how to construct vector of vector in boost interprocess

Question

I'm new boost interprocess, and I've read the quick guide on Creating vectors in shared memory. But this example only construct a vector<int>, in my use case I have to construct more complicated data structures (usually nested containers).

Let's take an example for vector<vector<int>>, I write a small example based on that quick guide

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

using namespace boost::interprocess;

//Define an STL compatible allocator of ints that allocates from the managed_shared_memory.
//This allocator will allow placing containers in the segment
typedef allocator<int, managed_shared_memory::segment_manager>  ShmemAllocator;
typedef allocator<vector<int , ShmemAllocator>, managed_shared_memory::segment_manager>  ShmemAllocator2D;

//Alias a vector that uses the previous STL-like allocator so that allocates
//its values from the segment
typedef vector<int, ShmemAllocator> MyVector;
typedef vector<MyVector, ShmemAllocator2D> My2DVector;

//Main function. For parent process argc == 1, for child process argc == 2
int main(int argc, char *argv[])
{
  if(argc == 1){ //Parent process
    //Remove shared memory on construction and destruction
    struct shm_remove
    {
      shm_remove() { shared_memory_object::remove("MySharedMemory"); }
      ~shm_remove(){ shared_memory_object::remove("MySharedMemory"); }
    } remover;

    //Create a new segment with given name and size
    managed_shared_memory segment(create_only, "MySharedMemory", 65536);

    //Initialize shared memory STL-compatible allocator
    const ShmemAllocator2D alloc_inst (segment.get_segment_manager());

    //Construct a vector named "MyVector" in shared memory with argument alloc_inst
    My2DVector *myvector = segment.construct<My2DVector>("MyVector")(alloc_inst);

    for (int i = 0; i < 10;++i) {
      myvector->emplace_back();
//      for (int j = i; j < 10;++j) {
//        myvector->back().push_back(j);
//      }
    }

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

    //Check child has destroyed the vector
    if(segment.find<MyVector>("MyVector").first)
      return 1;
  }
  else{ //Child process
    //Open the managed segment
    managed_shared_memory segment(open_only, "MySharedMemory");

    //Find the vector using the c-string name
    My2DVector *myvector = segment.find<My2DVector>("MyVector").first;

    //Use vector in reverse order
    for (auto &vec : *myvector){
      std::cout << "row" << std::endl;
      for (auto i:vec)
        std::cout << i << ", ";
      std::cout << std::endl;
    }


    //When done, destroy the vector from the segment
    segment.destroy<MyVector>("MyVector");
  }

  return 0;
};

However this code will give complie error:

/usr/include/boost/container/vector.hpp:301:54: error: no matching function for call to ‘boost::interprocess::allocator<int, boost::interprocess::segment_manager<char, boost::interprocess::rbtree_best_fit<boost::interprocess::mutex_family>, boost::interprocess::iset_index> >::allocator()’
       : Allocator(), m_start(), m_size(), m_capacity()

I suppose emplace_back should work like stl vector....

Answer 1

It is not as simple as you may think - you initialize the outer std::vector with an allocator. But when using emplace_back to create internal std::vector<int> compiler tries to construct it using default constructor (which does not exist due to using shared memory allocator). I see two possible solutions.

Pass allocator explicitly

Instead of

myvector->emplace_back();

use

ShmemAllocator2D ac(segment.get_segment_manager());
myvector->emplace_back(ac);

for every operation adding anything to the vector. Yet, this is pretty error prone, cumbersome and there is already a known solution to this issue called:

scoped_allocator_adaptor

Just replace:

typedef allocator<vector<int , ShmemAllocator>, managed_shared_memory::segment_manager>  ShmemAllocator2D;

with:

typedef scoped_allocator_adaptor<allocator<vector<int , ShmemAllocator>, managed_shared_memory::segment_manager>>  ShmemAllocator2D;

and it should work fine