C++: pointers Vs references

Web Developer

Hi,

Can someone provide a short and concise statement(s) on the difference
between pointers and references. A graphical representation (via links?) of
both would be much appreciated as well.
WD

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Jul 19 '05 #1

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14041

David White

Web Developer <no****@hotmail .com> wrote in message
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Hi,

Can someone provide a short and concise statement(s) on the difference
between pointers and references.
A pointer is an object containing the address in memory of another object; a
reference is an alias for another object.
A pointer can be null; a reference cannot.
A pointer can be re-seated (i.e., pointed at a different object); a
reference cannot.
A reference must be initialized (i.e., its refer-ee specified) when it is
created; a pointer can be uninitialized.
A graphical representation (via links?) of
both would be much appreciated as well.

I'm not sure what one would look like. Anyway, I am unable to provide one.

DW

Jul 19 '05 #2

John Harrison

"David White" <no@email.provi ded> wrote in message
news:ta******** ********@nasal. pacific.net.au. ..

Web Developer <no****@hotmail .com> wrote in message
news:3f******** **@news.iprimus .com.au...
Hi,

Can someone provide a short and concise statement(s) on the difference
between pointers and references.
A pointer is an object containing the address in memory of another object;

a reference is an alias for another object.
A pointer can be null; a reference cannot.
A pointer can be re-seated (i.e., pointed at a different object); a
reference cannot.
A reference must be initialized (i.e., its refer-ee specified) when it is
created; a pointer can be uninitialized.

Also a pointer is syntactically more convenient, how would you write an
assignment operator or copy constructor without using references?

john

Jul 19 '05 #3

John Harrison

Also a pointer is syntactically more convenient, how would you write an
assignment operator or copy constructor without using references?

I mean a REFERENCE is syntactically more convenient, of course.

john

Jul 19 '05 #4

Josephine Schafer

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:bh******** ****@ID-196037.news.uni-berlin.de...

"David White" <no@email.provi ded> wrote in message
news:ta******** ********@nasal. pacific.net.au. ..
Web Developer <no****@hotmail .com> wrote in message
news:3f******** **@news.iprimus .com.au...
Hi,

Can someone provide a short and concise statement(s) on the difference
between pointers and references.
A pointer is an object containing the address in memory of another object; a
reference is an alias for another object.
A pointer can be null; a reference cannot.
A pointer can be re-seated (i.e., pointed at a different object); a
reference cannot.
A reference must be initialized (i.e., its refer-ee specified) when it

is created; a pointer can be uninitialized.

Also a pointer is syntactically more convenient, how would you write an
assignment operator or copy constructor without using references?

Also there is no such thing as a null reference. This could imply that it is
more efficient to use references because you need
not test the validity of a reference before using it.
Pointers, on the other hand, should generally be tested against null.

Jul 19 '05 #5

David White

"Josephine Schafer" <js*@usa.net> wrote in message
news:bh******** ****@ID-192448.news.uni-berlin.de...

"David White" <no@email.provi ded> wrote in message
news:ta******** ********@nasal. pacific.net.au. ..
A pointer can be null; a reference cannot.
Also there is no such thing as a null reference.

You must have missed it :)

DW

Jul 19 '05 #6

Web Developer

> > Hi,
> >
> > Can someone provide a short and concise statement(s) on the difference > > between pointers and references.
>
> A pointer is an object containing the address in memory of another object;
a
> reference is an alias for another object.
> A pointer can be null; a reference cannot.
> A pointer can be re-seated (i.e., pointed at a different object); a
> reference cannot.
> A reference must be initialized (i.e., its refer-ee specified) when it > is created; a pointer can be uninitialized.

Wait a minute, in Java, an object reference also hold the memory address

of an object. Can you understand my confusion? I think my background Java may
curse me in C++.

I think I got it now. A reference gets you the value it references, but for
object references it does not give you the actual value but the memory
address of where the value lies, and in this latter case, it is EXACTLY the
same as a pointer.

Summary: object references in Java are pointers - they are just not
explicitly defined.

Note: I realise pointers don't behave like object references in Java.
Any comments appreciated.
Regards
WD

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Jul 19 '05 #7

Peter van Merkerk

> I think I got it now. A reference gets you the value it references,
but for

object references it does not give you the actual value but the memory
address of where the value lies, and in this latter case, it is EXACTLY the same as a pointer.

Summary: object references in Java are pointers - they are just not
explicitly defined.

Java references are closer to C++ pointers than to C++ references. A
difference between Java references and C++ pointers is that you can
perform arithmetric operations on C++ pointers e.g.:

int* a = new int[10];
int* b = a + 1;
int* c = b - 1;

*b = 2; // Modifies a[1]
*c = 1; // Modifies a[0]

However typically you want to avoid pointer arithmetric where ever you
can. If the pointer goes outside allocated memory sooner or later (if
you are unlucky) nasty things will happen.

It looks like you are trying to learn C++ in terms of Java. Even though
the syntax of those languages look deceptively similar, they are really
different languages with different semantics and idioms. It is better to
start C++ with a clear and open mind. C++ often requires a different
approach; and trying to do things the Java way usually doesn't work out
too well in C++.

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

Jul 19 '05 #8

ghl

"Web Developer" <no****@hotmail .com> wrote in message
news:3f******** @news.iprimus.c om.au...

> > Hi,
> >
> > Can someone provide a short and concise statement(s) on the difference > > between pointers and references.
>
> A pointer is an object containing the address in memory of another object;
a
> reference is an alias for another object.
> A pointer can be null; a reference cannot.
> A pointer can be re-seated (i.e., pointed at a different object); a
> reference cannot.
> A reference must be initialized (i.e., its refer-ee specified) when it > is created; a pointer can be uninitialized.

Wait a minute, in Java, an object reference also hold the memory address

of an object. Can you understand my confusion? I think my background Java may
curse me in C++.

It isn't clear what is meant by "the memory address of an object." Since an
object is a conceptual fiction and consists in reality of various control
structures, arrays, member data, etc. it is unclear as to just what exactly
a reference variable points to. Probably most implementation use it to
address the start of the structure containing all the object information,
but I don't know that structure myself.

I also believe that a reference in C++ is not necessarily an address of the
data object, but could be implemented in some other fashion (say, a handle
that refers to an address of the data object). The only requirement is that
the identifier used for the reference can be used interchangeably for the
declaration name of the object being referenced. Probably most
implementations simply use the address of the data object. The restrictions
on the C++ reference prohibit using it like a pointer, which does contain
the address of the data object pointed to.

Like a pointer, the Java reference variable contains an address, but the
language syntax checks the type of the primitive or object being stored into
it for compatibility. So, a Java reference cannot be used as a pointer; same
as in C++. In Java a reference variable can be set to null.

Since I am here to learn C++: can a reference in C++ be set to null? I
should think not, since it only stands for one object and cannot be changed.

Yes, these languages are different. It's a pity (in a way) that they use the
terms "reference" and "object" so differently.
--
Gary

Jul 19 '05 #9

Peter van Merkerk

> Since I am here to learn C++: can a reference in C++ be set to null? I

should think not, since it only stands for one object and cannot be changed.

You think right; C++ references cannot be 0, if were posible it would be
useless since C++ references cannot be reseated.
Yes, these languages are different. It's a pity (in a way) that they use the terms "reference" and "object" so differently.

I think it is a pitty Java plagiarises the C++ syntax while at the same time
having different semantics. This leads to unnecessary confusion for people
moving from C++ to Java and visa versa.

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

Jul 19 '05 #10

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