std::atomic<T>::fetch_add

 

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Threads thread (C++11) jthread (C++20) hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17) this_thread namespace get_id (C++11) yield (C++11) sleep_for (C++11) sleep_until (C++11) Cooperative cancellation stop_token (C++20) inplace_stop_token (C++26) never_stop_token (C++26) stop_source (C++20) inplace_stop_source (C++26)   stop_callback (C++20) inplace_stop_callback (C++26) stop_callback_for_t (C++26) stoppable_token (C++26) unstoppable_token (C++26) stoppable-source (C++26) stoppable-callback-for (C++26)   Mutual exclusion mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17) timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14) Generic lock management lock (C++11) lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) once_flag (C++11) call_once (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lockdefer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)(C++11)(C++11)(C++11) Condition variables condition_variable (C++11) condition_variable_any (C++11) notify_all_at_thread_exit (C++11) cv_status (C++11) Semaphores counting_semaphorebinary_semaphore (C++20)(C++20) Latches and Barriers latch (C++20) barrier (C++20) Futures promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11) launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11) Safe Reclamation rcu_obj_base (C++26) rcu_domain (C++26) rcu_default_domain (C++26) rcu_synchronize (C++26) rcu_barrier (C++26) rcu_retire (C++26) Hazard Pointers hazard_pointer_obj_base (C++26) hazard_pointer (C++26) make_hazard_pointer (C++26)

Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_fetch_maxatomic_fetch_max_explicit (C++26)(C++26) atomic_fetch_minatomic_fetch_min_explicit (C++26)(C++26) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20) Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20)

 

std::atomic

Member functions

atomic::atomic

atomic::operator=

atomic::is_lock_free

atomic::store

atomic::load

atomic::operator T

atomic::exchange

atomic::compare_exchange_strongatomic::compare_exchange_weak

atomic::wait (C++20)

atomic::notify_one (C++20)

atomic::notify_all (C++20)

Constants

atomic::is_always_lock_free (C++17)

Specialized member functions

Specialized for integral, floating-point (C++20) and pointer types

atomic::fetch_add

atomic::fetch_sub

atomic::operator+=atomic::operator-=

Specialized for integral and pointer types only

atomic::fetch_max (C++26)

atomic::fetch_min (C++26)

atomic::operator++atomic::operator++(int)atomic::operator--atomic::operator--(int)

Specialized for integral types only

atomic::fetch_and

atomic::fetch_or

atomic::fetch_xor

atomic::operator&=atomic::operator|=atomic::operator^=

 

member only of atomic<Integral > specializations and atomic<Floating > specializations(since C++20)
T fetch_add( T arg, std::memory_order order =                         std::memory_order_seq_cst ) noexcept;	(1)	(since C++11)
T fetch_add( T arg, std::memory_order order =                         std::memory_order_seq_cst ) volatile noexcept;	(2)	(since C++11)
member only of atomic<T*> partial specialization
T* fetch_add( std::ptrdiff_t arg,               std::memory_order order =                   std::memory_order_seq_cst ) noexcept;	(3)	(since C++11)
T* fetch_add( std::ptrdiff_t arg,               std::memory_order order =                   std::memory_order_seq_cst ) volatile noexcept;	(4)	(since C++11)

Atomically replaces the current value with the result of arithmetic addition 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 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 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)

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 overload (2) or (4) participates in overload resolution.

(since C++20)

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 <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 retroactively to previously published C++ standards.

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

See also

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