vector::at() in C++ STL

In C++, vector at() is a built-in method used to access an element in a vector using index. It is the only access method that performs bound checking before accessing the element to confirm whether the given index lies is within the vector.

Let’s take a quick look at a simple example that uses vector at() method:

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1, 3, 4, 9};

    // Accessing the element of second index
    cout << v.at(2);
    return 0;
}

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This article covers the syntax, usage, and common queries of vector at() method in C++:

Syntax of vector at()

The vector at() is a member method of std::vector class defined inside <vector> header file.

v.at(i);

Parameters

• i: 0-based position of the element in the vector.

Return Value

• Returns the reference to the element at given index.
• If the index is out of bounds, an std::out_of_range exception is thrown.

Examples of vector at()

The below examples illustrate the common uses of vector at() method:

Modifying Elements Using vector at()

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1, 3, 4, 9};

    // Modify the element at 2nd index
    v.at(2) = 7;
    cout << v.at(2);
  
    return 0;
}

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Catching Out-of-Range Exceptions with vector at()

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1, 3, 4, 9};

    try {
      
        // Attempting to access out of range index
        cout << v.at(5) << endl;
      
    } catch (const out_of_range& e) {
        cout << "Exception: " << e.what() << endl;
    }
  
    return 0;
}

Exception: vector::_M_range_check: __n (which is 5) >= this->size() (which is 4)