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std::vector<T,Allocator>::operator[]

From cppreference.com
< cpp‎ | container‎ | vector
 
 
 
std::vector
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
 
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.
(since C++26)

[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

[edit] See also

access specified element with bounds checking
(public member function) [edit]
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