprocess/containers/vector.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/archive/xml_oarchive.hpp>
#include <boost/archive/xml_iarchive.hpp>
#include <shmfw/serialization/interprocess_vector.hpp>
#include <stdlib.h>     /* srand, rand */
#include <time.h>       /* time */


using namespace boost::interprocess;
//Alias an STL-like allocator of ints that allocates ints from the segment
typedef allocator<int, managed_shared_memory::segment_manager>  ShmemAllocator;

//Alias a vector that uses the previous STL-like allocator
typedef vector<int, ShmemAllocator> MyVector;

typedef allocator<void, managed_shared_memory::segment_manager >                           void_allocator;

class MyStruct {
public:
    MyVector myVector;
    //Since void_allocator is convertible to any other allocator<T>, we can simplify
    //the initialization taking just one allocator for all inner containers.
    MyStruct ( const void_allocator &void_alloc )
        : myVector ( void_alloc )
    {}
    // Thats what I like to have       
    //MyStruct ()
    //    : myVector ( ?? )
    //{}
};

int main () {

    // I would like to have something like that working and also the shm stuff below
    // MyStruct x;


    managed_shared_memory segment;
    //A managed shared memory where we can construct objects
    //associated with a c-string
    try {
         segment =  managed_shared_memory( create_only, "MySharedMemory", 65536 );
    } catch (...){
        segment = managed_shared_memory( open_only, "MySharedMemory" );
    }

    //Initialize the STL-like allocator
    const ShmemAllocator alloc_inst ( segment.get_segment_manager() );

    MyStruct *myStruct_src =  segment.find_or_construct<MyStruct> ( "MyStruct" ) ( alloc_inst );
    srand (time(NULL));
    myStruct_src->myVector.push_back ( rand() );

    MyStruct *myStruct_des =  segment.find_or_construct<MyStruct> ( "MyStruct" ) ( alloc_inst );

    for ( size_t i = 0; i < myStruct_src->myVector.size(); i++ ) {
        std::cout << i << ": " << myStruct_src->myVector[i] << " = " << myStruct_des->myVector[i] << std::endl;
    if(myStruct_src->myVector[i] != myStruct_des->myVector[i]) {
      std::cout << "Something went wrong!" << std::endl;
    }
    }


    //segment.destroy<MyVector> ( "MyVector" );
    return 0;
}

Answer 1

If you change the allocator type, you change the container (such is the nature of compile-time template instantiation).

Technically, you could devise a type-erased allocator (à la std::function or boost::any_iterator) but this would probably result in abysmal performance. Also, it would still require all the allocators to correspond in all the statically known properties, reducing flexibility.

In reality, I suggest just templatizing MyStruct on the Allocator type to be used for any embedded containers. Then specifically take such an allocator in the constructor:

// Variant to use on the heap:
using HeapStruct  = MyStruct<std::allocator>;
// Variant to use in shared memory:
using ShmemStruct = MyStruct<BoundShmemAllocator>;

Demo Program:

#include <boost/interprocess/containers/vector.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/range/algorithm.hpp>
#include <iostream>
#include <cassert>

namespace bip = boost::interprocess;
template <typename T> 
    using BoundShmemAllocator = bip::allocator<T, bip::managed_shared_memory::segment_manager>;

///////////////////////////////////////////////////////////////
// Your MyStruct, templatized for an Allocator class template

template <template<typename...> class Allocator>
class MyStruct {
public:
    bip::vector<int,    Allocator<int>    > ints;
    bip::vector<double, Allocator<double> > doubles;

    MyStruct(const Allocator<void>& void_alloc = {})
        : ints(void_alloc),
          doubles(void_alloc)
    {}
};

// Variant to use on the heap:
using HeapStruct  = MyStruct<std::allocator>;
// Variant to use in shared memory:
using ShmemStruct = MyStruct<BoundShmemAllocator>;

//
///////////////////////////////////////////////////////////////

int main() {
    srand(time(NULL));

    // You can have something like this working: 
    HeapStruct x; // and also the shm stuff below
    std::generate_n(std::back_inserter(x.ints),    20, &std::rand);
    std::generate_n(std::back_inserter(x.doubles), 20, &std::rand);

    // A managed shared memory where we can construct objects
    bip::managed_shared_memory segment = bip::managed_shared_memory(bip::open_or_create, "MySharedMemory", 65536);
    BoundShmemAllocator<int> const shmem_alloc(segment.get_segment_manager());

    auto src = segment.find_or_construct<ShmemStruct>("MyStruct")(shmem_alloc);
    src->ints.insert(src->ints.end(),       x.ints.begin(),    x.ints.end());
    src->doubles.insert(src->doubles.end(), x.doubles.begin(), x.doubles.end());

    auto des = segment.find_or_construct<ShmemStruct>("MyStruct")(shmem_alloc);

    std::cout << "-------------------------";
    boost::copy(src->ints, std::ostream_iterator<int>(std::cout << "\nsrc ints: ", "; "));
    boost::copy(des->ints, std::ostream_iterator<int>(std::cout << "\ndes ints: ", "; "));
    std::cout << "\n-------------------------";
    boost::copy(src->doubles, std::ostream_iterator<double>(std::cout << "\nsrc doubles: ", "; "));
    boost::copy(des->doubles, std::ostream_iterator<double>(std::cout << "\ndes doubles: ", "; "));

    assert(src->ints.size()    == des->ints.size());
    assert(src->doubles.size() == des->doubles.size());
    assert(boost::mismatch(src->ints,    des->ints)    == std::make_pair(src->ints.end(),    des->ints.end()));
    assert(boost::mismatch(src->doubles, des->doubles) == std::make_pair(src->doubles.end(), des->doubles.end()));

    segment.destroy<ShmemStruct>("MyStruct");
}