队列没有保存更新的数据

Question

我有一个简单的多线程程序，其中一个线程正在填充队列，而另一个线程正在使用队列，我知道这个示例程序有问题，它正试图复制我的应用程序。我没有解释为什么会这样，我希望输出会弹出 1 、 2 等

#include <iostream>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <queue>
using namespace std;

class A{
    public :
    int y;
    A(int z):y(z){
        cout << "constructed =" << y << std::endl;
    }
    
    ~A(){
        cout << "destructed =" << y << std::endl;
    }

};
std::mutex lk;

std::queue<shared_ptr<A>> p;
std::condition_variable cv;
int ct = 1;
void waitfordata(){
    std::unique_lock<std::mutex> lock(lk);
    cv.wait(lock, []{return p.size();});
    cout << "popping  = " << p.front()->y << endl;
    p.pop();
}

void threadhandler(){
    while(1){
        waitfordata();
    }
}
int main()
{

    std::thread foo(threadhandler);
    
    std::shared_ptr<A> svc_resp = std::make_shared<A>(ct++);
    while(1){
        
        {
            std::lock_guard<std::mutex> lock(lk);
            p.push(svc_resp);
        }
        cv.notify_one();
        std::this_thread::sleep_for(std::chrono::seconds(1));
        std::shared_ptr<A> svc_resp = std::make_shared<A>(ct++);
    }

    return 0;
}

Ouput which I got
constructed =1
popping  = 1
constructed =2
destructed =2
popping  = 1
constructed =3
destructed =3
popping  = 1
constructed =4
destructed =4
popping  = 1

Ouput which I  was expecting

constructed =1
destructed =1  <===== why is 1 not destructed
popping  = 1
constructed =2
destructed =2
popping  = 2  <===== why is this not taking the next data 
constructed =3
destructed =3
popping  = 3  <===== why is this not taking the next data 
constructed =4
destructed =4
popping  = 4 <===== why is this not taking the next data

Answer 1

您在 main 中有两个个名为 svc_resp 的变量。第二个（在 while 循环的底部）将创建并短暂拥有一个 shared_ptr<A> ，然后立即销毁它。这说明了随着数量的增加而构造和破坏的输出。

第一个 svc_resp 变量，在 while 循环之前声明，作为 1 的 y 并且在每个循环中 re-pushed 到您的队列。

修复很简单：从第二个 svc_resp 声明中删除类型名称，将其变成赋值。

svc_resp = std::make_shared<A>(ct++);

队列没有保存更新的数据

Queue not holding the updated data

c++

smart-pointers