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std::atomic::fetch_add(3)      C++ Standard Libary     std::atomic::fetch_add(3)

NAME
     std::atomic::fetch_add - std::atomic::fetch_add

Synopsis
	member only of atomic<Integrala> specializations
	and atomic<Floatinga> specializations
	(since C++20)
	T  fetch_add(  T arg, std::memory_order order = 		     (1)
     (since C++11)
				std::memory_order_seq_cst ) noexcept;
	T fetch_add( T arg, std::memory_order order =
				std::memory_order_seq_cst )  volatile	     (2)
     (since C++11)
	noexcept;
	member only of atomic<T*> partial specialization
	T* fetch_add( std::ptrdiff_t arg,

		      std::memory_order  order	=			     (3)
     (since C++11)

			  std::memory_order_seq_cst ) noexcept;
	T* fetch_add( std::ptrdiff_t arg,

		      std::memory_order order  =			     (4)
     (since C++11)

			  std::memory_order_seq_cst ) volatile noexcept;

	Atomically  replaces the current value with the result of arithmetic ad-
     dition of the
	value and arg. That is, it performs atomic post-increment. The operation
     is a
	read-modify-write operation. Memory is affected according to  the  value
     of order.

	1,2)  For signed integral types, arithmetic is defined to use twoas com-
     plement
	representation. There are no undefined results.

	For floating-point types, the floating-point environment in effect may
	be different from the calling thread's floating-point environment. The
	operation need not  conform  to  the  corresponding  std::numeric_limits
     (since C++20)
	traits but is encouraged to do so. If the result is not a
	representable value for its type, the result is unspecified but the
	operation otherwise has no undefined behavior.

	3,4) The result may be an undefined address, but the operation otherwise
     has no
	undefined behavior.
	If T is not a complete object type, the program is ill-formed.

	It  is	deprecated  if	std::atomic<T>::is_always_lock_free is false and
     (since C++20)
	overload (2) or (4) participates in overload resolution.

Parameters
	arg   - the other argument of arithmetic addition
	order - memory order constraints to enforce

Return value
	The value immediately preceding the effects of this function in the mod-
     ification
	order of *this.

Example
     // Run this code

      #include <array>
      #include <atomic>
      #include <iostream>
      #include <thread>

      std::atomic<long long> data{10};
      std::array<long long, 5> return_values{};

      void do_work(int thread_num)
      {
	  long long val = data.fetch_add(1, std::memory_order_relaxed);
	  return_values[thread_num] = val;
      }

      int main()
      {
	  {
	      std::jthread th0{do_work, 0};
	      std::jthread th1{do_work, 1};
	      std::jthread th2{do_work, 2};
	      std::jthread th3{do_work, 3};
	      std::jthread th4{do_work, 4};
	  }

	  std::cout << "Result : " << data << '\n';

	  for (long long val : return_values)
	      std::cout << "Seen return value : " << val << std::endl;
      }

Possible output:
      Result : 15
      Seen return value : 11
      Seen return value : 10
      Seen return value : 14
      Seen return value : 12
      Seen return value : 13

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	  DR	Applied to		Behavior as published		    Cor-
     rect behavior
	P0558R1 C++11	   arithmetic permitted on pointers to (possibly    made
     ill-formed
			   cv-qualified) void or function

See also
	atomic_fetch_add	   adds  a  non-atomic value to an atomic object
     and obtains
	atomic_fetch_add_explicit the previous value of the atomic
	(C++11) 		  (function template)
	(C++11)
	operator++
	operator++(int) 	  increments or decrements the atomic  value  by
     one
	operator--		  (public member function)
	operator--(int)

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