NAME¶

std::atomic::operator++,++(int),--,--(int) - std::atomic::operator++,++(int),--,--(int)

Synopsis¶

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 (since C++20)
any volatile overload participates in overload resolution.

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¶

// Run this code

#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

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 made ill-formed
cv-qualified) void or function

See also¶

atomically adds the argument to the value stored in the atomic object and
fetch_add obtains the value held previously
(public member function)
atomically subtracts the argument from the value stored in the atomic
fetch_sub object and obtains the value held previously
(public member function)
operator+= adds to or subtracts from the atomic value
operator-= (public member function)
operator&= performs bitwise AND, OR, XOR with the atomic value
operator|= (public member function)
operator^=