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| Member functions | ||||
(C++20) | ||||
(C++20) | ||||
(C++20) | ||||
| Constants | ||||
(C++17) | ||||
| Specialized member functions | ||||
| Specialized for integral, floating-point(C++20) and pointer types | ||||
atomic::fetch_add | ||||
| Specialized for integral and pointer types only | ||||
(C++26) | ||||
(C++26) | ||||
| Specialized for integral types only | ||||
member only of atomic<Integral > specializationsand 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)volatilenoexcept; | (2) | (since C++11) |
member only of atomic<T*> partial specialization | ||
T* fetch_add(std::ptrdiff_t arg, std::memory_order order= | (3) | (since C++11) |
T* fetch_add(std::ptrdiff_t arg, std::memory_order order= | (4) | (since C++11) |
Atomically replaces the current value with the result of arithmetic addition of the value andarg. That is, it performs atomic post-increment. The operation is a read-modify-write operation. Memory is affected according to the value oforder.
For floating-point types, thefloating-point environment in effect may be different from the calling thread's floating-point environment. The operation need not conform to the correspondingstd::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 ifstd::atomic<T>::is_always_lock_free isfalse and overload(2) or(4) participates in overload resolution. | (since C++20) |
Contents |
| arg | - | the other argument of arithmetic addition |
| order | - | memory order constraints to enforce |
The value immediately preceding the effects of this function in themodification order of*this.
#include <array>#include <atomic>#include <iostream>#include <thread> std::atomic<longlong> data{10};std::array<longlong,5> return_values{}; void do_work(int thread_num){longlong 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(longlong val: return_values)std::cout<<"Seen return value : "<< val<<std::endl;}
Possible output:
Result : 15Seen return value : 11Seen return value : 10Seen return value : 14Seen return value : 12Seen return value : 13
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 |
(C++11)(C++11) | adds a non-atomic value to an atomic object and obtains the previous value of the atomic (function template)[edit] |
| increments or decrements the atomic value by one (public member function)[edit] |
