Pointer vs Reference

Question

What would be better practice when giving a function the original variable to work with   unsigned long x   4   void func1 unsigned long amp  val         val   5                func1 x     or   void func2 unsigned long  val          val   5    func2  amp x     IOW  Is there any reason to pick one over another

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A reference is similar to a pointer  except that you don   t need to use a prefix   to access the value referred to by the reference  Also  a reference cannot be made to refer to a different object after its initialization   References are particularly useful for specifying function arguments   for more information see  A Tour of C    by  Bjarne Stroustrup   2014  Pages 11-12

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Pointers   A pointer is a variable that holds a memory address   A pointer declaration consists of a base type  an    and the variable name  A pointer can point to any number of variables in lifetime A pointer that does not currently point to a valid memory location is given the value null  Which is zero   BaseType  ptrBaseType  BaseType objBaseType  ptrBaseType    amp objBaseType   The  amp  is a unary operator that returns the memory address of its operand  Dereferencing operator     is used to access the value stored in the variable which pointer points to      int nVar   7     int  ptrVar    amp nVar     int nVar2    ptrVar     Reference   A reference   amp   is like an alias to an existing variable   A reference   amp   is like a constant pointer that is automatically dereferenced  It is usually used for function argument lists and function return values  A reference must be initialized when it is created  Once a reference is initialized to an object  it cannot be changed to refer to another object  You cannot have NULL references  A const reference can refer to a const int  It is done with a temporary variable with value of the const  int i   3       integer declaration int   pi    amp i       pi points to the integer i int amp  ri   i       ri is refers to integer i     creation of reference and initialization

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This ultimately ends up being subjective  The discussion thus far is useful  but I don t think there is a correct or decisive answer to this  A lot will depend on style guidelines and your needs at the time   While there are some different capabilities  whether or not something can be NULL  with a pointer  the largest practical difference for an output parameter is purely syntax  Google s C   Style Guide  https   google github io styleguide cppguide html Reference Arguments   for example  mandates only pointers for output parameters  and allows only references that are const  The reasoning is one of readability  something with value syntax should not have pointer semantic meaning  I m not suggesting that this is necessarily right or wrong  but I think the point here is that it s a matter of style  not of correctness

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I really think you will benefit from establishing the following function calling coding guidelines    As in all other places  always be const-correct    Note  This means  among other things  that only out-values  see item 3  and values passed by value  see item 4  can lack the const specifier   Only pass a value by pointer if the value 0 NULL is a valid input in the current context    Rationale 1  As a caller  you see that whatever you pass in must be in a usable state  Rationale 2  As called  you know that whatever comes in is in a usable state  Hence  no NULL-check or error handling needs to be done for that value  Rationale 3  Rationales 1 and 2 will be compiler enforced  Always catch errors at compile time if you can   If a function argument is an out-value  then pass it by reference    Rationale  We don t want to break item 2     Choose  pass by value  over  pass by const reference  only if the value is a POD  Plain old Datastructure  or small enough  memory-wise  or in other ways cheap enough  time-wise  to copy    Rationale  Avoid unnecessary copies  Note  small enough and cheap enough are not absolute measurables

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Use a reference when you can  use a pointer when you have to  From C   FAQ   When should I use references  and when should I use pointers

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A reference is an implicit pointer  Basically you can change the value the reference points to but you can t change the reference to point to something else  So my  2 cents is that if you only want to change the value of a parameter pass it as a reference but if you need to change the parameter to point to a different object pass it using a pointer

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Pointers    Can be assigned nullptr  or NULL   At the call site  you must use  amp  if your type is not a pointer itself  making explicitly you are modifying your object  Pointers can be rebound    References    Cannot be null  Once bound  cannot change  Callers don t need to explicitly use  amp   This is considered sometimes bad because you must go to the implementation of the function to see if your parameter is modified

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Pass by const reference unless there is a reason you wish to change keep the contents you are passing in   This will be the most efficient method in most cases   Make sure you use const on each parameter you do not wish to change  as this not only protects you from doing something stupid in the function  it gives a good indication to other users what the function does to the passed in values  This includes making a pointer const when you only want to change whats pointed to

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Consider C  s out keyword  The compiler requires the caller of a method to apply the out keyword to any out args  even though it knows already if they are  This is intended to enhance readability  Although with modern IDEs I m inclined to think that this is a job for syntax  or semantic  highlighting

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If you have a parameter where you may need to indicate the absence of a value  it s common practice to make the parameter a pointer value and pass in NULL   A better solution in most cases  from a safety perspective  is to use boost  optional  This allows you to pass in optional values by reference and also as a return value      Sample method using optional as input parameter void PrintOptional const boost  optional lt std  string gt  amp  optional str        if  optional str               cout  lt  lt   optional str  lt  lt  std  endl            else              cout  lt  lt    no string    lt  lt  std  endl              Sample method using optional as return value boost  optional lt int gt  ReturnOptional bool return nothing        if  return nothing               return boost  optional lt int gt                return boost  optional lt int gt  42

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My rule of thumb is   Use pointers if you want to do pointer arithmetic with them  e g  incrementing the pointer address to step through an array  or if you ever have to pass a NULL-pointer   Use references otherwise

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If you have a parameter where you may need to indicate the absence of a value  it s common practice to make the parameter a pointer value and pass in NULL   A better solution in most cases  from a safety perspective  is to use boost  optional  This allows you to pass in optional values by reference and also as a return value      Sample method using optional as input parameter void PrintOptional const boost  optional lt std  string gt  amp  optional str        if  optional str               cout  lt  lt   optional str  lt  lt  std  endl            else              cout  lt  lt    no string    lt  lt  std  endl              Sample method using optional as return value boost  optional lt int gt  ReturnOptional bool return nothing        if  return nothing               return boost  optional lt int gt                return boost  optional lt int gt  42

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You should pass a pointer if you are going to modify the value of the variable  Even though technically passing a reference or a pointer are the same  passing a pointer in your use case is more readable as it  advertises  the fact that the value will be changed by the function

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Consider C  s out keyword  The compiler requires the caller of a method to apply the out keyword to any out args  even though it knows already if they are  This is intended to enhance readability  Although with modern IDEs I m inclined to think that this is a job for syntax  or semantic  highlighting

[c++] Pointer vs. Reference

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