deque<deque> empty after push_back a deque

Question

I tried to read lines from file, push every 5 lines into one deque dq, then push dq into a deque ques. The dq successively got 5 lines, but after push_back the dq into ques, the debuger shows that one empty queue was inserted into ques, shown below is the output from debugger.

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"

Any idea what's wrong?

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

Finally find the code works what I expected. The debug shows the ques' content empty. But actually, there are element in it. I can count it's content. Don't know why debugger doesn't show the content of a deque inside a deque.