spsc_queue with compile_time_sized_ringbuffer does not call destructors of remaining queue items when destroyed

[jwdevel]

Unlike runtime_sized_ringbuffer, compile_time_sized_ringbuffer does not have a custom destructor, and so it does not clean up any outstanding queue items when destroyed.

I am not a lockfree guru, so I do not know if there is some hidden limitation that makes this impossible, or if it is simply a bug.
I could not find any documentation for this behavior, so I suspect the latter.

I can make a PR for the proposed fix below, if it seems correct.

Here is a small test that fails unexpectedly for me:

int g_numObjs = 0;

struct Obj {
	Obj() {
		++g_numObjs;
	}
	Obj(const Obj&) {
		++g_numObjs;
	}
	~Obj() {
		--g_numObjs;
	}
};

typedef boost::lockfree::spsc_queue<
		Obj
		// Note: commenting out this line makes it into a
		// runtime_sized_ringbuffer, and the test passes.
		,boost::lockfree::capacity<10>
	> MyQueue;

TEST_CASE("boost spsc bug") {
	{
		REQUIRE(g_numObjs == 0);
		MyQueue queue;
		queue.push(Obj());
		REQUIRE(g_numObjs == 1);
	} // queue gets destroyed

	// FAILS
	REQUIRE(g_numObjs == 0);
}

If I add a destructor to compile_time_sized_ringbuffer, such as below (copied from runtime_sized_ringbuffer), then the test passes and everything seems fine:

~compile_time_sized_ringbuffer(void)
{
    // destroy all remaining items
    T out;
    while (pop(&out, 1)) {}
}

Note: I suppose another implemenation might be to call reset() from inside ~spsc_queue() itself, which would benefit from the optimization of not calling trivial destructors. Or maybe a similar optimization would be warranted in the destructors for compile_time_sized... and runtime_sized_... as well?

[jwdevel]

Note: using the fix I wrote above does add the requirement that T must be default-constructable (this requirement already existed with runtime_sized_ringbuffer, it would seem). Perhaps better would be:

while (consume_one([](const T&){})) {}

(assuming lambdas are available)

This same approach could be used for runtime_sized_ringbuffer, which might be nice both for consistency and because default-constructability it's a needless extra requirement.

[Tim Blechmann]

afaict this one is a duplicate of #66

[jwdevel]

Ah, so it is; sorry I missed that ( :
That's what I get for not pulling latest first.

If you're interested in a PR for a version that doesn't require T to be default constructable, I can send it (it's what I settled on locally), but it's small peanuts.