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

public member function

<deque>

std::deque::begin 
      iterator begin();const_iterator begin() const;
      iterator begin() noexcept;const_iterator begin() const noexcept;

Return iterator to beginning

Returns an iterator pointing to the first element in the deque container.

Notice that, unlike member deque::front, which returns a reference to the first element, this function returns a random access iterator pointing to it.

If the container is empty, the returned iterator value shall not be dereferenced.



Parameters none

Return Value An iterator to the beginning of the sequence container.

If the deque object is const-qualified, the function returns a const_iterator. Otherwise, it returns an iterator.

Member types iterator and const_iterator are random access iterator types (pointing to an element and to a const element, respectively).



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

int main ()
{
  std::deque<int> mydeque;

  for (int i=1; i<=5; i++) mydeque.push_back(i);

  std::cout << "mydeque contains:";

  std::deque<int>::iterator it = mydeque.begin();

  while (it != mydeque.end())
    std::cout << ' ' << *it++;

  std::cout << '\n';

  return 0;
}

Output: 
mydeque contains: 1 2 3 4 5


Complexity Constant.

Iterator validity No changes.

Data races The container is accessed (neither the const nor the non-const versions modify the container).
No contained elements are accessed by the call, but the iterator returned can be used to access or modify elements. Concurrently accessing or modifying different elements is safe.

Exception safetyNo-throw guarantee: this member function never throws exceptions.
The copy construction or assignment of the returned iterator is also guaranteed to never throw.

See also
deque::front
Access first element (public member function)
deque::end
Return iterator to end (public member function)
deque::rbegin
Return reverse iterator to reverse beginning (public member function)
deque::rend
Return reverse iterator to reverse end (public member function)

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