C++11读写公平实现

#include<iostream>
#include<mutex>
#include<condition_variable>
#include<thread>
#include<string>
using namespace std;
class semaphere{
	public: 
	semaphere(int n):m_count(n){}
	semaphere(const semaphere& s)=delete;
	semaphere & operator=(const semaphere& s)=delete;
	void Wait(){
		unique_lock<mutex> lc(m_mtx);
		m_cv.wait(lc,[&]{return m_count>0;});
		m_count--;
	}
	void Signal(){
		{
				unique_lock<mutex> lc(m_mtx);
				m_count++;
		}
		m_cv.notify_one();
	}
	private:
		mutex m_mtx;
		condition_variable m_cv;
		int m_count;
}; 
semaphere  mtx(1),rw(1),mtx_gongpin(1); //mtx  用于reader_count的控制       mtx_gongpin 用于读写公平的控制   
int reader_count=0;
string read_context="hello world";
int w_times=0;
void reader_0(){
	mtx_gongpin.Wait();
	
		mtx.Wait();
		reader_count++;
		if(reader_count==1) rw.Wait();
		mtx.Signal();
	mtx_gongpin.Signal();
//	cout<<"reding time"<<endl;
	cout<< read_context<<endl;
	this_thread::sleep_for(chrono::milliseconds(1000));
	mtx.Wait();
	reader_count--;
	if(reader_count==0) rw.Signal();
	mtx.Signal();
};
void write_0(){
	mtx_gongpin.Wait();
	rw.Wait();
//	cout<<"reding time"<<endl;
	this_thread::sleep_for(chrono::milliseconds(1000));
	read_context="hello world"+to_string(++w_times); 
	rw.Signal();
	mtx_gongpin.Signal();
}	 
int main(){
	thread  t_r[5];
	for(auto &t:t_r) t=thread(reader_0);
	thread  t_w(write_0);
	thread  t_r1[5];
	for(auto &t:t_r1) t=thread(reader_0);
	for(auto &t:t_r) t.join();
	for(auto &t:t_r1) t.join();
	t_w.join();
	thread  t_r3[5];
	for(auto &t:t_r3) t=thread(reader_0);
	for(auto &t:t_r3) t.join();
	return 0;
} 

在外层多加了一层锁，使得读者写着都需要去竞争这个锁，
读者需要加锁解锁两次是为了实现读者的并发

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