C++ Atomic::fetch_sub() function

The C++ std::atomic::fetch_sub() function is used to subtracts a specified value from an atomic object current value and returns the value it held before the subtraction. This function is important in multi-threaded environment to prevent race conditions when multiple threads access and modify shared variables concurrently.

Syntax

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

T fetch_sub (T val, memory_order sync = memory_order_seq_cst) volatile noexcept;
T fetch_sub (T val, memory_order sync = memory_order_seq_cst) noexcept;
or
T fetch_sub (ptrdiff_t val, memory_order sync = memory_order_seq_cst) volatile noexcept;
T fetch_sub (ptrdiff_t val, memory_order sync = memory_order_seq_cst) noexcept;

Parameters

• val − It indicates the value to subtract.
• sync − It indicates the synchronization mode for the operation.

Return Value

This function returns the contained value before the call.

Exceptions

This member function never throws exceptions.

Example

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

#include <iostream>
#include <atomic>
int main()
{
    std::atomic<int> x(11);
    int y = x.fetch_sub(4);
    std::cout << "Old value: " << y << std::endl;
    std::cout << "New value: " << x << std::endl;
    return 0;
}

Output

Output of the above code is as follows −

Old value: 11
New value: 7

Example

Consider the following example, where we are going to perform the fetch_sub() in multithreaded scenario.

#include <iostream>
#include <atomic>
#include <thread>
#include <vector>
std::atomic<int> a(49);
void b()
{
    for (int x = 0; x < 9; ++x) {
        a.fetch_sub(1);
    }
}
int main()
{
    std::vector<std::thread> c;
    for (int y = 0; y < 5; ++y) {
        c.emplace_back(b);
    }
    for (auto& t : c) {
        t.join();
    }
    std::cout << "Final value: " << a << std::endl;
    return 0;
}

Output

Following is the output of the above code −

Final value: 4