std::atomic_fetch_sub, std::atomic_fetch_sub_explicit
Min standard notice:
Header: <atomic>
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:
# Declarations
template< class T >
T atomic_fetch_sub( std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;
(since C++11)
template< class T >
T atomic_fetch_sub( volatile std::atomic<T>* obj,
typename std::atomic<T>::difference_type arg ) noexcept;
(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;
(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;
(since C++11)
# Parameters
obj: pointer to the atomic object to modifyarg: the value to subtract from the value stored in the atomic objectorder: the memory synchronization ordering
# Return value
The value immediately preceding the effects of this function in the modification order of *obj.
# Example
#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();
}
# Defect reports
| 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 |