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std::deque<T,Allocator>::operator[] - cppreference.com

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reference operator[]( size_type pos );

 (1)

const_reference operator[]( size_type pos ) const;

 (2)

Returns a reference to the element at specified location pos. No bounds checking is performed, unless the implementation is hardened(since C++26).

If pos < size() is false, the behavior is undefined.

(until C++26)

If pos < size() is false:

  • If the implementation is hardened, a contract violation occurs. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
  • If the implementation is not hardened, the behavior is undefined.
 (since C++26)

Parameters

pos - position of the element to return

Return value

Reference to the requested element.

Complexity

Constant.

Notes

Unlike std::map::operator[], this operator never inserts a new element into the container. Accessing a nonexistent element through this operator is undefined behavior, unless the implementation is hardened(since C++26).

Example

The following code uses operator[] to read from and write to a std::deque<int>:

#include <deque>
#include <iostream>

int main()
{
    std::deque<int> numbers{2, 4, 6, 8};
    
    std::cout << "Second element: " << numbers[1] << '\n';
    
    numbers[0] = 5;
    
    std::cout << "All numbers:";
    for (auto i : numbers)
        std::cout << ' ' << i;
    std::cout << '\n';
}

Output: 

Second element: 4
All numbers: 5 4 6 8

See also

access specified element with bounds checking
(public member function) [edit]