Why does enable_shared_from_this have a non-virtual destructor?

Question

I have a pet project with which I experiment with new features of C++11. While I have experience with C, I'm fairly new to C++. To train myself into best practices, (besides reading a lot), I have enabled some strict compiler parameters (using GCC 4.4.1):

-std=c++0x -Werror -Wall -Winline -Weffc++ -pedantic-errors

This has worked fine for me. Until now, I have been able to resolve all obstacles. However, I have a need for enable_shared_from_this, and this is causing me problems. I get the following warning (error, in my case) when compiling my code (probably triggered by -Weffc++):

base class ‘class std::enable_shared_from_this<Package>’ has a non-virtual destructor

So basically, I'm a bit bugged by this implementation of enable_shared_from_this, because:

• A destructor of a class that is intended for subclassing should always be virtual, IMHO.
• The destructor is empty, why have it at all?
• I can't imagine anyone would want to delete their instance by reference to enable_shared_from_this.

But I'm looking for ways to deal with this, so my question is really, is there a proper way to deal with this? And: am I correct in thinking that this destructor is bogus, or is there a real purpose to it?

Answer 1

A destructor of a class that is intended for subclassing should always be virtual, IMHO.

A virtual destructor in a base class is only needed if an instance of the derived class is going to be deleted via a pointer to the base class.

Having any virtual function in a class, including a destructor, requires overhead. Boost (and the TR1 and C++11 Standard Library) doesn't want to force you to have that overhead just because you need to be able to obtain a shared_ptr from the this pointer.

The destructor is empty, why have it at all?

If you don't have a user-defined constructor, the compiler provides one for you, so it doesn't really matter.

I can't imagine anyone would want to delete their instance by reference to enable_shared_from_this.

Exactly.

As for the compiler warning, I would ignore the warning or suppress it (with a comment in the code explaining why you are doing so). Occasionally, especially at "pedantic" warning levels, compiler warnings are unhelpful, and I'd say this is one of those cases.

Answer 2

I agree with Jame's description, but would add

A virtual destructor is only required if you want to destroy an instance of that class virtually. This is not always the case, however if a base class is not intended to be destroyed virtually it should protect against it

So I would change

A destructor of a class that is intended for subclassing should always be virtual, IMHO.

this to be:

A destructor of a class that is intended for subclassing should always be virtual or protected.