NAME¶

std::atomic_ref::atomic_ref - std::atomic_ref::atomic_ref

Synopsis¶

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.

Parameters¶

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

Example¶

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.

// Run this code

#include <atomic>
#include <iostream>
#include <numeric>
#include <thread>
#include <vector>

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)
++x;
};

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';
};

test_run(inc_atomically);
test_run(inc_directly);
}

Possible output:¶

sum = 40000000
sum = 39994973