std::vector::operator[](3)

💖 NAME 💖

okay bestie, welcome to the C++ Standard Libary page for std::vector::operator[](3) 💅 the title is std::vector::operator[](3), and the NAME line is literally:

std::vector::operator[] - std::vector::operator[]

---

✨ Synopsis ✨

reference operator[]( size_type pos );             (1) (constexpr
since C++20)
const_reference operator[]( size_type pos ) const; (2) (constexpr
since C++20)

Returns a reference to the element at specified location pos. No bounds checking is performed. 🫶

no bounds checking, bestie — it's giving raw speed with zero safety net 😭

---

🎀 Parameters 🎀

• pos — position of the element to return

---

🦋 Return value 🦋

Reference to the requested element. periodt ✨

---

⚡ Complexity ⚡

Constant. O(1) energy, no cap 🔥

---

🧚 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.

🚨 undefined behavior is not a cute little error message, bestie — it's the scary kind of "anything can happen" 💀

---

🌸 Example 🌸

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

// Run this code

#include <vector>
#include <iostream>

int main()
{
    std::vector<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';
}

// Since C++20 std::vector can be used in constexpr context:
#if defined(__cpp_lib_constexpr_vector) and defined(__cpp_consteval)
// Gets the sum of all primes in [0, N) using sieve of Eratosthenes
consteval auto sum_of_all_primes_up_to(unsigned N)
{
    if (N < 2)
        return 0ULL;

    std::vector<bool> is_prime(N, true);
    is_prime[0] = is_prime[1] = false;

    auto propagate_non_primality = [&](decltype(N) n)
    {
        for (decltype(N) m = n + n; m < is_prime.size(); m += n)
            is_prime[m] = false;
    };

    auto sum{0ULL};
    for (decltype(N) n{2}; n != N; ++n)
        if (is_prime[n])
        {
            sum += n;
            propagate_non_primality(n);
        }

    return sum;
} //< vector's memory is released here

static_assert(sum_of_all_primes_up_to(42) == 0xEE);
static_assert(sum_of_all_primes_up_to(100) == 0x424);
static_assert(sum_of_all_primes_up_to(1001) == 76127);
#endif

---

💀 Output 💀

Second element: 4
All numbers: 5 4 6 8

---

👑 See also 👑

• at — access specified element with bounds checking (public member function)

---

source: http://cppreference.com • date: 2024.06.10 • page: std::vector::operator[](3) 🫶💅✨