Is calling destructor manually always a sign of ba

Question

I was thinking: they say if you're calling destructor manually - you're doing something wrong. But is it always the case? Are there any counter-examples? Situations where it is neccessary to call it manually or where it is hard/impossible/impractical to avoid it?

Answer 1

No, Depends on the situation, sometimes it is legitimate and good design.

To understand why and when you need to call destructors explicitly, let's look at what happening with "new" and "delete".

To created an object dynamically, T* t = new T; under the hood: 1. sizeof(T) memory is allocated. 2. T's constructor is called to initialize the allocated memory. The operator new does two things: allocation and initialization.

To destroy the object delete t; under the hood: 1. T's destructor is called. 2. memory allocated for that object is released. the operator delete also does two things: destruction and deallocation.

One writes the constructor to do initialization, and destructor to do destruction. When you explicitly call the destructor, only the destruction is done, but not the deallocation.

A legitimate use of explicitly calling destructor, therefore, could be, "I only want to destruct the object, but I don't (or can't) release the memory allocation (yet)."

A common example of this, is pre-allocating memory for a pool of certain objects which otherwise have to be allocated dynamically.

When creating a new object, you get the chunk of memory from the pre-allocated pool and do a "placement new". After done with the object, you may want to explicitly call the destructor to finish the cleanup work, if any. But you won't actually deallocate the memory, as the operator delete would have done. Instead, you return the chunk to the pool for reuse.