std::atomic_fetch_sub

STD：：原子[医]去取[医]分，STD：：原子[医]去取[医]分[医]显式

Defined in header
	(1)	(since C++11)
template< class Integral > Integral atomic_fetch_sub( std::atomic<Integral>* obj, Integral arg
template< class Integral > Integral atomic_fetch_sub( volatile std::atomic<Integral>* obj, Integral arg
	(2)	(since C++11)
template< class Integral > Integral atomic_fetch_sub_explicit( std::atomic<Integral>* obj, Integral arg, std::memory_order order
template< class Integral > Integral atomic_fetch_sub_explicit( volatile std::atomic<Integral>* obj, Integral arg, std::memory_order order
	(3)	(since C++11)
template< class T > T* atomic_fetch_sub( std::atomic<T> obj, std::ptrdiff_t arg
template< class T > T* atomic_fetch_sub( volatile std::atomic<T> obj, std::ptrdiff_t arg
	(4)	(since C++11)
template< class T > T* atomic_fetch_sub_explicit( std::atomic<T> obj, std::ptrdiff_t arg, std::memory_order order
template< class T > T* atomic_fetch_sub_explicit( volatile std::atomic<T> obj, std::ptrdiff_t arg, std::memory_order order

执行原子减法。

1-2%29arg从所指的价值obj并返回值。obj以前持有。执行该操作时，就像执行了以下操作一样：

1%29obj->fetch_sub(arg)

2%29obj->fetch_sub(arg, order)

3-4%29原子地减少指针值，obj，通过arg，并返回值obj以前持有。执行该操作时，就像执行了以下操作一样：

3%29obj->fetch_sub(arg)

4%29obj->fetch_sub(arg, order)

参数

obj	-	pointer to the atomic object to modify
arg	-	the value to subtract from the value stored in the atomic object
order	-	the memory sycnhronization ordering for this operation: all values are permitted.

返回值

中此函数的效果之前的值。修改顺序成*obj...

例外

noexcept规格：

noexcept

可能的实施

第一版

*。

模板<class T>TypeName STD：：Enable[医]如果<std：：is[医]积分<T>*价值&%21 std：：is[医]相同<T，bool>：：value，T>：type原子[医]去取[医]分%28 STD：：原子<T>%2AOBJ，T Arg%29；{返回obj->提取[医]次%28 arg%29；}

第二版

模板<类T>T%2A原子[医]去取[医]分%28 STD：：原子<T%2A>%2AOBJ，STD：：ptrdiff[医]t arg%29{返回obj->获取[医]次%28 arg%29；}

例

多线程可以使用fetch_sub并发处理索引容器。

二次

#include <string>
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
#include <numeric>
 
const int N = 10000;
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 < 10; ++n) {
        v.emplace_back(reader, n
    }
    for (auto& t : v) {
        t.join(
    }
}

二次

产出：

二次

reader 2 processed item 10000
reader 6 processed item 9994
reader 4 processed item 9996
reader 6 processed item 9992
<....>
reader 0 done
reader 5 done
reader 3 done
reader 9 done

二次

另见

fetch_sub	atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously (public member function of std::atomic)
atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11)	adds a non-atomic value to an atomic object and obtains the previous value of the atomic (function template)

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