When is a C destructor called

Question

Basic Question  when does a program call a class  destructor method in C    I have been told that it is called whenever an object goes out of scope or is subjected to a delete  More specific questions   1  If the object is created via a pointer and that pointer is later deleted or given a new address to point to  does the object that it was pointing to call its destructor  assuming nothing else is pointing to it    2  Following up on question 1  what defines when an object goes out of scope  not regarding to when an object leaves a given  block    So  in other words  when is a destructor called on an object in a linked list   3  Would you ever want to call a destructor manually

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When you create an object with new  you are responsible for calling delete  When you create an object with make shared  the resulting shared ptr is responsible for keeping count and calling delete when the use count goes to zero  Going out of scope does mean leaving a block  This is when the destructor is called  assuming that the object was not allocated with new  i e  it is a stack object   About the only time when you need to call a destructor explicitly is when you allocate the object with a placement new

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Whenever you use  new   that is  attach an address to a pointer  or to say  you claim space on the heap  you need to  delete  it  1 yes  when you delete something  the destructor is called  2 When the destructor of the linked list is called  it s objects  destructor is called  But if they are pointers  you need to delete them manually  3 when the space is claimed by  new

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Pointers -- Regular pointers don t support RAII  Without an explicit delete  there will be garbage  Fortunately C   has auto pointers that handle this for you  Scope -- Think of when a variable becomes invisible to your program  Usually this is at the end of  block   as you point out  Manual destruction -- Never attempt this  Just let scope and RAII do the magic for you

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Others have already addressed the other issues  so I ll just look at one point  do you ever want to manually delete an object  The answer is yes   DavidSchwartz gave one example  but it s a fairly unusual one  I ll give an example that s under the hood of what a lot of C   programmers use all the time  std  vector  and std  deque  though it s not used quite as much   As most people know  std  vector will allocate a larger block of memory when if you add more items than its current allocation can hold  When it does this  however  it has a block of memory that s capable of holding more objects than are currently in the vector  To manage that  what vector does under the covers is allocate raw memory via the Allocator object  which  unless you specify otherwise  means it uses   operator new   Then  when you use  for example  push back to add an item to the vector  internally the vector uses a placement new to create an item in the  previously  unused part of its memory space  Now  what happens when if you erase an item from the vector  It can t just use delete -- that would release its entire block of memory  it needs to destroy one object in that memory without destroying any others  or releasing any of the block of memory it controls  for example  if you erase 5 items from a vector  then immediately push back 5 more items  it s guaranteed that the vector will not reallocate memory when you do so  To do that  the vector directly destroys the objects in the memory by explicitly calling the destructor  not by using delete  If  perchance  somebody else were to write a container using contiguous storage roughly like a vector does  or some variant of that  like std  deque really does   you d almost certainly want to use the same technique  Just for example  let s consider how you might write code for a circular ring-buffer   ifndef CBUFFER H INC  define CBUFFER H INC  template  lt class T gt  class circular buffer       T  data      unsigned read pos      unsigned write pos      unsigned in use      const unsigned capacity  public      circular buffer unsigned size            data  T   operator new size   sizeof T             read pos 0           write pos 0           in use 0           capacity size              void push T const  amp t               ensure there s room in buffer          if  in use    capacity               pop                construct copy of object in-place into buffer         new  amp data write pos     T t              keep pointer in bounds          write pos    capacity            in use                return oldest object in queue      T front             return data read pos                 remove oldest object from queue      void pop                 destroy the object          data read pos     T                keep pointer in bounds          read pos    capacity          --in use            circular buffer            first destroy any content     while  in use    0          pop            then release the buffer      operator delete data            endif  Unlike the standard containers  this uses operator new and operator delete directly  For real use  you probably do want to use an allocator class  but for the moment it would do more to distract than contribute  IMO  anyway

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1  If the object is created via a pointer and that pointer is later deleted or given a new address to point to  does the object that it was pointing to call its destructor  assuming nothing else is pointing to it     It depends on the type of pointers  For example  smart pointers often delete their objects when they are deleted  Ordinary pointers do not  The same is true when a pointer is made to point to a different object  Some smart pointers will destroy the old object  or will destroy it if it has no more references  Ordinary pointers have no such smarts  They just hold an address and allow you to perform operations on the objects they point to by specifically doing so      2  Following up on question 1  what defines when an object goes out of scope  not regarding to when an object leaves a given  block    So  in other words  when is a destructor called on an object in a linked list    That s up to the implementation of the linked list  Typical collections destroy all their contained objects when they are destroyed   So  a linked list of pointers would typically destroy the pointers but not the objects they point to   Which may be correct  They may be references by other pointers   A linked list specifically designed to contain pointers  however  might delete the objects on its own destruction   A linked list of smart pointers could automatically delete the objects when the pointers are deleted  or do so if they had no more references  It s all up to you to pick the pieces that do what you want      3  Would you ever want to call a destructor manually    Sure  One example would be if you want to replace an object with another object of the same type but don t want to free memory just to allocate it again  You can destroy the old object in place and construct a new one in place   However  generally this is a bad idea       pointer is destroyed because it goes out of scope     but not the object it pointed to  memory leak if  1     Foo  myfoo   new Foo  foo           pointer is destroyed because it goes out of scope     object it points to is deleted  no memory leak if 1     Foo  myfoo   new Foo  foo     delete myfoo        no memory leak  object goes out of scope if 1     Foo myfoo  foo

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Remember that Constructor of an object is called immediately after the memory is allocated for that object and whereas the destructor is called just before deallocating the memory of that object

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1  Objects are not created  via pointers   There is a pointer that is assigned to any object you  new   Assuming this is what you mean  if you call  delete  on the pointer  it will actually delete  and call the destructor on  the object the pointer dereferences  If you assign the pointer to another object there will be a memory leak  nothing in C   will collect your garbage for you   2  These are two separate questions  A variable goes out of scope when the stack frame it s declared in is popped off the stack  Usually this is when you leave a block  Objects in a heap never go out of scope  though their pointers on the stack may  Nothing in particular guarantees that a destructor of an object in a linked list will be called   3  Not really  There may be Deep Magic that would suggest otherwise  but typically you want to match up your  new  keywords with your  delete  keywords  and put everything in your destructor necessary to make sure it properly cleans itself up  If you don t do this  be sure to comment the destructor with specific instructions to anyone using the class on how they should clean up that object s resources manually

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To give a detailed answer to question 3  yes  there are  rare  occasions when you might call the destructor explicitly  in particular as the counterpart to a placement new  as dasblinkenlight observes   To give a concrete example of this    include  lt iostream gt   include  lt new gt   struct Foo       Foo int i     i i          int i      int main            Allocate a chunk of memory large enough to hold 5 Foo objects      int n   5      char  chunk   static cast lt char  gt    operator new sizeof Foo    n            Use placement new to construct Foo instances at the right places in the chunk      for int i 0  i lt n    i                new  chunk   i sizeof Foo   Foo i                 Output the contents of each Foo instance and use an explicit destructor call to destroy it      for int i 0  i lt n    i                Foo  foo   reinterpret cast lt Foo  gt  chunk   i sizeof Foo            std  cout  lt  lt  foo- gt i  lt  lt    n           foo- gt  Foo                  Deallocate the original chunk of memory        operator delete chunk        return 0      The purpose of this kind of thing is to decouple memory allocation from object construction

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Yes  a destructor  a k a  dtor  is called when an object goes out of scope if it is on the stack or when you call delete on a pointer to an object    If the pointer is deleted via delete then the dtor will be called  If you reassign the pointer without calling delete first  you will get a memory leak because the object still exists in memory somewhere  In the latter instance  the dtor is not called  A good linked list implementation will call the dtor of all objects in the list when the list is being destroyed  because you either called some method to destory it or it went out of scope itself   This is implementation dependent  I doubt it  but I wouldn t be surprised if there is some odd circumstance out there

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If the object is created not via a pointer for example A a1   A     the destructor is called when the object is destructed  always when the function where the object lies is finished for example   void func         A a1   A           finish   the destructor is called when code is execused to line  finish   If the object is created via a pointer for example A   a2   new A     the destructor is called when the pointer is deleted delete a2   If the point is not deleted by user explictly or given a new address before deleting it  the memory leak is occured  That is a bug  In a linked list  if we use std  list lt    we needn t care about the desctructor or memory leak because std  list lt   has finished all of these for us  In a linked list written by ourselves  we should write the desctructor and delete the pointer explictly Otherwise  it will cause memory leak  We rarely call a destructor manually  It is a function providing for the system  Sorry for my poor English

[c++] When is a C++ destructor called?

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Examples related to destructor