memory_order

Learn memory ordering with memory_order.

memory_order

memory_order sets the visibility and ordering guarantees of an atomic operation. The default std::memory_order_seq_cst is the strongest; all threads see the same order. Weaker orderings trade safety for speed but need care.

std::memory_order_relaxed guarantees only atomicity; no ordering. std::memory_order_acquire sees prior writes when reading; std::memory_order_release publishes changes made before writing. These two form producer-consumer synchronization. Beginners should use seq_cst.

acquire and release

When one thread does store(v, std::memory_order_release) and another reads with load(std::memory_order_acquire), the reader sees all changes the writer made before the release. This sets up producer-consumer synchronization.

Example: the producer prepares data, then ready.store(true, release). The consumer ready.load(acquire) sees true and knows the data is ready. This pair is the foundation of lock-free synchronization. Wrong ordering can introduce data races; use it carefully.

Exercise store/load with relaxed

Create std::atomic<int> a{0};. Write with a.store(1, std::memory_order_relaxed); and read with std::cout << a.load(std::memory_order_relaxed);. Include the <atomic> header.

Show example solution

#include <iostream>
#include <atomic>

int main() {
    std::atomic<int> a{0};
    a.store(1, std::memory_order_relaxed);
    std::cout << a.load(std::memory_order_relaxed);
    return 0;
}

Test yourself

Complete the memory_order constant

std::atomic<int> a{0};
a.store(1, std::____);

Show answer

std::atomic<int> a{0};
a.store(1, std::memory_order_relaxed);

What does memory_order_relaxed NOT guarantee?
- Atomicity
- Operation ordering
- Type safety
- Memory size
Show answer

Operation ordering — relaxed guarantees only atomicity; it has no ordering.
Match each ordering to its purpose

memory_order_relaxed

Atomicity only
memory_order_acquire

See on read
memory_order_release

Publish on write
memory_order_seq_cst

Strongest guarantee

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