std::atomic<T>::fetch_add

T fetch_add( T arg, 
             std::memory_order order = std::memory_order_seq_cst ) noexcept;

T fetch_add( T arg, 
             std::memory_order order = std::memory_order_seq_cst ) volatile noexcept;

T* fetch_add( std::ptrdiff_t arg, 
              std::memory_order order = std::memory_order_seq_cst ) noexcept;

T* fetch_add( std::ptrdiff_t arg, 
              std::memory_order order = std::memory_order_seq_cst ) volatile noexcept;

Atomically replaces the current value with the result of arithmetic addition of the value and arg. That is, it performs atomic post-increment. The operation is a read-modify-write operation. Memory is affected according to the value of order.

For signed Integral types, arithmetic is defined to use two’s complement representation. There are no undefined results.

For T* types, the result may be an undefined address, but the operation otherwise has no undefined behavior. The program is ill-formed if T is not an object type.

Parameters

Return value

The value immediately preceding the effects of this function in the modification order of *this.

Example

#include <iostream>
#include <thread>
#include <atomic>
#include <array>
 
std::atomic<long long> data{10};
std::array<long long, 5> return_values{};
 
void do_work(int thread_num)
{
    long long val = data.fetch_add(1, std::memory_order_relaxed);
    return_values[thread_num] = val;
}
 
int main()
{
    {
        std::jthread th0{do_work, 0};
        std::jthread th1{do_work, 1};
        std::jthread th2{do_work, 2};
        std::jthread th3{do_work, 3};
        std::jthread th4{do_work, 4};
    }
 
    std::cout << "Result : " << data << '\n';
 
    for (long long val : return_values) {
        std::cout << "Seen return value : " << val << std::endl;
    }
}

Result : 15
Seen return value : 11
Seen return value : 10
Seen return value : 14
Seen return value : 12
Seen return value : 13

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also