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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<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 noex- cept; 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 noex- cept; Atomically replaces the current value with the result of arithmetic addition of the value and arg. That is, it performs atomic post-increment. The oper- ation is read-modify-write operation. Memory is affected according to the value of order. For signed Integral types, arithmetic is defined to use twos comple- ment representation. There are no undefined results. For T* types, the result may be an undefined address, but the opera- tion otherwise has no undefined behavior. The program is ill-formed if T is not an object type. 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 traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the (since C++20) operation otherwise has no undefined behavior. The volatile-qualified versions are deprecated if std::atomic<T>::is_always_lock_free is false. 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> #include <array> 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 Correct behavior P0558R1 C++11 arithmetic permitted on pointers to cv void or made ill-formed function See also atomic_fetch_add adds a non-atomic value to an atomic ob- ject 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 2022.07.31 std::atomic::fetch_add(3)
NAME | Synopsis | Parameters | Return value | Example | Possible output: | See also
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