std::atomic<T>::fetch_add (3) - Linux Manuals

std::atomic<T>::fetch_add: std::atomic<T>::fetch_add

NAME

std::atomic<T>::fetch_add - std::atomic<T>::fetch_add

Synopsis

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

Atomically replaces the current value with the result of arithmetic addition of the value and arg. The operation is read-modify-write operation. Memory is affected according to the value of order.
For signed Integral types, arithmetic is defined to use two’s complement 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 be conform to the corresponding std::numeric_limits 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. (since C++20)

For T* types, the result may be an undefined address, but the operation otherwise has no undefined behavior. The program is ill-formed if T is not an object type.

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 modification_order of *this.

Example

// Run this code

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

  std::atomic<long long> data;
  void do_work()
  {
      data.fetch_add(1, std::memory_order_relaxed);
  }

  int main()
  {
      std::thread th1(do_work);
      std::thread th2(do_work);
      std::thread th3(do_work);
      std::thread th4(do_work);
      std::thread th5(do_work);

      th1.join();
      th2.join();
      th3.join();
      th4.join();
      th5.join();

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

Output:

  Result:5

Defect reports

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

DR Applied to Behavior as published Correct behavior
P0558R1 C++11 arithmetic permitted on pointers to cv void or function made ill-formed