Passing references to pointers in C

Question

As far as I can tell  there s no reason I shouldn t be allowed to pass a reference to a pointer in C    However  my attempts to do so are failing  and I have no idea why    This is what I m doing   void myfunc string  amp  val           Do stuff to the string pointer       sometime later                   string s      myfunc  amp s                  And I m getting this error      cannot convert parameter 1 from  std  string    to  std  string   amp

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Change it to     std  string s    std  string  pS    amp s    myfunc pS     EDIT   This is called ref-to-pointer and you cannot pass temporary address as a reference to function    unless it is const reference    Though  I have shown std  string  pS    amp s   pointer to a local variable   its typical usage would be   when you want the callee to change the pointer itself  not the object to which it points  For example  a function that allocates memory and assigns the address of the memory block it allocated to its argument must take a reference to a pointer  or a pointer to pointer   void myfunc string  amp  val      val is valid even after function call    val   new std  string  Test

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I know that it s posible to pass references of pointers  I did it last week  but I can t remember what the syntax was  as your code looks correct to my brain right now  However another option is to use pointers of pointers   Myfunc String   s

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I have just made use of a reference to a pointer to make all the pointers in a deleted binary tree except the root safe  To make the pointer safe we just have to set it to 0  I could not make the function that deletes the tree  keeping only the root  to accept a ref to a pointer since I am using the root  this pointer  as the first input to traverse left and right   void BinTree  safe tree BinTree    amp vertex         if   vertex  0         base case         safe tree vertex- gt left         left subtree              safe tree vertex- gt right         right subtree               delete vertex      using this delete causes an error  since they were deleted on the fly using inorder LVR  If inorder LVR does not perform delete to the nodes  then  use delete vertex          vertex 0      making a safe pointer            end in   Bottom line  a reference to a pointer is invalid when the formal parameter is the  this  pointer

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myfunc  string  amp  val   this itself doesn t make any sense   string  amp  val  implies  string val    and  amp  cancels each other  Finally one can not pas string variable to a function  string val    Only basic data types can be passed to a function  for other data types need to pass as pointer or reference   You can have either string amp  val or string  val to a function

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Your function expects a reference to an actual string pointer in the calling scope  not an anonymous string pointer  Thus   string s  string   s    amp s  myfunc  s     should compile just fine   However  this is only useful if you intend to modify the pointer you pass to the function  If you intend to modify the string itself you should use a reference to the string as Sake suggested  With that in mind it should be more obvious why the compiler complains about you original code  In your code the pointer is created  on the fly   modifying that pointer would have no consequence and that is not what is intended  The idea of a reference  vs  a pointer  is that a reference always points to an actual object

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Try   void myfunc string amp  val           Do stuff to the string pointer       sometime later                   string s      myfunc s                  or  void myfunc string  val           Do stuff to the string pointer       sometime later                   string s      myfunc  amp s

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The problem is that you re trying to bind a temporary to the reference  which C   doesn t allow unless the reference is const   So you can do one of either the following   void myfunc string  amp  val           Do stuff to the string pointer     void myfunc2 string  const amp  val           Do stuff to the string pointer    int main      sometime later                   string s      string  ps    amp s       myfunc  ps        OK because ps is not a temporary     myfunc2   amp s       OK because the parameter is a const amp                  return 0

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amp s produces temporary pointer to string and you can t make reference to temporary object

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Welcome to C  11 and rvalue references    include  lt cassert gt   include  lt string gt   using std  string   void myfunc string  amp  amp  val        assert  amp val       assert val       assert val- gt c str            Do stuff to the string pointer       sometime later  int main                     string s      myfunc  amp s                  Now you have access to the value of the pointer  referred to by val   which is the address of the string   You can modify the pointer  and no one will care  That is one aspect of what an rvalue is in the first place   Be careful  The value of the pointer is only valid until myfunc   returns  At last  its a temporary

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EDIT   I experimented some  and discovered thing are a bit subtler than I thought   Here s what I now think is an accurate answer    amp s is not an lvalue so you cannot create a reference to it unless the type of the reference is reference to const   So for example  you cannot do  string    amp r    amp s    but you can do  string   const  amp r    amp s    If you put a similar declaration in the function header  it will work   void myfunc string   const  amp a            There is another issue  namely  temporaries   The rule is that you can get a reference to a temporary only if it is const   So in this case one might argue that  amp s is a temporary  and so must be declared const in the function prototype   From a practical point of view it makes no difference in this case    It s either an rvalue or a temporary   Either way  the same rule applies    However  strictly speaking  I think it is not a temporary but an rvalue   I wonder if there is a way to distinguish between the two    Perhaps it is simply defined that all temporaries are rvalues  and all non-lvalues are temporaries   I m not an expert on the standard    That being said  your problem is probably at a higher level   Why do you want a reference to the address of s   If you want a reference to a pointer to s  you need to define a pointer as in  string  p    amp s  myfunc p     If you want a reference to s or a pointer to s  do the straightforward thing

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