C++ .at()

The C++ .at() function accesses the element at a given index in a deque while performing bounds checking. If the index is out of range, the function throws an std::out_of_range exception. This makes it a safer alternative to the subscript operator [], which offers no safety checks.

Syntax

deque.at(pos)

Parameters:

• pos: A zero-based index of the element to access. Must be within the valid range of the deque.

Return value:

Returns a reference to the element at the given position.

Exceptions:

Throws std::out_of_range if pos is invalid.

Example 1: Accessing Elements Safely

In this example, .at() retrieves elements from valid positions in the deque:

#include <iostream>
#include <deque>
using namespace std;

int main() {
  deque<int> numbers = {10, 20, 30, 40};

  cout << "Element at index 2: " << numbers.at(2) << endl;
  cout << "Element at index 0: " << numbers.at(0) << endl;

  return 0;
}

Copy to clipboard
Copy to clipboard

The output of this code is:

Element at index 2: 30
Element at index 0: 10

Copy to clipboard
Copy to clipboard

The .at() function returns elements at positions 2 and 0 with built-in bounds checking.

Example 2: Handling Out-of-Range Access

In this example, accessing index 5 triggers an exception because the index is outside the deque’s valid range:

#include <iostream>
#include <deque>
using namespace std;

int main() {
  deque<string> names = {"Ava", "Mira", "Leo"};

  try {
    cout << names.at(5) << endl;
  } catch (const out_of_range& e) {
    cout << "Error: " << e.what() << endl;
  }

  return 0;
}

Copy to clipboard
Copy to clipboard

The output of this code is:

Error: deque::_M_range_check: __n (which is 5)>= this->size() (which is 3)

Copy to clipboard
Copy to clipboard

Codebyte Example

This example demonstrates safe element access and shows how .at() behaves when an invalid index is used:

Visit us
Visit us
Hide code
Code
Output
Hide output
Loading...
Copy to your clipboard
Copy Codebyte
Run

Frequently Asked Questions

1. What is the use of deque in C++?

A deque (double-ended queue) stores elements in a dynamic sequence where insertions and deletions at both the front and back are efficient. It supports random access like a vector but provides faster operations at the beginning of the structure.

2. What is a std::deque?

std::deque is a standard container that provides a dynamic array-like structure with fast operations at both ends. It is implemented as segmented memory blocks, which allows it to grow without relocating all elements like a vector.

3. How to check if deque is empty in C++?

A deque can be checked for emptiness using the .empty() method, which returns true if it contains no elements and false otherwise. This check is constant time and works for all standard containers.