std::atomic_fetch_sub, std::atomic_fetch_sub_explicit
From cppreference.com
| Defined in header <atomic>
|
||
template< class T >
T atomic_fetch_sub( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;
|
(1) | (since C++11) |
template< class T >
T atomic_fetch_sub( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;
|
(2) | (since C++11) |
template< class T >
T atomic_fetch_sub_explicit( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,
std::memory_order order ) noexcept;
|
(3) | (since C++11) |
template< class T >
T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg,
std::memory_order order ) noexcept;
|
(4) | (since C++11) |
Performs atomic subtraction. Atomically subtracts arg from the value pointed to by obj and returns the value obj held previously. The operation is performed as if the following was executed:
1,2)
obj->fetch_sub(arg)3,4)
obj->fetch_sub(arg, order)If std::atomic<T> has no fetch_sub member (this member is only provided for integral, floating-point(since C++20) and pointer types except bool), the program is ill-formed.
Parameters
| obj | - | pointer to the atomic object to modify |
| arg | - | the value to subtract from the value stored in the atomic object |
| order | - | the memory synchronization ordering |
Return value
The value immediately preceding the effects of this function in the modification order of *obj.
Example
Multiple threads may use std::atomic_fetch_sub to concurrently process an indexed container.
Run this code
#include <atomic>
#include <iostream>
#include <numeric>
#include <string>
#include <thread>
#include <vector>
const int N = 50;
std::atomic<int> cnt;
std::vector<int> data(N);
void reader(int id)
{
for (;;)
{
int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
if (idx >= 0)
std::cout << "reader " << std::to_string(id) << " processed item "
<< std::to_string(data[idx]) << '\n';
else
{
std::cout << "reader " << std::to_string(id) << " done\n";
break;
}
}
}
int main()
{
std::iota(data.begin(), data.end(), 1);
cnt = data.size() - 1;
std::vector<std::thread> v;
for (int n = 0; n < 5; ++n)
v.emplace_back(reader, n);
for (auto& t : v)
t.join();
}
Output:
reader 2 processed item 50
reader 1 processed item 44
reader 4 processed item 46
<....>
reader 0 done
reader 4 done
reader 3 done
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 | exact type match was required becauseT was deduced from multiple arguments
|
T is only deducedfrom obj
|