push_back a deque后deque<deque>为空

Question

我尝试从文件中读取行，每5行推入一个deque dq，然后将dq推入一个deque队列。dq连续得到5行，但是在将dq push_back成ques之后，调试器显示一个空队列被插入到ques中，如下所示是调试器的输出。

ques    std::__1::deque<std::__1::deque<std::__1::basic_string<char,            std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >, std::__1::allocator<std::__1::deque<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > > >   size=1  
[0] std::__1::deque<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >    size=5  
   [0]          
   [1]          
   [2]          
   [3]          
   [4]          

dq  std::__1::deque<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > >    size=5  
   [0]=std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >    "CCCGGGT"   
   [1]  std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >   "CCCCGCA"   
   [2]  std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >   "GGGCCTC"   
   [3]  std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >   "GGGGGGC"   
   [4]  std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >   "TTTGCCT"

知道出什么问题了吗？

deque<deque<string>>ques;
deque<string>  dq;
while (std::getline(inputstream, line))
{
    if(linecount<5)
    {
        dq.push_back(line);
        linecount++;
    }
    else if (linecount == 5)
    {
          deque<string>  dq2;
          dq2 = dq;
          ques.push_back(dq2);
        linecount=0;
        dq.clear();
    }
}

Answer 1

最后找到我所期望的代码。调试显示ques的内容为空。但实际上，它是有元素的。我可以数一数它的内容。不知道为什么调试器不显示双队列中的双队列的内容。