Deleting a pointer in C

Question

Context  I m trying to wrap my head around pointers  we just saw them a couple of weeks ago in school and while practicing today I ran into a silly  issue  it can be super straightforward to you but I have little to none programming experience   I ve seen quite a few questions over in SO about deleting pointers but they all seem to be related to deleting a class and not a  simple  pointer  or whatever the proper term might be   here s the code I m trying to run    include  lt iostream gt    using namespace std   int main       int myVar         myPointer     myVar   8    myPointer    amp myVar     cout  lt  lt   delete-ing pointers    lt  lt  endl    cout  lt  lt   Memory address     lt  lt  myPointer  lt  lt  endl        Seems I can t  just  delete it  as it triggers an error    delete myPointer    cout  lt  lt   myPointer     lt  lt  myPointer  lt  lt  endl       Error  a out 14399  malloc      error for object 0x7fff61e537f4       pointer being freed was not allocated          set a breakpoint in malloc error break to debug      Abort trap  6       Using the new keyword befor deleting it works  but      does it really frees up the space     myPointer   new int    delete myPointer    cout  lt  lt   myPointer     lt  lt  myPointer  lt  lt  endl       myPointer continues to store a memory address        Using NULL before deleting it  seems to work     myPointer   NULL    delete myPointer    cout  lt  lt   myPointer     lt  lt  myPointer  lt  lt  endl       myPointer returns 0       So my questions are    Why won t the first case work  Seems the most straightforward use to use and delete a pointer  The error says the memory wasn t allocated but  cout  returned an address  On the second example the error is not being triggered but doing a cout of the value of myPointer still returns a memory address  Does  3 really work  Seems to work to me  the pointer is no longer storing an address  is this the proper way to delete a pointer    Sorry for the long question  wanted to make this as clear as possible  also to reiterate  I have little programming experience  so if someone could answer this using layman s terms  it would be greatly appreciated

User · Answer

There is a rule in C    for every new there is a delete     Why won t the first case work  Seems the most straightforward use to use and delete a pointer  The error says the memory wasn t allocated but  cout  returned an address    new is never called  So the address that cout prints is the address of the memory location of myVar  or the value assigned to myPointer in this case  By writing    myPointer    amp myVar    you say      myPointer   The address of where the data in myVar is stored    On the second example the error is not being triggered but doing a cout of the value of myPointer still returns a memory address    It returns an address that points to a memory location that has been deleted  Because first you create the pointer and assign its value to myPointer  second you delete it  third you print it  So unless you assign another value to myPointer  the deleted address will remain    Does  3 really work  Seems to work to me  the pointer is no longer storing an address  is this the proper way to delete a pointer    NULL equals 0  you delete 0  so you delete nothing  And it s logic that it prints 0 because you did   myPointer   NULL    which equals   myPointer   0

User · Answer

int value   ptr   value   8  ptr    amp value     ptr points to value  which lives on a stack frame     you are not responsible for managing its lifetime   ptr   new int  delete ptr     yes this is the normal way to manage the lifetime of    dynamically allocated memory  you new ed it  you delete it   ptr   nullptr  delete ptr     this is illogical  essentially you are saying delete nothing

User · Answer

I believe you re not fully understanding how pointers work  When you have a pointer pointing to some memory there are three different things you must understand  - there is  what is pointed  by the pointer  the memory  - this memory address - not all pointers need to have their memory deleted  you only need to delete memory that was dynamically allocated  used new operator    Imagine   int  ptr   new int      ptr has the address of the memory     at this point  the actual memory doesn t have anything   ptr   8     you re assigning the integer 8 into that memory  delete ptr     you are only deleting the memory     at this point the pointer still has the same memory address  as you could      notice from your 2nd test  but what inside that memory is gone    When you did  ptr   NULL     you didn t delete the memory    you re only saying that this pointer is now pointing to  nowhere      the memory that was pointed by this pointer is now lost    C   allows that you try to delete a pointer that points to null but it doesn t actually do anything  just doesn t give any error

User · Answer

You are trying to delete a variable allocated on the stack  You can not do this Deleting a pointer does not destruct a pointer actually  just the memory occupied is given back to the OS  You can access it untill the memory is used for another variable  or otherwise manipulated  So it is good practice to set a pointer to NULL  0  after deleting  Deleting a NULL pointer does not delete anything

User · Answer

1  amp  2  myVar   8    not dynamically allocated  Can t call delete on it  myPointer   new int    dynamically allocated  can call delete on it    The first variable was allocated on the stack  You can call delete only on memory you allocated dynamically  on the heap  using the new operator   3     myPointer   NULL    delete myPointer    The above did nothing at all  You didn t free anything  as the pointer pointed at NULL      The following shouldn t be done   myPointer   new int  myPointer   NULL    leaked memory  no pointer to above int delete myPointer    no point at all   You pointed it at NULL  leaving behind leaked memory  the new int you allocated   You should free the memory you were pointing at  There is no way to access that allocated new int anymore  hence memory leak     The correct way   myPointer   new int  delete myPointer    freed memory myPointer   NULL    pointed dangling ptr to NULL     The better way   If you re using C    do not use raw pointers  Use smart pointers instead which can handle these things for you with little overhead  C  11 comes with several

User · Answer

Pointers are similar to normal variables in that you don t need to delete them  They are removed from memory at the end of a functions execution and or the end of the program   You can however use pointers to allocate a  block  of memory  for example like this   int  some integers   new int 20000    This will allocate memory space for 20000 integers  Useful  because the Stack has a limited size and you might want to mess about with a big load of  ints  without a stack overflow error   Whenever you call new  you should then  delete  at the end of your program  because otherwise you will get a memory leak  and some allocated memory space will never be returned for other programs to use  To do this   delete    some integers    Hope that helps

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