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std::atomic<T>::fetch_add

From cppreference.com
< cpp‎ | atomic‎ | atomic
 
 
 
 
member only of atomic<Integral>(C++11) and atomic<Floating>(C++20) template specializations
(1)
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
(2)
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 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.

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 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.

The volatile-qualified versions are deprecated if std::atomic<T>::is_always_lock_free is false.

(since C++20)

Contents

[edit] Parameters

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

[edit] Return value

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

[edit] 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;
    }
}

Possible output:

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

[edit] 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

[edit] See also

adds a non-atomic value to an atomic object and obtains the previous value of the atomic
(function template) [edit]
increments or decrements the atomic value by one
(public member function) [edit]