explicit atomic_ref( T& obj );
(1) (since C++20)
atomic_ref( const atomic_ref& ref ) noexcept;
(2) (since C++20)

Constructs a new atomic_ref object.

1) Constructs an atomic_ref object referencing the object obj. The behavior is undefined if obj is not aligned to required_alignment.
2) Constructs an atomic_ref object referencing the object referenced by ref.


obj - object to reference
ref - another atomic_ref object to copy from


The program increments the values in a container using several threads. Then the final sum is printed. Non-atomic access may "loss" the results of some operations due to data-races.

#include <atomic>
#include <thread>
#include <vector>
#include <numeric>
#include <iostream>
int main()
    using Data = std::vector<char>;
    auto inc_atomically = [](Data& data) {
        for (Data::value_type& x : data) {
            auto xx = std::atomic_ref<Data::value_type>(x);
            ++xx;  // atomic read-modify-write
    auto inc_directly = [](Data& data) {
        for (Data::value_type& x : data)
    auto test_run = [](const auto Fun) {
        Data data(10'000'000);
            std::jthread j1 {Fun, std::ref(data)};
            std::jthread j2 {Fun, std::ref(data)};
            std::jthread j3 {Fun, std::ref(data)};
            std::jthread j4 {Fun, std::ref(data)};
        std::cout << "sum = " << std::accumulate(cbegin(data), cend(data), 0) << '\n';

Possible output:

sum = 40000000
sum = 39994973

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