Semaphore built from mutex in C++11

#include <mutex>

#include <condition_variable>

using namespace std;

class semaphore{

private:

    mutex mtx;

    condition_variable cv;

    int count;

public:

    semaphore(int count_ = ):count(count_){;}

    void notify()

    {

        unique_lock<mutex> lck(mtx);

        ++count;

        cv.notify_one();

    }

    void wait()

    {

        unique_lock<mutex> lck(mtx);

        while(count == ){

            cv.wait(lck);

        }

        count--;

    }

};