Why don't I need to check if references are invalid/null?

Question

Reading http://www.cprogramming.com/tutorial/references.html, it says:

In general, references should always be valid because you must always initi

Answer 1

I think you could benefit from a simple parallelism:

• T & is similar to T * const
• T const & is similar to T const * const

References are very similar to const in their intent, they carry a meaning and thus help write clearer code, but don't provide different runtime behavior.

Now to answer your question: yes it is possible that a reference be null or invalid. You can test for a null reference (T& t = ; if (&t == 0)) but it should not happen >> by contract a reference is valid.

When to use reference vs pointer ? Use a pointer if:

• you wish to be able to change the pointee
• you wish to express the possible nullity

In any other case, use a reference.

Some examples:

// Returns an object corresponding to the criteria
// or a special value if it cannot be found
Object* find(...); // returns 0 if fails

// Returns an object corresponding to the criteria
// or throw "NotFound" if it cannot be found
Object& find(...); // throw NotFound

Passing arguments:

void doSomething(Object* obj)
{
  if (obj) obj->doSomething();
}

void doSomething(Object& obj) { obj.do(); obj.something(); }

Attributes:

struct Foo
{
  int i;
  Bar* b; // No constructor, we need to initialize later on
};

class Foo
{
public:
  Foo(int i, Bar& b): i(i), b(b) {}
private:
  int i;
  Bar& b; // will always point to the same object, Foo not Default Constructible
};

class Other
{
public:
  Other(Bar& b): b(&b) {} // NEED to pass a valid object for init

  void swap(Other& rhs);  // NEED a pointer to be able to exchange

private:
  Bar* b;
};

Functionally references and pointers play the very same role. It's just a matter of contract. And unfortunately, both can invoke Undefined Behavior if you delete the object they refer to, there's no winner there ;)