Re: [boost] Boost.Threads, N2178, N2184, et al

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-03-27 13:44:39

Howard Hinnant wrote:

> New use case:
>
> vector<thread> v;
> ... fill with threads ...
> for (auto i = v.begin(), e = v.end(); i != e; ++i)
>     if (i_feel_like_it)
>         i->detach();
> ...
> // need new thread now, go look for a detached one and use it
> auto i = v.begin();
> for (auto e = v.end(); i != e; ++i)
>     if (i->is_detached())  // N2184 spells this i->get_id() ==
> thread::id()
>         break;
> if (i != v.end())
>    *i = std::thread(f);  // recycle detached thread
> else
>    v.push_back(std::thread(f));

I was thinking of something along these lines, but it still seems contrived 
to me. There is no reason to reuse an std::thread _object_ because you gain 
no efficiency from that (as opposed to reusing a thread).

So I'd use something like:

> for (auto i = v.begin(), e = v.end(); i != e; ++i)
>     if (i_feel_like_it)
>         i->detach();
            remove_i_from_v();

and then just add new threads with push_back... If I'm not interested in *i, 
I see no reason to keep it in the vector - if I use v.size() as an 
indication of the number of my active threads, this would skew the results.

There's also:

for (auto i = v.begin(), e = v.end(); i != e; ++i)
    if( i->try_join() )
        remove_i_from_v();

(ignoring invalidation for the purposes of the pseudocode :-) )

which is useful if I want to keep an eye on 'my' threads but need v.size() 
to not include dead ones. In this case, I can decide whether a detached 
thread still counts against 'mine' by leaving or not leaving it in v after 
detach.

In a less forgiving model such as N2178, the original example is just

erase_if( v, feel_like_it_predicate );

since it has an implicit detach, although I admit that cancel on destroy is 
useful in other scenarios and have no strong preference for one over the 
other.