C++ Atomic::operator--() function.

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The C++ std::atomic::operator--() function is used for atomic decrement operations on atomic variables. This ensures that the decrement operation is performed atomically, meaning it is thread-safe and free from race conditions.

The operator--() function can be used in both pre-decrement and post-decrement.

Syntax

Following is the syntax for std::atomic::operator--() function.

T operator--() volatile noexcept;
T operator--() noexcept;
or	
T operator-- (int) volatile noexcept;
T operator-- (int) noexcept;

Parameters

It does not accepts any parameter.

Return Value

It returns the value of the atomic variable before the modification.

Exceptions

This function never throws exceptions.

Example

In the following example, we are going to consider the basic usage of operator--() function.

#include <iostream>
#include <atomic>
int main()
{
    std::atomic<int> x(8);
    --x;
    std::cout << "Result : " << x.load() << std::endl;
    return 0;
}

Output

Output of the above code is as follows −

Result : 7

Example

Consider the following example, where we are going to use the decrement in the loop.

#include <iostream>
#include <atomic>
int main()
{
    std::atomic<int> x(4);
    for (int i = 0; i < 3; ++i) {
        --x;
    }
    std::cout << "Result : " << x.load() << std::endl;
    return 0;
}

Output

Following is the output of the above code −

Result : 1

Example

Let's look at the following example, where we are going to use the decrement in a multi thread environment.

#include <iostream>
#include <atomic>
#include <thread>
#include <vector>
void a(std::atomic<int>& b)
{
    --b;
}
int main()
{
    std::atomic<int> x(20);
    std::vector<std::thread> x1;
    for (int y = 0; y < 13; ++y) {
        x1.push_back(std::thread(a, std::ref(x)));
    }
    for (auto& t : x1) {
        t.join();
    }
    std::cout << "Result : " << x.load() << std::endl;
    return 0;
}

Output

If we run the above code it will generate the following output −

Result : 7