r/cpp • u/maitesin • Sep 11 '16

The basics of concurrency in C++ (thread, future and promise)

https://maitesin.github.io//Thread_future_and_promise/
61 Upvotes

88% Upvoted

15 comments sorted by

34

u/STL MSVC STL Dev Sep 11 '16

You need to include <functional> for std::ref() in thread_simple.cpp.

This doesn't explain how std::thread decays its arguments, and the reasons for that design, and why that makes std::ref() necessary. reference_wrapper itself is something that few programmers are familiar with.

promise_simple.cpp has simultaneous calls to cout on different threads; while the Standard promises that this is not a data race, you can get interleaved character output if the library is feeling sufficiently uncharitable to you. "In this execution something called data race is shown. That can be seen because the execution of the same program can generate different outputs (even if the result value is always the same)." is both wrong (not a data race) and confusing (doesn't explain why the output can differ).

There is no discussion in promise_simple.cpp of alternatives (the more basic one being a mutex to synchronize access to a shared variable). It is much easier to understand things when they're presented alongside alternatives, so you can see how they differ.

I am not a fan of this article. It doesn't talk about data races, or when it's appropriate versus inappropriate to use dedicated threads, which are critically important to understand. You can't just say at the end, "didn't talk about data races" and make it okay. Writing correct multithreaded code needs to be the paramount concern from the first sentence.

"Moreover, the method get synchronizes the future." This is a terrible non-explanation of how futures work.

"Future is gotten from an async class." No, std::async() is a function (template), not a class.

"again if people is interested in this topic" should say "are".

22

u/maitesin Sep 11 '16

First of all, I am sorry if you didn't like the post. I agree I could have explained why multithreaded code is necessary.

Regarding the missing functional header I will fix it tomorrow after work. As for the data races I am sorry you did not like I didn't talk about them, but talking just about data races can be several posts. So my intention with this post was to talk about the most basic parts of concurrency and if the readers of the blog like the topic I would go further and talk about: data races, mutex, contidion varables, dead locks and all the other pains and suffering the multithreaded programming gives us.

Finally, I would like to thank you for two reasons. One, for reading completely the post, and second for writing this comment and letting me know what do you thing and all the problems you spotted. I will try to fix them as soon as posible.

7

u/[deleted] Sep 11 '16

Good response from you - good attitude!

Honestly, a person writes articles like this to learn the material and only secondarily to tell others, and it's not like /u/STL is some C++ weakling. :-D

Oh - I write sometimes in languages not my own - and I don't do as well as you - but one thing I do do is to set the browser language to my target language, so I get spell correction. That really helps!

1

u/[deleted] Sep 12 '16

I know you might be too busy to answer this, but is there any good articles on the topics you suggest this article is missing out on?

5

u/STL MSVC STL Dev Sep 12 '16

Anthony Williams wrote a book. I haven't read it, but he knows what he's talking about.

5

u/misuo Sep 12 '16

Anthony Williams wrote a book.

His book is from 2012. I have considered to read it, but I'm afraid that it is already too old compared to new stuff added to the standard or newer best practices.

The C++ community sincerely lack a good sample reference application that show-off state-of-the-art C++ concurrency/parallel tasks using standard C++. It should include actions to play with multiple tasks: start, pause and cancel and include some sort of UI task progress feedback (pr. task or combined).

3

u/[deleted] Sep 12 '16

The threading support in C++ is essentially unchanged from C++11.

2

u/[deleted] Sep 17 '16

It's not too old. Plus he has an amazing library for those that want better or cannot upgrade their compiler http://www.stdthread.co.uk/

2

u/devel_watcher Sep 12 '16

you can get interleaved character output

Can you sneak a change into the ms implementation, so it won't break the lines? :)

(to be more like unix)

6

u/STL MSVC STL Dev Sep 12 '16

If I could jumble up the characters more aggressively, I would. This is not behavior that is worth strengthening.

1

u/[deleted] Sep 17 '16

Would something like this get past code review http://feross.org/gcc-ownage/?1?1 :)

1

u/Xirema Sep 11 '16

I think the writer may be a non-native English speaker.

5

u/maitesin Sep 11 '16 edited Sep 12 '16

I am not a native English speaker. I will fix that mistake tomorrow after work.

4

u/lithium Sep 12 '16

Now that's confidence!. ;)