CODEWITHSUNDEEP
c++casting
user3612643
user3612643

Reputation: 5772

Downcasting/upcasting via void*

I have a class hierarchy that involves virtual functions, multiple inheritance, but not virtual inheritance. The whole hierarchy roots in a base class B. A class appears at most in the hierarchy.

Now, I am using a library where I can only pass and receive back void* (some “handles” basically).

In which circumstances is it safe/legal/defined to cast between instances of my hierarchy and void*?

Should I always upcast to B* before passing void* and vice versa?

Will (D*) (B*) (void*) (B*) d be equal to d if d is an instance of D* and D a subclass of B?

Upvotes: 1

Views: 458

Answers (1)

n. m. could be an AI
n. m. could be an AI

Reputation: 119877

You must cast from void* to the same exact type you cast to void* to begin with. This is guaranteed to be safe. Other casts from void* lead to UB.

Once you have a non-void pointer, normal casting rules for pointers of a class hierarchy apply.

Using your example, 

(D*) (B*) (void*) (B*) d

is OK, but 

(D*) (void*) (B*) d
(B*) (void*) d

are not.

Upvotes: 5

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