std::vector<T,Allocator>::at
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reference at( size_type pos );
(1)
(constexpr since C++20)
const_reference at( size_type pos ) const;
(2)
(constexpr since C++20)
Returns a reference to the element at specified location pos, with bounds checking.
If pos is not within the range of the container, an exception of type std::out_of_range is thrown.
[edit] Parameters
pos
-
position of the element to return
[edit] Return value
Reference to the requested element.
[edit] Exceptions
std::out_of_range if pos >= size().
[edit] Complexity
Constant.
[edit] Example
Run this code
#include <chrono> #include <cstddef> #include <iostream> #include <vector> #include <stdexcept> int main() { std::vector <int> data{1, 2, 4, 5, 5, 6}; // Set element 1 data.at(1) = 88; // Read element 2 std::cout << "Element at index 2 has value " << data.at(2) << '\n'; std::cout << "data size = " << data.size() << '\n'; try { // Try to set an element at random position >= size() auto moon_phase = [] { return std::chrono::system_clock::now ().time_since_epoch().count() % 8; }; data.at(data.size() + moon_phase()) = 13; } catch(const std::out_of_range & ex) { std::cout << ex.what() << '\n'; } // Print final values std::cout << "data:"; for (int elem : data) std::cout << ' ' << elem; std::cout << '\n'; }
Possible output:
Element at index 2 has value 4 data size = 6 vector::_M_range_check: __n (which is 8) >= this->size() (which is 6) data: 1 88 4 5 5 6