CODEWITHSUNDEEP
multithreadingboostasynchronousboost-asio
Oliver K
Oliver K

Reputation: 295

ASIO strand::wrap does it not have to serialize in order?

I'm lost on the distinction between posting using strand::wrap and a strand::post? Seems like both guarantee serialization yet how can you serialize with wrap and not get consistent order? Seems like they both would have to do the same thing. When would I use one over the other?

Here is a little more detail pseudo code:

    mystrand(ioservice);
    mystrand.post(myhandler1);
    mystrand.post(myhandler2);

this guarantees my two handlers are serialized and executed in order even in a thread pool.

Now, how is that different from below?

    ioservice->post(mystrand.wrap(myhandler1));
    ioservice->post(mystrand.wrap(myhandler2));

Seems like they do the same thing? Why use one over the other? I see both used and am trying to figure out when one makes more sense than the other.

Upvotes: 5

Views: 1287

Answers (2)

Mudzot
Mudzot

Reputation: 136

This way

mystrand(ioservice);
mystrand.post(myhandler1);
mystrand.post(myhandler2);

myhandler1 is guaranteed by mystrand to be executed before myhandler2

but 

ioservice->post(mystrand.wrap(myhandler1));
ioservice->post(mystrand.wrap(myhandler2));

the execution order is the order of executing wrapped handlers, which io_service::post does not guarantee.

Upvotes: 3

John Zwinck
John Zwinck

Reputation: 249404

wrap creates a callable object which, when called, will call dispatch on a strand. If you don't call the object returned by wrap, nothing much will happen at all. So, calling the result of wrap is like calling dispatch. Now how does that compare to post? According to the documentation, post differs from dispatch in that it does not allow the passed function to be invoked right away, within the same context (stack frame) where post is called.

So wrap and post differ in two ways: the immediacy of their action, and their ability to use the caller's own context to execute the given function.

I got all this by reading the documentation.

Upvotes: 2

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