Is calling destructor manually always a sign of bad design

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

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There are cases when they are necessary   In code I work on I use explicit destructor call in allocators  I have implementation of simple allocator that uses placement new to return memory blocks to stl containers  In destroy I have     void destroy  pointer p           destroy objects by calling their destructor     p- gt  T          while in construct     void construct  pointer p  const T amp  value           initialize memory with placement new      undef new       new  PVOID p  T value         there is also allocation being done in allocate   and memory deallocation in deallocate    using platform specific alloc and dealloc mechanisms  This allocator was used to bypass doug lea malloc and use directly for example LocalAlloc on windows

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All answers describe specific cases  but there is a general answer   You call the dtor explicitly every time you need to just destroy the object  in C   sense  without releasing the memory the object resides in   This typically happens in all the situation where memory allocation   deallocation is managed independently from object construction   destruction  In those cases construction happens via placement new upon an existent chunk of memory  and destruction happens via explicit dtor call   Here is the raw example       char buffer sizeof MyClass              MyClass  p   new buffer MyClass       p- gt dosomething         p- gt  MyClass                 MyClass  p   new buffer MyClass       p- gt dosomething         p- gt  MyClass             Another notable example is the default std  allocator when used by std  vector  elements are constructed in vector during push back  but the memory is allocated in chunks  so it pre-exist the element contruction  And hence  vector  erase must destroy the elements  but not necessarily it deallocates the memory  especially if new push back have to happen soon       It is  bad design  in strict OOP sense  you should manage objects  not memory  the fact objects require memory is an  incident    it is  good design  in  low level programming   or in cases where memory is not taken from the  free store  the default operator new buys in   It is bad design if it happens randomly around the code  it is good design if it happens locally to classes specifically designed for that purpose

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I have never come across a situation where one needs to call a destructor manually  I seem to remember even Stroustrup claims it is bad practice

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Any time you need to separate allocation from initialization  you ll need placement new and explicit calling of the destructor manually   Today  it s rarely necessary  since we have the standard containers  but if you have to implement some new sort of container  you ll need it

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What about this   Destructor is not called if an exception is thrown from the constructor  so I have to call it manually to destroy handles that have been created in the constructor before the exception   class MyClass     HANDLE h1 h2    public    MyClass            handles have to be created first     h1 SomeAPIToCreateA        h2 SomeAPIToCreateB                try                   if error            throw MyException                      catch              this- gt  MyClass          throw               MyClass         SomeAPIToDestroyA h1       SomeAPIToDestroyB h2

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I have another situation where I think it is perfectly reasonable to call the destructor   When writing a  Reset  type of method to restore an object to its initial state  it is perfectly reasonable to call the Destructor to delete the old data that is being reset   class Widget   private       char  pDataText   NULL          int   idNumber    0         public      void Setup     pDataText   new char 100          Widget        delete pDataText                  void Reset                 Widget blankWidget          this- gt  Widget           Manually delete the current object using the dtor          this   blankObject     Copy a blank object to the this-object

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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

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I found 3 occasions where I needed to do this    allocating deallocating objects in memory created by memory-mapped-io or shared memory when implementing a given C interface using C    yes this still happens today unfortunately  because I don t have enough clout to change it   when implementing allocator classes

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No  you shouldn t call it explicitly because it would be called twice  Once for the manual call and another time when the scope in which the object is declared ends   Eg       Class c    c  Class        If you really need to perform the same operations you should have a separate method   There is a specific situation in which you may want to call a destructor on a dynamically allocated object with a placement new but it doesn t sound something you will ever need

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Calling the destructor manually is required if the object was constructed using an overloaded form of operator new    except when using the  std  nothrow  overloads   T  t0   new std  nothrow  T    delete t0     OK  std  nothrow overload  void  buffer   malloc sizeof T    T  t1   new buffer  T    t1- gt  T       required  delete t1 would be wrong free buffer     Outside managing memory on a rather low level as above calling destructors explicitly  however  is a sign of bad design  Probably  it is actually not just bad design but outright wrong  yes  using an explicit destructor followed by a copy constructor call in the assignment operator is a bad design and likely to be wrong    With C   2011 there is another reason to use explicit destructor calls  When using generalized unions  it is necessary to explicitly destroy the current object and create a new object using placement new when changing the type of the represented object  Also  when the union is destroyed  it is necessary to explicitly call the destructor of the current object if it requires destruction

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As quoted by the FAQ  you should call the destructor explicitly when using placement new      This is about the only time you ever explicitly call a destructor    I agree though that this is seldom needed

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Found another example where you would have to call destructor s  manually  Suppose you have implemented a variant-like class that holds one of several types of data   struct Variant       union           std  string str          int num          bool b             enum Type   Str  Int  Bool   type       If the Variant instance was holding a std  string  and now you re assigning a different type to the union  you must destruct the std  string first  The compiler will not do that automatically

[c++] Is calling destructor manually always a sign of bad design?

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