std::atomic::fetch_add (3) Linux Manual Page

std::atomic<T>::fetch_add – std::atomic<T>::fetch_add

Synopsis

member only of atomic<Integral>(C++11) and atomic<Floating>(C++20) template specializations

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;

member only of atomic<T*> template specialization                                           (1)

T* fetch_add( std::ptrdiff_t arg,

std::memory_order order = std::memory_order_seq_cst ) noexcept;                                 (2)

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 floating-point types, the floating-point_environment in effect may be different from the calling thread’s floating-point environment. The operation need not be conform to the corresponding std::numeric_limits traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the operation otherwise has no undefined behavior. (since C++20)
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

arg – the other argument of arithmetic addition
order – memory order constraints to enforce

Return value

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

Example

// Run this code

#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';
}

Output:

  Result:5

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 arithmetic permitted on pointers to cv void or function made ill-formed

See also

atomic_fetch_add
atomic_fetch_add_explicit adds a non-atomic value to an atomic object and obtains the previous value of the atomic
                          (function template)
(C++11)
(C++11)