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. The operation is 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>
 
std::atomic<long long> data;
void do_work()
{
    data.fetch_add(1, std::memory_order_relaxed);
}
 
int main()
{
    std::thread th1(do_work);
    std::thread th2(do_work);
    std::thread th3(do_work);
    std::thread th4(do_work);
    std::thread th5(do_work);
 
    th1.join();
    th2.join();
    th3.join();
    th4.join();
    th5.join();
 
    std::cout << "Result:" << data << '\n';
}

Result:5

Defect reports

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

See also