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std::atomic...::atomic_ref(3)  C++ Standard Libary std::atomic...::atomic_ref(3)

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 be-
     havior 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

http://cppreference.com 	   2024.06.10	   std::atomic...::atomic_ref(3)
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