difference between pointers and references

I was reading up on the differences between pointers and references because even though they operate similarly, i.e. through indirection, they have very different mechanics and semantics.

I couldn't find a direct answer searching the forums because I think the keyword null in modern OOP languages (i.e. C#, Java) is implemented differently then the NULL macro in C++, C which is defined as 0.

So, I perused Stack Overflow and I found this post:

Difference between pointer variable and reference variable in C++ - Stack Overflow

And what caught my eye was point number 4 in the answer to the post given by Brian R. Bondy.

"4) Pointer can be assigned NULL directly, whereas reference cannot. If you try hard enough, and you know how, you can make the address of a reference NULL. Likewise, if you try hard enough you can have a reference to a pointer, and then that reference can contain NULL."

Code:

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int *p = NULL;
int &r = NULL; <--- compiling error

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Now this confused me a little because I was weaned on OOP languages such as Java and C# and from what I read in books, when an object is created using the new operator, memory is allocated for the object on the heap and a reference is returned and stored in the object variable.

If this is the case then how come the following code in C# is acceptable as well as Java?

Code:

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Square square = new Square();
square = null;

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If anyone can provide and explanation, it would ease my mind greatly because I'll be thinking about it all the time if I don't get closure.

And for further evidence, according to the MSDN C# reference:

The null keyword is a literal that represents a null reference, one that does not refer to any object.

Source: null (C# Reference)

Java and C# are not C++!
References MUST be bound to something (they cannot be NULL or rather nullptr, as it should be today). They cannot be reassigned either.
Java mixes pointers and references into one concept called a reference. Dunno about C#.
Pointers can be null, in which case it is a null pointer (nullptr_t).

Furthermore, new returns a pointer in C++, not a reference.

I know that, I was just wondering how it would be done in those languages since it's allowed in C# and Java to have a null reference, but in C++ it isn't.

What exactly do you mean by "how it would be done"?

I'm just wondering why references can't be assigned a null value in C++, where in modern OOP languages, you can assign a null reference to an object variable.

I mean it seems strange to make it possible in languages like C# or Java, but not possible (unless you try really hard) in C++.

I guess it's not really in scope here. I was just curious to see if someone could provide a conjecture.

I think this is a null reference, although the behaviour should be undefined according to the ISO definition.

Code:

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        int *pointer = NULL;
        int &null_ref = *pointer;
        cout<<&null_ref;

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A reference is just an alias of a variable.

Quote:

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Originally Posted by jshowa

I'm just wondering why references can't be assigned a null value in C++, where in modern OOP languages, you can assign a null reference to an object variable.

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If it is not permitted to have a NULL reference, then a huge class of potential problems simply evaporates.

Quote:

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I mean it seems strange to make it possible in languages like C# or Java, but not possible (unless you try really hard) in C++.

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If you want something that can refer to an object, or to nothing at all, then use a pointer. It's not like there's missing functionality in the language.

Quote:

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Originally Posted by jshowa

I'm just wondering why references can't be assigned a null value in C++, where in modern OOP languages, you can assign a null reference to an object variable.

I mean it seems strange to make it possible in languages like C# or Java, but not possible (unless you try really hard) in C++.

I guess it's not really in scope here. I was just curious to see if someone could provide a conjecture.

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Code:

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int *ptr = NULL, &ref = *ptr;

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do you want a reference to reference nothing? What is the point of having such a reference? Oo

If that is the case, a null pointer should be more than enough.

Also, are you saying that C++ is incomplete compared to your so called "modern languages"? I would never say such a thing while using "Java" as an argument, where you can't even overload an operator or use pointer arithmetics

Quote:

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Originally Posted by like_no_other

I think this is a null reference, although the behaviour should be undefined according to the ISO definition.

Code:

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        int *pointer = NULL;
        int &null_ref = *pointer;
        cout<<&null_ref;

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A reference is just an alias of a variable.

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In any platform I've used, that would blow up as soon as it evaluates *pointer, which is the same as *NULL.

The whole concept is pointless. Anything declared in source has a predefined area to hold it's value, whether that space be in the x segment or the stack. Using a different, non-predetermined area would break globals, the temporary nature of locals, and any form of referencing done through linking.

Despite the fact x feature isn't possible in C/C++, I'll bet that if you dig deep enough, you'll find C/C++ at the core of whatever language has x feature. Assuming it wasn't coded in asm, in that case, you should be expecting endless incompatibilities.

Quote:

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Originally Posted by User Name:

Despite the fact x feature isn't possible in C/C++

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In this case, the feature exists in both C and C++, except that we call them pointers*, and they have different syntax and are more powerful than the references of those other languages.

* that said, Java has this thing named NullPointerException, so they arguably recognise references as pointers without pointer syntax and pointer arithmetic.

Yeah, that always makes me laugh. Why would they have a NullPointerException if they're so against calling their crippled pointers 'pointers'? Shouldn't they call them NullReferenceExceptions? :D

Quote:

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Originally Posted by Dante Wingates

Also, are you saying that C++ is incomplete compared to your so called "modern languages"? I would never say such a thing while using "Java" as an argument, where you can't even overload an operator or use pointer arithmetics

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I never said I was comparing anything, I'm just confused on why a null reference is consider unacceptable in C++, but acceptable in C# and Java.

You can also overload operators in C#: C# Operator Overloading..|..C# Help – C# Tutorials and Resources

Quote:

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Originally Posted by laserlight

* that said, Java has this thing named NullPointerException, so they arguably recognize references as pointers without pointer syntax and pointer arithmetic.

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Does this mean that the whole concept of a reference doesn't really exist in Java? The only time I've got these is with objects that have no established reference. If this is the case, then are pointers really returned when the new keyword is used instead of a reference?

References in java are glorified pointers. The only difference is that the "->" syntax is replaced with ".".

Quote:

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Originally Posted by jshowa

Does this mean that the whole concept of a reference doesn't really exist in Java? The only time I've got these is with objects that have no established reference. If this is the case, then are pointers really returned when the new keyword is used instead of a reference?

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What is the "whole concept of a reference"? References in Java are indeed reference types in that a reference variable refers to some object. Yet, they are different and in a way more limited than references in C++, yet in a way more powerful than references in C++. They behave like pointers, yet they lack pointer arithmetic and do not need to be dereferenced like a pointer. Does the new keyword in Java return a pointer? Conceptually, I'd say no, yet in a way it does, insofar as references in Java are pointers without pointer syntax.

Quote:

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Originally Posted by laserlight

What is the "whole concept of a reference"? References in Java are indeed reference types in that a reference variable refers to some object. Yet, they are different and in a way more limited than references in C++, yet in a way more powerful than references in C++. They behave like pointers, yet they lack pointer arithmetic and do not need to be dereferenced like a pointer. Does the new keyword in Java return a pointer? Conceptually, I'd say no, yet in a way it does, insofar as references in Java are pointers without pointer syntax.

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Great, now I have a headache from all that bouncing around! :p

The basic concept of a reference in C++ is that it is an alternate name for an object. It is therefore not possible to have a null reference, as having an alternate name for something non-existant is meaningless. All the names refer to nothing.

One consequence of this concept, which allows plenty of optimisations by the compiler, is that a reference is assumed to refer to something that exists, IF it is used. For example, the compiler is not required to add runtime checks to ensure a reference is valid - it is assumed to be valid, so doesn't need to be checked. That is the case because any act that creates an invalid reference (eg a null reference, a dangling reference) is specified to have undefined behaviour. If the act of creating something is undefined, the result of using it is also undefined.

A pointer, on the other hand, is something that optionally contains the address of an object. This means a pointer may be NULL, indicating it points at no object. Dereferencing such a pointer is, in itself, undefined behaviour (and, since creating a null reference requires dereferencing a null pointer, that explains why creating a null reference gives undefined behaviour - let alone using that null reference). The difference is that the programmer can check if a pointer is null before dereferencing it, and hence avoid that form of undefined behaviour.

Because of this, a C++ reference is sometimes described as a glorified pointer. It is not. At most, it is a deliberately constrained pointer. The constraints can't eliminate programmer error (there is no upper bound on programmer stupidity, and there is always opportunity for a programmer to deliberately cause error) but they can help reduce programmer error.

The concepts of a reference in other languages are a bit different.

As laserlight noted, Java's concept of a reference has some attributes that, in C++, are only associated with pointers. In effect, it might be said, that the Java designers sought to publically eliminate pointers as an "unsafe construct" but then found they had to add to quietly implement some of those unsafe features into their reference type. A Java reference therefore allows more dangerous practices (i.e. opportunities for programmer error) than does a C++ reference. That's not an indictment of Java - it's simply a realistic result in programming language design when a marketing imperative ("we are creating a safer C++!!!!") intersects with pragmatics ("if something is too safe it can be unusable").

Had to read your post a couple times in order to have it sink in grumpy.

It's a very interesting conundrum to say the least, that languages who are so adamant about eliminating pointers, have to end up implementing them in the first place in order for the language to even work.

I guess I'll never understand marketing...