NAME¶

std::atomic_fetch_add,std::atomic_fetch_add_explicit - std::atomic_fetch_add,std::atomic_fetch_add_explicit

Synopsis¶

Defined in header <atomic>
template< class T >

T atomic_fetch_add( std::atomic<T>* obj, (1) (since C++11)

typename std::atomic<T>::difference_type arg )
noexcept;
template< class T >

T atomic_fetch_add( volatile std::atomic<T>* obj, (2) (since C++11)

typename std::atomic<T>::difference_type arg )
noexcept;
template< class T >

T atomic_fetch_add_explicit( std::atomic<T>* obj,
typename (3) (since C++11)
std::atomic<T>::difference_type arg,

std::memory_order order ) noexcept;
template< class T >

T atomic_fetch_add_explicit( volatile std::atomic<T>* obj,
typename (4) (since C++11)
std::atomic<T>::difference_type arg,

std::memory_order order ) noexcept;

Performs atomic addition. Atomically adds arg to 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_add(arg)
3,4) obj->fetch_add(arg, order)

If std::atomic<T> has no fetch_add 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 add to 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¶

Single-writer/multiple-reader lock can be made with std::atomic_fetch_add. Note that
this simplistic implementation is not lockout-free.

// Run this code

#include <atomic>
#include <chrono>
#include <iostream>
#include <string>
#include <thread>
#include <vector>

using namespace 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.

const int N = 5; // four concurrent readers are allowed
std::atomic<int> cnt(N);

std::vector<int> data;

void reader(int id)
{
for (;;)
{
// lock
while (std::atomic_fetch_sub(&cnt, 1) <= 0)
std::atomic_fetch_add(&cnt, 1);

// read
if (!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);

// pause
std::this_thread::sleep_for(1ms);
}
}

void writer()
{
for (int n = 0; n < 25; ++n)
{
// lock
while (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);

// pause
std::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 0
reader 2 sees 0
reader 3 sees 0
reader 1 sees 0
<...>
reader 2 sees 24
reader 4 sees 24
reader 1 sees 24

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 because T is only deduced
T was deduced from multiple arguments from obj

See also¶

atomically adds the argument to the value stored in the
fetch_add atomic object and obtains the value held previously
(public member function of std::atomic<T>)
atomic_fetch_sub subtracts a non-atomic value from an atomic object and
atomic_fetch_sub_explicit obtains the previous value of the atomic
(C++11) (function template)
(C++11)
C documentation for
atomic_fetch_add,
atomic_fetch_add_explicit