std::deque<T,Allocator>::at

#include <chrono>
#include <cstddef>
#include <iostream>
#include <deque>
#include <stdexcept>

int main()
{
    std::deque<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';
}

Возможный вывод:

Element at index 2 has value 4
data size = 6
deque::_M_range_check: __n (which is 8) >= this->size() (which is 6)
data: 1 88 4 5 5 6