#include <memory>
#include <mutex>

struct some_resource {
  void do_something() {}
};

std::shared_ptr<some_resource> resource_ptr;
std::mutex resource_mutex;
void
foo()
{
  std::unique_lock<std::mutex> lk(resource_mutex);
  if (!resource_ptr) {
    resource_ptr.reset(new some_resource);
  }
  lk.unlock();
  resource_ptr->do_something();
}

int
main()
{
  foo();
}