cast shared_ptr to shared_ptr

#include <iostream>
#include <memory>

int main () {
  std::shared_ptr<int> foo;
  std::shared_ptr<const int> bar;

  foo = std::make_shared<int>(10);

  bar = std::const_pointer_cast<const int>(foo);

  std::cout << "*bar: " << *bar << '\n';
  *foo = 20;
  std::cout << "*bar: " << *bar << '\n';

  return 0;
}