Shared_Ptr Implementation Without Polymorphic Class Virtual Destructor
In the debate between Armen Tsirunyan and Daniel Lidström on the necessity of virtual destructors for shared_ptr implementation, it is indeed possible to design a shared_ptr that does not require such destructors.
Technical Implementation
The key to this implementation lies in type erasure. Shared_ptr manages not only reference counters but also a deleter object stored in the same memory block. The type of this deleter is distinct from that of shared_ptr, allowing for flexibility in managing objects with different dynamic types.
A templated constructor is introduced:
template<class t> class shared_ptr { public: ... template<class y> explicit shared_ptr(Y* p); ... };</class></class>
When constructing a shared_ptr with a pointer of a derived class (e.g., shared_ptr
C 11 Standard Requirements
The C 11 standard explicitly defines the requirements for this constructor:
- The pointer p must be convertible to T* and Y must be a complete type.
- The expression "delete p" must be well-formed, have well-defined behavior, and not throw exceptions.
For the destructor:
- If shared_ptr is empty or shares ownership, there are no side effects.
- If shared_ptr owns an object with a deleter d, d(p) is called.
- Otherwise, if shared_ptr owns a pointer p, "delete p" is invoked.
Thus, the shared_ptr implementation utilizes type erasure and carefully manages deleters to effectively dispose of objects with differing dynamic types, even without requiring virtual destructors in polymorphic classes.
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