std::atomic<T>::fetch_add
atomic<Integral
>
specializationsand
atomic<Floating
>
specializations(since C++20)std::memory_order_seq_cst ) noexcept;
std::memory_order_seq_cst ) volatile noexcept;
atomic<T*>
partial specialization std::memory_order order =
std::memory_order order =
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.
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)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)[edit] Parameters
[edit] Return value
The value immediately preceding the effects of this function in the modification order of *this.
[edit] Example
#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
[edit] 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 |
[edit] See also
(function template) [edit]