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std::vector::operator[](3) | C++ Standard Libary | std::vector::operator[](3) |
NAME¶
std::vector::operator[] - std::vector::operator[]
Synopsis¶
reference operator[]( size_type pos ); (until C++20)
constexpr reference operator[]( size_type pos ); (since C++20)
const_reference operator[]( size_type pos ) const; (until C++20)
constexpr const_reference operator[]( size_type pos ) const; (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)
2022.07.31 | http://cppreference.com |