Summary - Atomic

Combine atomic, memory_order and compare_exchange to write a counter.

Summary - Atomic

In this group you met five tools of atomic operations: shared values with std::atomic, ordering guarantees with memory_order, CAS with compare_exchange_strong, spinlocks with atomic_flag and atomic arithmetic with fetch_add. They all live in the <atomic> header.

Together these tools let you write lock-free, data-race-free parallel code. Now you will write a small task that combines what you learned: two atomic additions with fetch_add and a print of the result.

Exercise Summary: two fetch_add

Create std::atomic<int> a{0};. Do two atomic additions with a.fetch_add(7); and a.fetch_add(3);. Print the result with std::cout << a.load();. Include the <atomic> header.

Show example solution

#include <iostream>
#include <atomic>

int main() {
    std::atomic<int> a{0};
    a.fetch_add(7);
    a.fetch_add(3);
    std::cout << a.load();
    return 0;
}

Test yourself

Which tool is the simplest atomic type?
- std::atomic<int>
- std::atomic_flag
- std::future
- std::mutex
Show answer

std::atomic_flag — std::atomic_flag is the simplest atomic type; it has only two states.
Match each tool to its purpose

std::atomic<int>

Shared counter
memory_order_relaxed

Weak ordering
compare_exchange_strong

CAS swap
fetch_add

Atomic addition

Complete the fetch_add line

std::atomic<int> a{0};
a.____(7);
a.fetch_add(3);
std::cout << a.load();

Show answer

std::atomic<int> a{0};
a.fetch_add(7);
a.fetch_add(3);
std::cout << a.load();

Next lesson: Singleton

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