Use std::vector in WaitForMultipleObjects()

Question

I have an std::vector of handle objects. I have to wait on these handle objects for using in the WaitForMultipleObjects function. Since it's a vector, I am getting an error while using it in WaitForMultipleObjects:

std::vector<HANDLE> events;
// ...
WaitForMultipleObjects(events.size(), events, true, INFINITE);

Is there any way to do this?

As @Joachim said, the second parameter should be a pointer, so why not send `&events[0]` instead of `events`? — Eitan T, Jun 19 '12 at 08:27

score 4 · Accepted Answer · edited May 30 '19 at 12:31

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Preferably, if you've got an up to date version of STL, you should use:

WaitForMultipleObjects(events.size(), events.data(), true, INFINITE);

With older STL's, you can use &events[0] if .data() isn't available as a method on vector.

edited May 30 '19 at 12:31

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answered Jun 19 '12 at 08:48

Scott Langham

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Some programmer dude · Answer 2 · 2012-06-19T08:45:48.033

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If you look at the documentation for WaitForMultipleObject you will see that the second argument is a pointer, not a std::vector. The std::vector class can not be used instead of a pointer or native array.

~~The only~~One way you can do is to create a temporary "array", and copy all the handles to it, and use that as argument.

Another way, as suggested by Charles, is to use &vector[0] or as suggested by Tony to use vector.data() (if available).

edited Jun 19 '12 at 08:45

answered Jun 19 '12 at 08:25

Some programmer dude

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Only way? What about `&events[0]`. – CB Bailey Jun 19 '12 at 08:28
@CharlesBailey That will work, but is it portable and "officially sanctioned" (i.e. defined by the standard)? – Some programmer dude Jun 19 '12 at 08:30
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@JoachimPileborg, yes, standard guarantees that `std::vector`'s elements are consecutive in memory. – unkulunkulu Jun 19 '12 at 08:34
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Agree that `&events[0]` is perfectly fine; also you can you `events.data()` - see http://en.cppreference.com/w/cpp/container/vector/data – Tony Delroy Jun 19 '12 at 08:36
But i have to wait on all the handle objects in the vector. – Aneesh Narayanan Jun 19 '12 at 08:36
@TonyDelroy, wow! that's nice, didn't know about the `vector::data` :( – unkulunkulu Jun 19 '12 at 08:37
@AneeshNarayanan, you specify the number of the elements as the first parameter, so it will wait for all of them. – unkulunkulu Jun 19 '12 at 08:39
@unkulunkulu std::vector::data is new with C++11, so if you STL implementation is old, then it might not be available. – haffax Jun 19 '12 at 08:41
My install of VisualStudio 2010 allows .data() and I've been using it successfully for a while. – Scott Langham Jun 19 '12 at 08:46
This is weird, the cppreference doesn't mark it as C++11. There's also http://stackoverflow.com/questions/6212572/whats-the-status-of-stdvectordata – unkulunkulu Jun 19 '12 at 08:48

unkulunkulu · Answer 3 · 2012-06-19T08:47:18.313

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You should do it like this

WaitForMultipleObjects(events.size(), &events[0], true, INFINITE);

This is portable and the-way-to-do-it.

edited Jun 19 '12 at 08:47

answered Jun 19 '12 at 08:35

unkulunkulu

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score 1 · Answer 4 · edited May 23 '17 at 12:32

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The canonical way to get to the underlying vector buffer is

&events[0]

So you can do this:

WaitForMultipleObjects(events.size(), &events[0], true, INFINITE);

Also see this similar question.

edited May 23 '17 at 12:32

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answered Jun 19 '12 at 08:49

sharptooth

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Use std::vector in WaitForMultipleObjects()

4 Answers4