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& val) {
     val          


        

Answer 1

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 = &objBaseType;
  

• The & 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 = &nVar;
     int nVar2 = *ptrVar;
  

Reference

• A reference (&) is like an alias to an existing variable.

• A reference (&) 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 = &i;    //pi points to the integer i
  int& ri = i;    //ri is refers to integer i – creation of reference and initialization
  

Answer 2

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.

Answer 3

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.

Answer 4

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<std::string>& optional_str)
{
    if (optional_str)
    {
       cout << *optional_str << std::endl;
    }
    else
    {
       cout << "(no string)" << std::endl;
    }
}

// Sample method using optional as return value
boost::optional<int> ReturnOptional(bool return_nothing)
{
    if (return_nothing)
    {
       return boost::optional<int>();
    }

    return boost::optional<int>(42);
}

Answer 5

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?"

Answer 6

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.