placement new

main.cc

#include <iostream>
#include <vector>

struct C {
  C(int a, int b) : a_{a}, b_{b} { std::cout << "ctor" << std::endl; }
  ~C() { std::cout << "dtor" << std::endl; }

  int a_;
  int b_;
} __attribute__((packed));

int main() {
  {
    std::cout << "constructing C on the stack" << std::endl;
    C{0, 1};
  }

  std::cout << std::endl;

  {
    std::cout << "using placement new" << std::endl;
    std::vector<uint8_t> v(sizeof(C));

    std::cout << "size of v: " << v.size() << std::endl;

    new (v.data()) C{2, 3};
  }

  return 0;
}