Allocate two std::vectors after each other on memory in nested data structure

Question

Let's say I have a data structure that looks something like this:

#include <vector>
#include <cstdef>

struct one {
    one(size_t n) : vec(n) {}
    std::vector<int> vec;
    // + some other stuff
};


struct entity {
    entity(size_t n) : One(n), Two(n) {}
    one One;
    one Two;
};

I want to have vec from Two allocated on memory right after vec from One ends, as it can make differences in performance. In C this would be easy by using pointer arrays. However I was wondering if there is a c++ style (using std::vectors) solution for that, maybe adapting the constructors?

Answer 1

You can use the span for this

#include <algorithm>
#include <iostream>
#include <memory>
#include <ranges>
#include <span>

struct Part {
    Part(int *ptr, std::size_t n)
        : span(ptr, n)
    {
        std::ranges::fill(span, std::rand());
    }

    std::span<int> span;
};

struct Entity {
    Entity(std::size_t n)
        : mem(new int[n * 2]),
          first(mem.get(), n),
          second(mem.get() + n, n)
    {}

    std::unique_ptr<int[]> mem;
    Part first;
    Part second;
};

int main()
{
    Entity ent(10);
    const auto print = [](int a){ std::cout << a << std::endl; };
    std::ranges::for_each(ent.first.span, print);
    std::ranges::for_each(ent.second.span, print);
}

example in compiler explorer