出队时的奇怪现象(strange phenomenon while dequeuing)

 我创建了一个队列矢量。 队列中有10个向量和大约1000个元素。 然后我尝试以循环方式将所有向量中的一个元素从队列中出列。 但我观察到一个非常奇怪的现象。 我看到一些向量有2个元素出列，有些没有。 
 
struct pkt{
    int data;
    int time;
};

//Input queues are already filled with 1000 packets
std::vector<std::queue<pkt>> per_port_input_queue;
std::vector<std::queue<pkt>> per_port_output_queue;

// Dequeing one element at a time in a round-robin fashion
for(int i=0;i<5; i++)
{
decoded_pkt=per_port_input_queue[i].front();
send_to_output(decoded_pkt);
per_port_input_queue[decoded_pkt.data].pop();
}

void send_to_output(pkt decoded_pkt)
{
     per_port_output_queue[decoded_pkt.data].push(decoded_pkt);

}
 
 您可以在下面找到观察到的输出和我的预期输出。 
 
========================================================
Port       I/p (t=0)  (after dequing        My
                           5 pkt)       Expected result
========================================================
 0         1000          1000               999
 1         1000          1000               999
 2         1000          1000               999
 3         1000          998                999
 4         1000          1000               999
 5         1000          1000              1000
 6         1000          999               1000
 7         1000          999               1000
 8         1000          1000              1000
 9         1000          999               1000
 
 有人可以澄清为什么会发生这种情况以及如何避免这种情况。 在c ++中使用队列是否有问题 

I have created a vector of queues. There are 10 vectors and around 1000 elements in the queue. Then I try to dequeue one element from the in a queue across all the vector in a round-robin fashion. But I observe a very strange phenomenon. I see that some of the vectors have 2 elements dequeued and some have none. 
 
struct pkt{
    int data;
    int time;
};

//Input queues are already filled with 1000 packets
std::vector<std::queue<pkt>> per_port_input_queue;
std::vector<std::queue<pkt>> per_port_output_queue;

// Dequeing one element at a time in a round-robin fashion
for(int i=0;i<5; i++)
{
decoded_pkt=per_port_input_queue[i].front();
send_to_output(decoded_pkt);
per_port_input_queue[decoded_pkt.data].pop();
}

void send_to_output(pkt decoded_pkt)
{
     per_port_output_queue[decoded_pkt.data].push(decoded_pkt);

}
 
Below you can find the observed output and my expected output.
 
========================================================
Port       I/p (t=0)  (after dequing        My
                           5 pkt)       Expected result
========================================================
 0         1000          1000               999
 1         1000          1000               999
 2         1000          1000               999
 3         1000          998                999
 4         1000          1000               999
 5         1000          1000              1000
 6         1000          999               1000
 7         1000          999               1000
 8         1000          1000              1000
 9         1000          999               1000
 
Can somebody clarify me why this occurs and how to avoid this. Is there some problem with using queues in c++

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