std::atomic<T>::fetch_add

 

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Threads thread (C++11) jthread (C++20) stop_token (C++20) stop_source (C++20) stop_callback (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) 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) 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_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20)

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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

atomic::fetch_add

atomic::fetch_sub

atomic::fetch_and

atomic::fetch_or

atomic::fetch_xor

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

atomic::operator+=atomic::operator-=atomic::operator&=atomic::operator|=atomic::operator^=

 

member only of atomic<Integral>(C++11) and atomic<Floating>(C++20) template specializations
	(1)
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
		(2)
T* fetch_add( std::ptrdiff_t arg,               std::memory_order order = std::memory_order_seq_cst ) noexcept;
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. That is, it performs atomic post-increment. 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 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.

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. The volatile-qualified versions are deprecated if std::atomic<T>::is_always_lock_free is false.

(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 <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 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

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)