std::enable_shared_from_this, non-virtual destructor and public inheritance

2
votes

The std::enable_shared_from_this class is a (template) mixin, recommended for use to enable creating shared pointers from a given object (or its address), which all have common ownership of the object.

The thing is, that if you have an class T which:

  1. Has virtual methods
  2. inherits from std::enable_shared_from_this<T> (and the inheritance must be public as detailed at the link above; otherwise the mixin is useless)
  3. Gets compiled with GCC with -Wnon-virtual-dtor (perhaps also with clang, I'm not sure)

you get warnings about the non-virtual destructor of std::enable_shared_from_this.

My question is - where is the fault here? That is...

  • Should std::enable_shared_from_this have a virtual destructor? (I don't think so)
  • Should the non-virtual-destructor warning employ some criterion for when it is emitted (if at all enabled, that is)?
  • Should the destructor of std::enable_shared_from_this be made protected? (And will this even work?)
  • Should classes with this mixin not have virtual methods at all?

I'm confused.

1
c++compiler-warningslibstdc++virtual-destructorenable-shared-from-this
Posible dup of stackoverflow.com/questions/2571850/… ? – Marshall Clow
@MarshallClow: It's definitely related, but the discussion there does not consider two of the possible alternatives I mentioned. I'm particularly interested in the "protected dtor" alternative not mentioned there. – einpoklum

1 Answers

1
votes

There is no fault; your code is fine. It's merely a false-positive. The point of a warning is to detect pieces of code which, although valid C++, usually indicate problems. But "usually" does not mean "always", so most warnings have cases of false-positives, where it thinks there is misuse when there actually is not.

Should std::enable_shared_from_this have a virtual destructor?

No code is expected to delete a pointer to enable_shared_from_this. So no.

Should the non-virtual-destructor warning employ some criterion for when it is emitted (if at all enabled, that is)?

It's not reasonable for a compiler to know everything about what you're intending to do. It's just seeing something that's usually a problem, which you've decided to have it flag. In this case, it's not a problem.

Should the destructor of std::enable_shared_from_this be made protected?

No.

Should classes with this mixin not have virtual methods at all?

No.