Casting shared_ptr to shared_ptr

Question

I have a structure:

struct Params {
   std::shared_ptr<void> user_data;
   /* ... */
};

I want to use it like this:

int main() {
  std::shared_ptr<SpecializedParams> sp(new SpecializedParams(100));
  Params params;
  /* ... */
  params.user_data = std::static_pointer_cast<void>(sp); 
  /* ... */
  std::shared_ptr<SpecializedParams> sp2 = 
    std::static_pointer_cast<SpecializedParams>(
      params.user_data
    );
  /* ... */
  return 0;
}

Is this valid and safe?

You can't `delete` a void pointer, so your code shouldn't even compile. — Kerrek SB, Jul 24 '12 at 05:52
@KerrekSB shared pointers to void are certainly valid. See here for more details: http://stackoverflow.com/questions/5913396/why-do-stdshared-ptrvoid-work — Michael Anderson, Jul 24 '12 at 06:26
@MichaelAnderson: Oh OK, the deleter is deduced from the argument, not from the pointee type - interesting! — Kerrek SB, Jul 24 '12 at 06:55
You might want to use `static_pointer_cast` without `std::`. It will still work due to argument-dependent lookup. — sellibitze, Jul 25 '12 at 09:01

score 8 · Accepted Answer · answered Jul 24 '12 at 06:56

The code, that actual deletes the shared object is determined when the shared pointer is created (that's why you need a complete type, when constructing the shared_ptr and not, when destructing the shared_ptr). Thus even when your shared_ptr is the last pointer that points to your SpecializedParams object, the object will get correctly destroyed.

score 3 · Answer 2 · answered Jul 24 '12 at 04:55

3

This should be safe as the void casted item is a shared_ptr too. It will add a reference to the existing element and it will not be released until the void casted item goes away.

answered Jul 24 '12 at 04:55

ThirdOne

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2

The reason it works is because of the hidden deleter function stored with the `shared_ptr` object ... Otherwise simply calling `delete` on a `void` pointer will not correctly deallocate the memory resources of the object being managed by the `shared_ptr`. – Jason Jul 24 '12 at 12:29

Casting shared_ptr to shared_ptr

2 Answers2