Reason to Pass a Pointer by Reference in C

Question

Under which circumstances would you want to use code of this nature in c     void foo type   amp in         void fii       type  choochoo          foo choochoo

User · Answer

Another situation when you may need this is if you have stl collection of pointers and want to change them using stl algorithm. Example of for_each in c++98.

struct Storage {
  typedef std::list<Object*> ObjectList;
  ObjectList objects;

  void change() {
    typedef void (*ChangeFunctionType)(Object*&);
    std::for_each<ObjectList::iterator, ChangeFunctionType>
                 (objects.begin(), objects.end(), &Storage::changeObject);
  }

  static void changeObject(Object*& item) {
    delete item;
    item = 0;
    if (someCondition) item = new Object();
  } 

};

Otherwise, if you use changeObject(Object* item) signature you have copy of pointer, not original one.

User · Answer

You would want to pass a pointer by reference if you have a need to modify the pointer rather than the object that the pointer is pointing to   This is similar to why double pointers are used  using a reference to a pointer is slightly safer than using pointers

User · Answer

I have had to use code like this to provide functions to allocate memory to a pointer passed in and return its size because my company  object  to me using the STL   int iSizeOfArray int   amp piArray        piArray   new int iNumberOfElements               return iNumberOfElements       It is not nice  but the pointer must be passed by reference  or use double pointer   If not  memory is allocated to a local copy of the pointer if it is passed by value which results in a memory leak

User · Answer

One example is when you write a parser function and pass it a source pointer to read from  if the function is supposed to push that pointer forward behind the last character which has been correctly recognized by the parser  Using a reference to a pointer makes it clear then that the function will move the original pointer to update its position   In general  you use references to pointers if you want to pass a pointer to a function and let it move that original pointer to some other position instead of just moving a copy of it without affecting the original

User · Answer

50  of C   programmers like to set their pointers to null after a delete   template lt typename T gt      void moronic delete T  amp  p        delete p      p   nullptr      Without the reference  you would only be changing a local copy of the pointer  not affecting the caller

User · Answer

David s answer is correct  but if it s still a little abstract  here are two examples    You might want to zero all freed pointers to catch memory problems earlier  C-style you d do   void freeAndZero void   ptr        free  ptr        ptr   0     void  ptr   malloc             freeAndZero  amp ptr     In C   to do the same  you might do   template lt class T gt  void freeAndZero T   amp ptr        delete ptr      ptr   0     int  ptr   new int        freeAndZero ptr    When dealing with linked-lists - often simply represented as pointers to a next node    struct Node       value t value      Node  next       In this case  when you insert to the empty list you necessarily must change the incoming pointer because the result is not the NULL pointer anymore  This is a case where you modify an external pointer from a function  so it would have a reference to pointer in its signature   void insert Node   amp list                if  list  list   new Node                    There s an example in this question

[c++] Reason to Pass a Pointer by Reference in C++?

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