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

public member function

<deque>

std::deque::front 
      reference front();const_reference front() const;

Access first element

Returns a reference to the first element in the deque container.

Unlike member deque::begin, which returns an iterator to this same element, this function returns a direct reference.

Calling this function on an empty container causes undefined behavior.



Parameters none

Return value A reference to the first element in the deque container.

If the deque object is const-qualified, the function returns a const_reference. Otherwise, it returns a reference.

Member types reference and const_reference are the reference types to the elements of the container (see deque member types).



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

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

  mydeque.push_front(77);
  mydeque.push_back(20);

  mydeque.front() -= mydeque.back();

  std::cout << "mydeque.front() is now " << mydeque.front() << '\n';

  return 0;
}

Output: 
mydeque.front() is now 57


Complexity Constant.

Iterator validity No changes.

Data races The container is accessed (neither the const nor the non-const versions modify the container).
The first element is potentially accessed or modified by the caller. Concurrently accessing or modifying other elements is safe.

Exception safety If the container is not empty, the function never throws exceptions (no-throw guarantee).
Otherwise, it causes undefined behavior.

See also
deque::back
Access last element (public member function)
deque::begin
Return iterator to beginning (public member function)
deque::push_front
Insert element at beginning (public member function)
deque::pop_front
Delete first element (public member function)

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