C++ functions: ampersand vs asterisk

Question

Let\'s say you have a function that modifies a variable.

Should you write it like this: void myfunc(int *a) or like this void myfunc(int &a)

Answer 1

One difference, like has been mentioned before, is you can't pass a null reference, but you can pass a null pointer.

Another thing, also mentioned already, when you call f(a,b) there could be confusion if the caller doesn't know that f could potentially change the value for b

However, yet another issue, which is rather subtle, but I still ran into it, is the semantics of references.

Pointers are passed by value, but references are not.

Which means, if you pass a parameter by pointer, you can change the pointer and make it point to something else.

Consider this:

void f1_ptr( type * a )
{
    a = new type(); //no change to passed parameters, you're changing the pointer which was passed by value
}

void f2_ptr( type * a )
{
    *a = some_other_value; //now you're changing the value of the parameter that was passed

   //or, if type is a class or struct:

   a->some_method_that_modifies_object(); //again, changing the parameter that was passed
}

But, when passing by reference, you can't change the reference to refer to another value. Once the reference is set, it can't be changed.

void f3_ref( type& a )
{
    a = type(); //the referred variable has also changed
}

//....

type obj = type( params );

f3_ref( obj ); //obj now changed

f1_ptr( &obj ); //obj doesn't change

f2_ptr( &obj ); //obj now changed