Meaning of and in C

Question

I found these symbols in a function declaration several times  but I don t know what they mean   Example   void raccogli dati double    amp  V  double   p  int N       int ultimo   3     V   new double    N 2      for int i 0  i  lt  N 2  i           V i    new double N 2   std    clog  lt  lt   digita    lt  lt  N 2 - i                   lt  lt    valori per la parte superiore della matrice V          for int j i  j  lt  N 2  j           std    cin  gt  gt  V i  j   p ultimo    0     V i  j     sqrt p i  0  p j  0            for int i 1  i  lt  N 2  i         for int j 0  j  lt  i  j            V i  j    V j  i

User · Answer

That s passing a pointer by reference rather than by value  This for example allows altering the pointer  not the pointed-to object  in the function is such way that the calling code sees the change   Compare   void nochange  int  pointer     passed by value      pointer       change will be discarded once function returns    void change  int  amp  pointer     passed by reference      pointer       change will persist when function returns

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First is a reference to a pointer  second is a reference to a pointer to a pointer  See also FAQ on how pointers and references differ   void foo int  amp  x  int   amp  y           modifying x or y here will modify a or b in main    int main         int val   42      int  a     amp val      int   b    amp a       foo a  b       return 0

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amp  signifies the receiving the pointer by reference  It means it is an alias for the passing parameter  So  it affects the passing parameter    include  lt iostream gt  using namespace std   void foo int  ptr        ptr   new int 50         Modifying the pointer to point to a different location     cout  lt  lt   In foo  t   lt  lt   ptr  lt  lt    n        delete ptr      void bar int   amp  ptr        ptr   new int 80         Modifying the pointer to point to a different location     cout  lt  lt   In bar  t   lt  lt   ptr  lt  lt    n           Deleting the pointer will result the actual passed parameter dangling   int main         int temp   100       int  p    amp temp        cout  lt  lt   Before foo  t   lt  lt   p  lt  lt    n       foo p        cout  lt  lt   After foo  t   lt  lt   p  lt  lt    n        cout  lt  lt   Before bar  t   lt  lt   p  lt  lt    n       bar p        cout  lt  lt   After bar  t   lt  lt   p  lt  lt    n        delete p       return 0      Output   Before foo  100 In foo  50 After foo   100 Before bar  100 In bar  80 After bar   80

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Typically  you can read the declaration of the variable from right to left  Therefore in the case of int  ptr    it means that you have a Pointer   to an Integer variable int  Also when it s declared int   ptr2   it is a Pointer variable   to a Pointer variable   pointing to an Integer variable int   which is the same as   int     ptr2     Now  following the syntax by declaring int  amp  rPtr   we say it s a Reference  amp  to a Pointer   that points to a variable of type int  Finally  you can apply again this approach also for int   amp  rPtr2  concluding that it signifies a Reference  amp  to a Pointer   to a Pointer   to an Integer int

User · Answer

That is taking the parameter by reference  So in the first case you are taking a pointer parameter by reference so whatever modification you do to the value of the pointer is reflected outside the function  Second is the simlilar to first one with the only difference being that it is a double pointer  See this example   void pass by value int  p          Allocate memory for int and store the address in p     p   new int     void pass by reference int  amp  p        p   new int     int main         int  p1   NULL      int  p2   NULL       pass by value p1     p1 will still be NULL after this call     pass by reference p2     p2  s value is changed to point to the newly allocate memory      return 0

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This   amp  in theory as well as in practical its possible and called as reference to pointer variable  and it s act like same  This   amp  combination is used in as function parameter for  pass by  type defining  unlike    can also be used for declaring a double pointer variable  The passing of parameter is divided into pass by value  pass by reference  pass by pointer  there are various answer about  pass by  types available  however the basic we require to understand for this topic is   pass by reference --  generally operates on already created variable refereed while passing to function e g fun int  amp a    pass by pointer --  Operates on already initialized  pointer variable variable address  passing to function e g fun int  a    auto addControl      SomeLabel   label  SomeControl   control         label   new SomeLabel       control   new SomeControl         few more operation further      addControl  amp m label1  amp m control1   addControl  amp m label2  amp m control2   addControl  amp m label3  amp m control3     in the above example this is the real life problem i came across  i am trying to init few pointer variable from the lambda function and for that we need to pass it by double pointer  so that comes with d-referencing of pointer for its all usage inside of that lambda   while passing pointer in function which takes double pointer  you need to pass reference to the pointer variable   so with this same thing reference to the pointer variable    amp   this combination helps  in below given way for the same example i have mentioned above   auto addControl      SomeLabel  amp  label  SomeControl  amp  control            label   new SomeLabel          control   new SomeControl             few more operation further          addControl m label1 m control1   addControl m label2 m control2   addControl m label3 m control3     so here you can see that you neither require d-referencing nor we require to pass reference to pointer variable while passing in function  as current pass by type is already reference to pointer   Hope this helps  -

User · Answer

To understand those phrases let s look at the couple of things   typedef double Foo  void fooFunc Foo  amp  bar           So that s passing a double by reference   typedef double  Foo  void fooFunc Foo  amp  bar           now it s passing a pointer to a double by reference   typedef double   Foo  void fooFunc Foo  amp  bar           Finally  it s passing a pointer to a pointer to a double by reference   If you think in terms of typedefs like this you ll understand the proper ordering of the  amp  and   plus what it means

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An int  is a pointer to an int  so int  amp  must be a reference to a pointer to an int  Similarly  int   is a pointer to a pointer to an int  so int   amp  must be a reference to a pointer to a pointer to an int

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