Add (default) virtual destructor

[aromanro]

Since Protothread is an abstract class and one must derive from it and implement Run, it should have a virtual destructor.

[benhoyt]

Hi @aromanro, it's been a while since I've done C++ seriously. Can you give a concrete protothread example where this would matter?

To fix it, I guess it'd be a matter of adding virtual ~Protothread() {} to the public section of the class?

Or, should we make the destructor protected and non-virtual? Do we even need to allow "polymorphic deletion through a base poitner"? See discussion at http://www.gotw.ca/publications/mill18.htm, which says:

"Either: a) you want to allow polymorphic deletion through a base pointer, in which case the destructor must be virtual and public; or b) you don't, in which case the destructor should be nonvirtual and protected, the latter to prevent the unwanted usage."

[aromanro]

It would matter anywhere you have several classes derived from Protothread, but in order to handle them 'the same', you use Protothread pointers. Deleting them could result in leaks. Here is an explanation on stackoverflow: https://stackoverflow.com/questions/461203/when-to-use-virtual-destructors

I guess having a public 'virtual ~Protothread() = default;' should be enough.

I did the change in this pull request for a repo that uses Protothreads: luteberget/simcpp#5

[benhoyt]

How does #8 look? I preferred old-school { } over = default to keep it simple (no need for C++11 here).

[aromanro]

It's perfectly fine that way, too :)