When Pass by reference becomes mandatory over pass by address/pointer

Question

Can someone please give some examples where pass by reference is preferred over pass by pointer.

Basically both offer the same functionality but I want to know where pass by reference becomes mandatory.

Also, please suggest how to decide to use either of the two.

Please suggest with some basic examples.

Possible duplicate: http://stackoverflow.com/questions/2550377/when-pass-by-pointer-is-preferred-to-pass-by-reference-in-c — Konrad, Apr 10 '12 at 12:01
Bjarne has answered the direct reason for adding references to C++ in his style guide. http://www2.research.att.com/~bs/bs_faq2.html#pointers-and-references — chris, Apr 10 '12 at 12:06

iammilind · Accepted Answer · 2012-04-10T12:03:45.090

3

Copy constructor and Assignment operator.

struct A {
  ...
  A(const A& copy);
  A& operator = (const A& copy);
};

These are mandatory situations. Apart from these there can be other situations also (e.g. overloading other operators like +, <, > and so on).

Also, please suggest how to decide to use either of the two.

I prefer references (more C++ style) at most of the places and if a reference can't be used, I tend to use pointers (more C style).
Reference are somewhat analogical representation of a const pointer; i.e. A& is similar as A* const. Thus they provide some in-built safety against overwriting the location accidently.

edited Apr 10 '12 at 12:03

answered Apr 10 '12 at 11:58

iammilind

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Actually Pass by reference is not mandatory for the assignment-operator (from the standard it seems return-by-reference isn't either). One could simply pass by value which might be advantageous for copy eliasion when using a copy-and-swap idiom. – Grizzly Apr 10 '12 at 12:07

score 1 · Answer 2 · answered Apr 10 '12 at 11:58

1

My simple rule of thumb would be always to prefer pass by reference where it was possible. If you need the ability to pass a null value, then you have to use pass by pointer. Otherwise use pass by reference.

answered Apr 10 '12 at 11:58

David Heffernan

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score 1 · Answer 3 · answered Apr 10 '12 at 11:59

1

When overloading operators it is mandatory to pass references. For example :

class A {
    A& operator=(const A &other);
}

answered Apr 10 '12 at 11:59

giorashc

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score 1 · Answer 4 · answered Apr 10 '12 at 12:10

Copy constructors must take their argument by reference.

Operator overloads must take their arguments by value or reference, not by pointer.

For normal functions, the choice is yours; although you must use a pointer if you want to accept null arguments or dynamic arrays.

Some people prefer to use references whenever possible; there is no need to check whether it's null, and no scope for confusion about whether it refers to a single object or an array, or whether the function might delete the object. Others prefer to use pointers, as it gives a visible indication at the call site that the function might change the object.

When Pass by reference becomes mandatory over pass by address/pointer

4 Answers4