Update MPMCQueue.h and add [[nodiscard]] attributes to emty(), idx(), turn(), size()

[ghost]

Add [[nodiscard]] attributes

[rigtorp]

Nice, I was going to add this myself. The question is if I should bump the required C++ standard to C++17 which includes [[nodiscard]] or use macros to support C++11.

[ghost]

I was under the impression that unknown attributes are ignored by major compilers. After some research it turns out this is a c++ 17 feature. Macros are better suited then.

[rigtorp]

This approach seems to work well rigtorp/SPSCQueue#31

[ghost]

Done.
Also, if not too persnickety, I see no reason to #include a pure C header <stdlib.h> instead of <cstdlib>, especially as the other two C headers (cassert, cstddef) were included that way.

[rigtorp]

Done. Also, if not too persnickety, I see no reason to #include a pure C header <stdlib.h> instead of , especially as the other two C headers (cassert, cstddef) were included that way.

Yes I agree.

Shouldn't try_pop, try_push etc also have NODISCARD?