In C++, you can insert elements into a deque using the insert() method. The insert() method can be used to insert elements at any position within the deque, and it takes two arguments: an iterator indicating the position where the new element should be inserted, and the value of the new element.

Here's an example of how to use the insert() method to insert elements into a deque:

#include <iostream>
#include <deque>

int main() {
  std::deque<int> d {1, 2, 3, 4};
  
  // Insert a new element at the beginning of the deque
  d.insert(d.begin(), 0);
  
  // Insert a new element at the end of the deque
  d.insert(d.end(), 5);
  
  // Insert a new element at position 2 in the deque
  d.insert(d.begin() + 2, 10);
  
  // Print the elements of the deque
  for (auto it = d.begin(); it != d.end(); ++it) {
    std::cout << *it << " ";
  }
  std::cout << std::endl;
  
  return 0;
}

In this example, we create a deque d with four elements, and then we use the insert() method to insert three new elements into the deque: one at the beginning, one at the end, and one at position 2. We then use a for loop to print the elements of the deque.

The output of this program is:

0 1 10 2 3 4 5

As you can see, the new elements have been successfully inserted into the deque at the specified positions.