Namespaces
Variants
Views
Actions

std::atomic<T>::operator++,++(int),--,--(int)

From cppreference.com
< cpp‎ | atomic‎ | atomic
 
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++20)
this_thread namespace
(C++11)
(C++11)
(C++11)
Mutual exclusion
(C++11)
(C++11)  
Generic lock management
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Condition variables
(C++11)
Semaphores
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
Safe Reclamation
(C++26)
(C++26)
Hazard Pointers





Atomic types
(C++11)
(C++20)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
 
 
member only of atomic<Integral > specializations
T operator++() noexcept;
(1) (since C++11)
T operator++() volatile noexcept;
(2) (since C++11)
T operator++( int ) noexcept;
(3) (since C++11)
T operator++( int ) volatile noexcept;
(4) (since C++11)
T operator--() noexcept;
(5) (since C++11)
T operator--() volatile noexcept;
(6) (since C++11)
T operator--( int ) noexcept;
(7) (since C++11)
T operator--( int ) volatile noexcept;
(8) (since C++11)
member only of atomic<T*> partial specialization
T* operator++() noexcept;
(9) (since C++11)
T* operator++() volatile noexcept;
(10) (since C++11)
T* operator++( int ) noexcept;
(11) (since C++11)
T* operator++( int ) volatile noexcept;
(12) (since C++11)
T* operator--() noexcept;
(13) (since C++11)
T* operator--() volatile noexcept;
(14) (since C++11)
T* operator--( int ) noexcept;
(15) (since C++11)
T* operator--( int ) volatile noexcept;
(16) (since C++11)

Atomically increments or decrements the current value. The operation is read-modify-write operation.

  • operator++() performs atomic pre-increment. Equivalent to return fetch_add(1) + 1;.
  • operator++(int) performs atomic post-increment. Equivalent to return fetch_add(1);.
  • operator--() performs atomic pre-decrement. Equivalent to return fetch_sub(1) - 1;.
  • operator--(int) performs atomic post-decrement. Equivalent to return fetch_sub(1);.
1-8) For signed integral types, arithmetic is defined to use two’s complement representation. There are no undefined results.
9-16) The result may be an undefined address, but the operations otherwise have no undefined behavior.
If T is not a complete object type, the program is ill-formed.

It is deprecated if std::atomic<T>::is_always_lock_free is false and any volatile overload participates in overload resolution.

(since C++20)

Contents

[edit] Return value

operator++() and operator--() return the value of the atomic variable after the modification. Formally, the result of incrementing/decrementing the value immediately preceding the effects of this function in the modification order of *this.

operator++(int) and operator--(int) return the value of the atomic variable before the modification. Formally, the value immediately preceding the effects of this function in the modification order of *this.

[edit] Notes

Unlike most pre-increment and pre-decrement operators, the pre-increment and pre-decrement operators for atomic types do not return a reference to the modified object. They return a copy of the stored value instead.

[edit] Example

#include <atomic>
#include <chrono>
#include <iomanip>
#include <iostream>
#include <mutex>
#include <random>
#include <string>
#include <thread>
 
std::atomic<int> atomic_count{0};
 
std::mutex cout_mutex;
int completed_writes{0};
 
constexpr int global_max_count{72};
constexpr int writes_per_line{8};
constexpr int max_delay{100};
 
template<int Max>
int random_value()
{
    static std::uniform_int_distribution<int> distr{1, Max};
    static std::random_device engine;
    static std::mt19937 noise{engine()};
    static std::mutex rand_mutex;
    std::lock_guard lock{rand_mutex};
    return distr(noise);
}
 
int main()
{
    auto work = [](const std::string id)
    {
        for (int count{}; (count = ++atomic_count) <= global_max_count;)
        {
            std::this_thread::sleep_for(
                std::chrono::milliseconds(random_value<max_delay>()));
 
            // print thread `id` and `count` value
            {
                std::lock_guard lock{cout_mutex};
 
                const bool new_line = ++completed_writes % writes_per_line == 0;
 
                std::cout << id << std::setw(3) << count << "  "
                          << (new_line ? "\n" : "") << std::flush;
            }
        }
    };
 
    std::jthread j1(work, "░"), j2(work, "▒"), j3(work, "▓"), j4(work, "█");
}

Possible output:

▒  2  ░  1  ▒  5  ▒  7  █  4  ░  6  ▓  3  ▒  8  
▓ 11  █  9  ▓ 13  ░ 10  █ 14  ▒ 12  ░ 16  ░ 19  
▓ 15  ▒ 18  ▓ 21  ▒ 22  █ 17  █ 25  ▒ 24  █ 26  
░ 20  ░ 29  ▒ 27  ▓ 23  ▒ 31  ▒ 33  ▓ 32  █ 28  
░ 30  ░ 37  ▒ 34  ▓ 35  █ 36  █ 41  ▓ 40  ▒ 39  
░ 38  ▓ 43  █ 42  ▓ 46  ▓ 48  █ 47  █ 50  ░ 45  
▒ 44  ▒ 53  ▒ 54  ▓ 49  ▒ 55  █ 51  ▒ 57  █ 58  
░ 52  ▓ 56  ░ 61  ▒ 59  █ 60  ▓ 62  ▒ 64  ░ 63  
░ 68  ▓ 66  █ 65  █ 71  ▒ 67  ▓ 70  ░ 69  █ 72

[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 (possibly cv-qualified) void or function made ill-formed

[edit] See also

atomically adds the argument to the value stored in the atomic object and obtains the value held previously
(public member function) [edit]
atomically subtracts the argument from the value stored in the atomic object and obtains the value held previously
(public member function) [edit]
adds to or subtracts from the atomic value
(public member function) [edit]
performs bitwise AND, OR, XOR with the atomic value
(public member function) [edit]