std::vector<T,Allocator>::operator[]
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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.
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.
[edit] Parameters
pos
-
position of the element to return
[edit] Return value
Reference to the requested element.
[edit] Complexity
Constant.
[edit] 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).
[edit] 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