Request for more efficient way to get exception_ptr from future

[Niall Douglas]

Vicente,

Here is that Boost.Thread feature request for AFIO I mentioned a while ago. Here is the code example:

		shared_future<std::shared_ptr<detail::async_io_handle>> *thisresult=state.first->outsharedstates[idx].get();
		// This seems excessive but I don't see any other legal way to extract the exception ...
		bool success=false;
		try
		{
			// Always must wait on the others, because some may be between decrementing the
			// atomic and returning their result to here. Any waiting is likely short.
			thisresult->get();
			success=true;
		}
		catch(...)
		{
			exception_ptr e(afio::make_exception_ptr(afio::current_exception()));
			assert(e);
			complete_async_op(s.out[idx].first, s.out[idx].second, e);
		}
		if(success)
			complete_async_op(s.out[idx].first, s.out[idx].second);

Taken from ​https://github.com/BoostGSoC/boost.afio/blob/master/libs/afio/src/afio.cpp#L1027

Basically, we need to scan a sequence of futures for any exception states which right now means trying to get() from each and try...catch to grab any exception_ptr's. As this is inefficient, is there any chance of adding a new member function to future and shared_future: get_exception()? This is identical to get(), except it returns any exception state instead of throwing it. If there is no exception state, it returns a null exception_ptr.

In other words, a very simple new feature, but one which lets us eliminate a lot of needless try...catch stanzas.

My thanks in advance Vicente.

Niall

[viboes]

Committed revision [85644].

[viboes]

Committed revision [85666].