Semaphore using c++11 condition_variable and mutex, SpinLock using atomic_flag

Semaphore.cpp

#include <future>

struct SpinLock {
    std::atomic_flag spin_flag = ATOMIC_FLAG_INIT;
  
    void lock() {
        for (size_t k = 0; spin_flag.test_and_set(std::memory_order_acquire); k++)
        {
            if (k < 4)
            {
                continue;
            }
            int ns = 1;
            if (k < 32)
            {
                ns = 0;
            }

            std::this_thread::sleep_for(std::chrono::nanoseconds(ns));
        }
    }

    void unlock() {
        spin_flag.clear(std::memory_order_release);
    }
};

struct Semaphore
{
    int count = 0;
    std::condition_variable cond_var;

    std::mutex mutex;

    void wait() {
        std::unique_lock<std::mutex> lock(mutex);
        while (!count)
        {
            cond_var.wait(lock);
        }
        count--;
    }

    void notify() {
        std::unique_lock<std::mutex> lock(mutex);
        ++count;
        cond_var.notify_one();
    }
};