auto new_foo(std::size_t size) -> std::tuple<std::unique_ptr<char>, int*>
{
    auto buffer = std::make_unique<char>(size * sizeof(int) + 8);
    auto begin = static_cast<int*>(static_cast<void*>(buffer.get() + 4));
    return std::make_tuple(std::move(buffer), begin);
}

struct X {
    std::unique_ptr<char> buffer_{nullptr};
    int* begin_{nullptr};
    std::size_t size_{0};

    X(std::size_t size) : size_{size}
    {
        auto foo = new_foo(size);
        buffer_ = std::unique_ptr<char>{std::get<0>(foo)};
        begin_ = std::get<1>(foo);
    }
};

X(std::size_t size)
        : buffer_{ ??? },
          begin_{ ??? },
          size_{size}
    {
    }

X(std::size_t size)
        : buffer_{std:get<0>(new_foo(size)},
          begin_{std:get<1>(new_foo(size)},
          size_{size}
    {
    }

auto {x,y,z} = f();