What is a C++ atomic variable?

Question

Here is the documentation for std::atomic: http://en.cppreference.com/w/cpp/atomic/atomic

Reading other questions what I've gathered is it generates some sort of special machine instructions (i.e. lock add) and that as a result you don't have to use a mutex around the variable.

What I want to know is how it works and what are the limitations to using them?

Any answer you get is going to be really long. A good start is to study up on what lock-free means. From there you can start to read the C++ standard starting with 29 Atomic operations library [atomics], and there's a lot to digest. — AndyG
– AndyG, Commented Jan 11, 2017 at 18:39
it does more than generate special machine instructions. It also prevents load/store reordering. The link to the videos in the answer below really should be followed - you'll be amazed at what you learn. You'll need 2 hours. — Richard Hodges
– Richard Hodges, Commented Jan 11, 2017 at 21:27
Thank you. I will watch the videos. I didn't realize it was so complicated. — tylerjw
– tylerjw, Commented Jan 12, 2017 at 14:35

Richard · Accepted Answer · 2017-01-11 19:13:27Z

1

This topic really could fill a book. Likely you won't get any clear answers in this format.

Check out Herb Sutters videos, these are really the clearest discussions I've seen on the topic:

https://herbsutter.com/2013/02/11/atomic-weapons-the-c-memory-model-and-modern-hardware/

See the links for "Part 1" and "Part 2".

answered Jan 11, 2017 at 19:13

Richard

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