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

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 ) ; .

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 .

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.

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, "█");
}

▒  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

Defect reports

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

See also