NAME¶

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.

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.

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)