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Showing content from https://cplusplus.com/reference/deque/deque/push_front/ below:

public member function

<deque>

std::deque::push_front 
void push_front (const value_type& val);
void push_front (const value_type& val);void push_front (value_type&& val);

Insert element at beginning

Inserts a new element at the beginning of the deque container, right before its current first element. The content of val is copied (or moved) to the inserted element.

This effectively increases the container size by one.



Parameters
val
Value to be copied (or moved) to the inserted element.
Member type value_type is the type of the elements in the container, defined in deque as an alias of its first template parameter (T).

Return value none

The storage for the new elements is allocated using the container's allocator, which may throw exceptions on failure (for the default allocator, bad_alloc is thrown if the allocation request does not succeed).



Example 
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// deque::push_front
#include <iostream>
#include <deque>

int main ()
{
  std::deque<int> mydeque (2,100);     // two ints with a value of 100
  mydeque.push_front (200);
  mydeque.push_front (300);

  std::cout << "mydeque contains:";
  for (std::deque<int>::iterator it = mydeque.begin(); it != mydeque.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

Output:


Complexity Constant.

Iterator validity All iterators related to this container are invalidated. Pointers and references to elements in the container remain valid, referring to the same elements they were referring to before the call.

Data races The container is modified.
No existing elements are accessed (although see iterator validity above).

Exception safetyStrong guarantee: if an exception is thrown, there are no changes in the container.
If allocator_traits::construct is not supported with val as argument, it causes undefined behavior.

See also
deque::push_back
Add element at the end (public member function)
deque::pop_front
Delete first element (public member function)
deque::insert
Insert elements (public member function)

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