class Widget {
private:
  void forbidden();
};

void hijack(Widget& w) {
  w.forbidden();  // ERROR!
}

class Calculator {
  float current_val = 0.f;
 public:
   void clear_value() { current_val = 0.f; };
   float value() const {
     return current_val;
   };

   void add(float x) { current_val += x; };
   void multiply(float x) { current_val *= x; };
};

using Operation = void (Calculator::*)(float);
Operation op1 = &Calculator::add;
Operation op2 = &Calculator::multiply;
Calculator calc{};
(calc.*op1)(123.0f); // Calls add
(calc.*op2)(10.0f);  // Calls multiply

class Widget {
 public:
  static auto forbidden_fun() {
    return &Widget::forbidden;
  }
 private:
  void forbidden();
};

void hijack(Widget& w) {
  using ForbiddenFun = void (Widget::*)();
  ForbiddenFun const forbidden_fun = Widget::forbidden_fun();

  // Calls a private member function on the Widget
  // instance passed in to the function.
  (w.*forbidden_fun)();
}

// The first template parameter is the type
// signature of the pointer-to-member-function.
// The second template parameter is the pointer
// itself.
template <
  typename ForbiddenFun,
  ForbiddenFun forbidden_fun
>
struct HijackImpl {
  static void apply(Widget& w) {
    // Calls a private method of Widget
    (w.*forbidden_fun)();
  }
};

// Explicit instantiation is allowed to refer to
// `Widget::forbidden` in a scope where it's not
// normally permissible.
template struct HijackImpl<
  decltype(&Widget::forbidden),
  &Widget::forbidden
>;

void hijack(Widget& w) {
  HijackImpl<decltype(&Widget::forbidden), &Widget::forbidden>::apply(w);
}

// HijackImpl is the mechanism for injecting the
// private member function pointer into the
// hijack function.
template <
  typename ForbiddenFun,
  ForbiddenFun forbidden_fun
>
class HijackImpl {
  // Definition of free function inside the class
  // template to give it access to the
  // forbidden_fun template argument.
  // Marking hijack as a friend prevents it from
  // becoming a member function.
  friend void hijack(Widget& w) {
    (w.*forbidden_fun)();
  }
};
// Declaration in the enclosing namespace to make
// hijack available for name lookup.
void hijack(Widget& w);

// Explicit instantiation of HijackImpl template
// bypasses access controls in the Widget class.
template class
HijackImpl<
  decltype(&Widget::forbidden),
  &Widget::forbidden
>;

namespace {
// This is a *different* type in every translation
// unit because of the anonymous namespace.
struct TranslationUnitTag {};
}

void hijack(Widget& w);

template <
  typename Tag,
  typename ForbiddenFun,
  ForbiddenFun forbidden_fun
>
class HijackImpl {
  friend void hijack(Widget& w) {
    (w.*forbidden_fun)();
  }
};

// Every translation unit gets its own unique
// explicit instantiation because of the
// guaranteed-unique tag parameter.
template class HijackImpl<
  TranslationUnitTag,
  decltype(&Widget::forbidden),
  &Widget::forbidden
>;

struct alignas(64) spinlock {
  std::atomic<bool> lock_ = {0};

  void lock() noexcept {
    for (;;) {
      // Optimistically assume the lock is free on the first try
      if (!lock_.exchange(true, std::memory_order_acquire)) {
        return;
      }
      // Wait for lock to be released without generating cache misses
      while (lock_.load(std::memory_order_relaxed)) {
        // Issue X86 PAUSE or ARM YIELD instruction to reduce contention between
        // hyper-threads
        __builtin_ia32_pause();
      }
    }
  }

  bool try_lock() noexcept {
    // First do a relaxed load to check if lock is free in order to prevent
    // unnecessary cache misses if someone does while(!try_lock())
    return !lock_.load(std::memory_order_relaxed) &&
           !lock_.exchange(true, std::memory_order_acquire);
  }

  void unlock() noexcept {
    lock_.store(false, std::memory_order_release);
  }
};

struct TicketSpinLock {
    /**
     * Attempt to grab the lock:
     * 1. Get a ticket number
     * 2. Wait for it
     */
    void enter() {
        const auto ticket = next_ticket.fetch_add(1, std::memory_order_relaxed);

        while (true) {
            const auto currently_serving = now_serving.load(std::memory_order_acquire);
            if (currently_serving == ticket) {
                break;
            }

            const size_t previous_ticket = ticket - currently_serving;
            const size_t delay_slots = BACKOFF_MIN * previous_ticket;

            while (delay_slots--) {
                spin_wait();
            }
        }
    }
    static inline void spin_wait(void) {
    #if (COMPILER == GCC || COMPILER == LLVM)
        /* volatile here prevents the asm block from being moved by the optimiser: */
        asm volatile("pause" ::: "memory");
    #elif (COMPILER == MVCC)
        __mm_pause();
    #endif
    }

    /**
     * Since we're in the critical section, no one can modify `now_serving`
     * but this thread. We just want the update to be atomic. Therefore we can use
     * a simple store instead of `now_serving.fetch_add()`:
     */
    void leave() {
        const auto successor = now_serving.load(std::memory_order_relaxed) + 1;
        now_serving.store(successor, std::memory_order_release);
    }

    /* These are aligned on a cache line boundary in order to avoid false sharing: */
    alignas(CACHELINE_SIZE) std::atomic_size_t now_serving = {0};
    alignas(CACHELINE_SIZE) std::atomic_size_t next_ticket = {0};
};

static_assert(sizeof(TicketSpinLock) == 2*CACHELINE_SIZE,
    "TicketSpinLock members may not be aligned on a cache-line boundary");