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Defined in header <atomic> | ||
template<class T> T atomic_fetch_add(std::atomic<T>* obj, | (1) | (since C++11) |
template<class T> T atomic_fetch_add(volatilestd::atomic<T>* obj, | (2) | (since C++11) |
template<class T> T atomic_fetch_add_explicit(std::atomic<T>* obj, | (3) | (since C++11) |
template<class T> T atomic_fetch_add_explicit(volatilestd::atomic<T>* obj, | (4) | (since C++11) |
Performs atomic addition. Atomically addsarg to the value pointed to byobj and returns the valueobj held previously. The operation is performed as if the following was executed:
Ifstd::atomic<T> has nofetch_add member (this member is only provided forintegral,floating-point(since C++20) andpointer types exceptbool), the program is ill-formed.
Contents |
| obj | - | pointer to the atomic object to modify |
| arg | - | the value to add to the value stored in the atomic object |
| order | - | the memory synchronization ordering |
The value immediately preceding the effects of this function in themodification order of*obj.
Single-writer/multiple-reader lock can be made withstd::atomic_fetch_add. Note that this simplistic implementation is not lockout-free.
#include <atomic>#include <chrono>#include <iostream>#include <string>#include <thread>#include <vector> usingnamespace std::chrono_literals; // meaning of cnt:// 5: readers and writer are in race. There are no active readers or writers.// 4...0: there are 1...5 active readers, The writer is blocked.// -1: writer won the race and readers are blocked. constint N=5;// four concurrent readers are allowedstd::atomic<int> cnt(N); std::vector<int> data; void reader(int id){for(;;){// lockwhile(std::atomic_fetch_sub(&cnt,1)<=0) std::atomic_fetch_add(&cnt,1); // readif(!data.empty())std::cout<<("reader "+std::to_string(id)+" sees "+std::to_string(*data.rbegin())+'\n');if(data.size()==25)break; // unlock std::atomic_fetch_add(&cnt,1); // pausestd::this_thread::sleep_for(1ms);}} void writer(){for(int n=0; n<25;++n){// lockwhile(std::atomic_fetch_sub(&cnt, N+1)!= N) std::atomic_fetch_add(&cnt, N+1); // write data.push_back(n);std::cout<<"writer pushed back "<< n<<'\n'; // unlock std::atomic_fetch_add(&cnt, N+1); // pausestd::this_thread::sleep_for(1ms);}} int main(){std::vector<std::thread> v;for(int n=0; n< N;++n) v.emplace_back(reader, n); v.emplace_back(writer); for(auto& t: v) t.join();}
Output:
writer pushed back 0reader 2 sees 0reader 3 sees 0reader 1 sees 0<...>reader 2 sees 24reader 4 sees 24reader 1 sees 24
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 | exact type match was required becauseT was deduced from multiple arguments | T is only deducedfromobj |
| atomically adds the argument to the value stored in the atomic object and obtains the value held previously (public member function of std::atomic<T>)[edit] | |
(C++11)(C++11) | subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic (function template)[edit] |
C documentation foratomic_fetch_add,atomic_fetch_add_explicit | |
