Question

I am using dque to push some elements that I read from a file to the back of the deque but when I print them from the front I only get the last element

  fgets(line,100,file);
  qu.push_back(line);
  fgets(line,100,file);
  qu.push_back(line);
  fgets(line,100,file);
  qu.push_back(line);
  fgets(line,100,file);
  qu.push_back(line);

  while(!qu.empty())
            {
                puts(qu.front());
                qu.pop_front();
            }

sample input

a
b
c
d

output

d
d
d
d

Thank you

Answer 1

You haven’t provided the declarations for line and qu, but I can guess they are

char line[100];
std::dequeue<char*> qu;

This means qu stores pointers to an external (to qu) buffer with characters in it. In your program, you have just a single buffer (line), whose address gets pushed repeatedly into qu, but whose contents get overwritten with each fgets call.

While you are learning C++, try to stay away from pointers as much as you can. So, instead of using char* for strings, use std::string:

std::string line;
std::deque<std::string> qu;

getline(std::cin, line);
qu.push_back(line);
getline(std::cin, line);
qu.push_back(line);
getline(std::cin, line);
qu.push_back(line);
getline(std::cin, line);
qu.push_back(line);

while(!qu.empty())
{
    std::cout << qu.front() << std::endl;
    qu.pop_front();
}