Usecase presented here: https://naveenspace7.github.io/2020/06/29/allocators.html

shared_allocator.cpp

#include <iostream>
#include <vector>
#include <algorithm>

#include <sys/ipc.h>
#include <sys/shm.h>

using namespace std;

void * shared_mem;

template<typename type>
struct MyAlloc : allocator<type>
{
    type* allocate(size_t size)
    {
        cout << "Allocate function called with size " << size << endl;
        return new(shared_mem) type[size];
    }

    template <typename U> struct rebind
    {
        typedef MyAlloc<U> other;
    };
};

void * setup_shm()
{
    int shmid = shmget(777, 100, 0644 | IPC_CREAT);

    shared_mem = shmat(shmid, (void*)(0), 0);

    if (shared_mem == (char *) (-1)) perror("error creating shared mem\n");

    return shared_mem;
}

int main()
{
    cout << "Starting process 1" << endl;

    void * shm_addr = setup_shm();

    vector<int, MyAlloc<int>> v1(7,7);

    for_each(begin(v1), end(v1), [](const int& i) { cout << i << ' ';});
    cout << endl;
}