std::atomic...ub_explicit(3)

💅 std::atomic...ub_explicit(3) 💅

okay bestie, welcome to the std::atomic...ub_explicit(3) page from the C++ Standard Libary manual — yes, “Libary” is literally how it’s spelled in the header, and we are keeping it 😭. This is the group chat for std::atomic_fetch_sub and std::atomic_fetch_sub_explicit, and they are iconic little atomic subtractors.

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🌸 NAME 🌸

std::atomic_fetch_sub,std::atomic_fetch_sub_explicit — that’s the whole name, bestie. Both functions, one page, no cap.

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✨ SYNOPSIS ✨

Defined in header <atomic>.

(1) — since C++11:

template< class T >
T atomic_fetch_sub( std::atomic<T>* obj,
                    typename std::atomic<T>::difference_type arg ) noexcept;

(2) — since C++11:

template< class T >
T atomic_fetch_sub( volatile std::atomic<T>* obj,
                    typename std::atomic<T>::difference_type arg ) noexcept;

(3) — since C++11:

template< class T >
T atomic_fetch_sub_explicit( std::atomic<T>* obj,
                             typename std::atomic<T>::difference_type arg,
                             std::memory_order order ) noexcept;

(4) — since C++11:

template< class T >
T atomic_fetch_sub_explicit( volatile std::atomic<T>* obj,
                             typename std::atomic<T>::difference_type arg,
                             std::memory_order order ) noexcept;

Performs atomic subtraction. Atomically subtracts arg from 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_sub(arg)
3,4) obj->fetch_sub(arg, order)

bestie, careful: if std::atomic<T> has no fetch_sub member (this member is only provided for integral, floating-point (since C++20) and pointer types except bool), the program is ill-formed. 😭

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🫶 PARAMETERS 🫶

• obj — pointer to the atomic object to modify
• arg — the value to subtract from the value stored in the atomic object
• order — the memory synchronization ordering

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👑 RETURN VALUE 👑

The value immediately preceding the effects of this function in the modification order of *obj. literally the previous value, periodt 💅

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🧚 EXAMPLE 🧚

Multiple threads may use std::atomic_fetch_sub to concurrently process an indexed container.

// Run this code

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

const int N = 50;
std::atomic<int> cnt;
std::vector<int> data(N);

void reader(int id)
{
    for (;;)
    {
        int idx = atomic_fetch_sub_explicit(&cnt, 1, std::memory_order_relaxed);
        if (idx >= 0)
            std::cout << "reader " << std::to_string(id) << " processed item "
                      << std::to_string(data[idx]) << '\n';
        else
        {
            std::cout << "reader " << std::to_string(id) << " done\n";
            break;
        }
    }
}

int main()
{
    std::iota(data.begin(), data.end(), 1);
    cnt = data.size() - 1;

    std::vector<std::thread> v;
    for (int n = 0; n < 5; ++n)
        v.emplace_back(reader, n);
    for (auto& t : v)
        t.join();
}

🖨️ OUTPUT 🖨️

reader 2 processed item 50
reader 1 processed item 44
reader 4 processed item 46
<....>
reader 0 done
reader 4 done
reader 3 done

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💀 DEFECT REPORTS 💀

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

• DR P0558R1 — applied to C++11.
  • Behavior as published: exact type match was required because T is only deduced from obj
  • Correct behavior: T was deduced from multiple arguments

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🌈 SEE ALSO 🌈

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

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mwah 💋 this page is from http://cppreference.com, dated 2024.06.10, and the page tag is std::atomic...ub_explicit(3). bye bestie, go subtract atomically and slay ✨