References are not addresses

發行項
02/17/2009

[NOTE: Based on some insightful comments I have updated this article to describe more clearly the relationships between references, pointers and addresses. Thanks to those who commented.]

I review a fair number of C# books; in all of them of course the author attempts to explain the difference between reference types and value types. Unfortunately, most of them do so by saying something like "a variable of reference type stores the address of the object". I always object to this. The last time this happened the author asked me for a more detailed explanation of why I always object, which I shall share with you now:

We have the abstract concept of "a reference". If I were to write about "Beethoven's Ninth Symphony", those two-dozen characters are not a 90-minute long symphonic masterwork with a large choral section. They're a reference to that thing, not the thing itself. And this reference itself contains references -- the word "Beethoven" is not a long-dead famously deaf Romantic Period composer, but it is a reference to one.

Similarly in programming languages we have the concept of "a reference" distinct from "the referent".

The inventor of the C programming language, oddly enough, chose to not have the concept of references at all. Rather, Ritchie chose to have "pointers" be first-class entities in the language. A pointer in C is like a reference in that it refers to some data by tracking its location, but there are more smarts in a pointer; you can perform arithmetic on a pointer as if it were a number, you can take the difference between two pointers that are both in the interior of the same array and get a sensible result, and so on.

Pointers are strictly "more powerful" than references; anything you can do with references you can do with pointers, but not vice versa. I imagine that's why there are no references in C -- it's a deliberately austere and powerful language.

The down side of pointers-instead-of-references is that pointers are hard for many novices to understand, and make it very very very easy to shoot yourself in the foot.

Pointers are typically implemented as addresses. An address is a number which is an offset into the "array of bytes" that is the entire virtual address space of the process (or, sometimes, an offset into some well-known portion of that address space -- I'm thinking of "near" vs. "far" pointers in win16 programming. But for the purposes of this article let's assume that an address is a byte offset into the whole address space.) Since addresses are just numbers you can easily perform pointer arithmetic with them.

Now consider C#, a language which has both references and pointers. There are some things you can only do with pointers, and we want to have a language that allows you to do those things (under carefully controlled conditions that call out that you are doing something that possibly breaks type safety, hence "unsafe".) But we also do not want to force anyone to have to understand pointers in order to do programming with references.

We also want to avoid some of the optimization nightmares that languages with pointers have. Languages with heavy use of pointers have a hard time doing garbage collection, optimizations, and so on, because it is infeasible to guarantee that no one has an interior pointer to an object, and therefore the object must remain alive and immobile.

For all these reasons we do not describe references as addresses in the specification. The spec just says that a variable of reference type "stores a reference" to an object, and leaves it completely vague as to how that might be implemented. Similarly, a pointer variable stores "the address" of an object, which again, is left pretty vague. Nowhere do we say that references are the same as addresses.

So, in C# a reference is some vague thing that lets you reference an object. You cannot do anything with a reference except dereference it, and compare it with another reference for equality. And in C# a pointer is identified as an address.

By contrast with a reference, you can do much more with a pointer that contains an address. Addresses can be manipulated mathematically; you can subtract one from another, you can add integers to them, and so on. Their legal operations indicate that they are "fancy numbers" that index into the "array" that is the virtual address space of the process.

Now, behind the scenes, the CLR actually does implement managed object references as addresses to objects owned by the garbage collector, but that is an implementation detail. There's no reason why it has to do that other than efficiency and flexibility. C# references could be implemented by opaque handles that are meaningful only to the garbage collector, which, frankly, is how I prefer to think of them. That the "handle" happens to actually be an address at runtime is an implementation detail which I should neither know about nor rely upon. (Which is the whole point of encapsulation; the client doesn't have to know.)

I therefore have three reasons why authors should not explain that "references are addresses".

1) It's close to a lie. References cannot be treated as addresses by the user, and in fact, they do not necessarily contain an address in the implementation. (Though our implementation happens to do so.)

2) It's an explanation that explains nothing to novice programmers. Novice programmers probably do not know that an "address" is an offset into the array of bytes that is all process memory. To understand what an "address" is with any kind of depth, the novice programmer already has to understand pointer types and addresses -- basically, they have to understand the memory model of many implementations of C. This is one of those "it's clear only if it's already known" situations that are so common in books for beginners.

3) If these novices eventually learn about pointer types in C#, their confused understanding of references will probably make it harder, not easier, to understand how pointers work in C#. The novice could sensibly reason "If a reference is an address and a pointer is an address, then I should be able to cast any reference to a pointer in unsafe code, right? " But you cannot.

If you think of a reference is actually being an opaque GC handle then it becomes clear that to find the address associated with the handle you have to somehow "fix" the object. You have to tell the GC "until further notice, the object with this handle must not be moved in memory, because someone might have an interior pointer to it". (There are various ways to do that which are beyond the scope of this screed.)

Basically what I'm getting at here is that an understanding of the meaning of "addresses" in any language requires a moderately deep understanding of the memory model of that language. If an author does not provide an explanation of the memory model of either C or C#, then explaining references in terms of addresses becomes an exercise in question begging. It raises more questions than it answers.

This is one of those situations where the author has the hard call of deciding whether an inaccurate oversimplification serves the larger pedagogic goal better than an accurate digression or a vague hand-wave.

In the counterfactual world where I am writing a beginner C# book, I would personally opt for the vague hand-wave. If I said anything at all I would say something like "a reference is actually implemented as a small chunk of data which contains information used by the CLR to determine precisely which object is being referred to by the reference". That's both vague and accurate without implying more than is wise.

Comments

Anonymous
February 17, 2009
Nice explanation. The phrase "objects owned by the garbage collector" caught my eye. Can you recommend/suggest any resources that dig into this concept of managed objects being under the ownership of the GC? I had always thought of the GC knowing about or managing objects rather than making the mental shift to it owning the objects as such. This probably shows my lack of understanding about the GC.
Anonymous
February 17, 2009
@Adrian: MSDN magazine had a series on the GC which was pretty good: http://msdn.microsoft.com/en-us/magazine/bb985010.aspx
Anonymous
February 17, 2009
I actually disagree. "Reference is an address" is a simple but powerful mental model. It is sufficient to explain semantics of assignment, field modification, method argument passing, etc. Without it, the novice has to memorize a bunch of weird rules. Sure, the address is not static, and it may not even be implemented as such. By the time the user cares, they will be able to grok it.
Anonymous
February 17, 2009
I think the use of the term address is reasonable. It's not a perfect analogy, but I'd compare it more to an street address or an IP address rather than a pointer. Really though, there's hardly any semantic distinction: both are intrinsically meaningless short bits of identifying information whose sole purpose is to find other information. It's not a coincidence that references are implemented as pointers; since they both serve almost the same purpose on the same architecture, it's natural they'll be implemented almost identically. The implementation of references as pointers isn't an implementation detail, it's inherent in what references are. The languages tries to protect you from inadvertent mistakes via the fixed statement, but it won't prevent a fixed pointer from being (unwisely) used outside of the fixed scope; and direct usage of the GCHandle type makes this kind of conversion even less obvious. In terms of pedagogy, I think the value of the inaccurate oversimplification is that it is indeed a simplification. It's hard enough as is to learn new things, but the larger the number of new concepts, the harder it becomes. If you need to explain both references and pointers, then treating the commonalities first might be easier to grasp than focusing on the distinctions. Then again, you probably don't need to explain both to a real novice anyhow. I suspect most readers of (beginner) programming books aren't first time programmers, but people that have seen similar constructs in other languages. If that's you're readership, the distinction might be a useful detail. To a complete beginner, an address, a pointer, a reference, an identifying description are all going to appear to be very similar. A vague hand wave might be confusing, an oversimplification might be misleading; there's no free lunch. But when it comes to C#, which doesn't allow pointer arithmetic on references (at least not with the unsafe and fixed keywords), wouldn't those pointer-oriented keywords be a better place to clarify the distinction? Confusing references and pointers seems harmless enough...
Anonymous
February 17, 2009
Well, would it help to point out to these authors out section 25 of the C# spec (unsafe code) is a conditional part of the standard, that conforming implementations are not required to implement unsafe code, pointers, or any of the other paraphernalia surrounding it? From this it naturally follows that MS's particular implementation and CLR are certainly not the only targets to consider. Further, it should be possible to compile/run C# for target environments where pointers simply do not exist and references are opaque first-class objects. Example environments include the Java VM, lisp machines, Parrot, or even a javascript engine with a C# compiler equivalent to Google's GWT Java-to-Javascript compiler.[0] I find it somewhat scary that there are people writing books on C# who do not understand this. [0] http://en.wikipedia.org/wiki/Google_Web_Toolkit
Anonymous
February 17, 2009
It's all a bit of a mine-field. Depending on your background, you might assume "pointer" to mean the C concept of a numerical offset from the beginning of an address-space, or you might assume it to be an opaque token like in Pascal. (Although I think most popular Pascals gave in and allowed pointer arithmetic.) You might understand "reference" to be an opaque token as in .net, or "something that's maybe sort of like a dereferenced pointer" as in C++. "Address" might have a precise technical meaning or simply mean "a piece of information that can be used to unambiguously find a thing". You're definitely right that you can't explain references in terms of addresses without first explaining quite what you mean by addresses! I often wonder how I'd do at learning programming from scratch in a modern language like C#. Although I now have a good understanding of reference and value types, I feel that I got there by first understanding value types and pointers (without pointer arithmetic) in languages like Pascal, and then understanding reference types by analogy to pointers. To me reference semantics seem quite advanced to learn straight away, and I wonder how they are best explained to those who have never learned another programming language.
Anonymous
February 17, 2009
"Pointers are strictly "more powerful" than references; anything you can do with references you can do with pointers, but not vice versa. I imagine that's why there are no references in C -- it's a deliberately austere and powerful language." Can you give an example for something that can be done with pointer and not with references, except indexing into arrays? Sure. The obvious example is "pointers to pointers". In C# you can have a variable which contains a reference to an object -- one level of indirection. And you can pass the variable as an argument of a method that has a ref parameter, so that's two levels of indirection. But with pointers you can get arbitrarily deep levels of indirection; you can have a pointer to a pointer to a pointer to a pointer to an int if you want. Another example is being able to compare two references for reference equality. In C# you can take two references and use System.Object.ReferenceEquals to test to see if they refer to the same object. But consider the following: void M1(ref int x, ref int y) { ... }
void M2(int* x, int* y) { ... }

int abc = 123;
M1(ref abc, ref abc);
M2(&abc, &abc); In M1 there is no code that you can write in C# that can tell you whether you are in this case or not -- you have no way of knowing if x and y are refs to the same variable. In M2 you can just compare the pointers for equality and you'll know. -- Eric
Anonymous
February 17, 2009
> A pointer in C is little more than a number which refers to a specific index into an array the size of all memory available. Since we're nitpicking here already, I have to point out that this is incorrect - in ISO C & C++, a pointer is not an index into "an array the size of all memory available" - in fact, intimately tied to the object, or an array of objects, from which it was produced, and any use of it outside that scope usually leads to U.B., with a few exceptions. For example, you can't take addresses of two locals and compare them with < or > operators, nor can you subtract one from another - both are U.B. according to the spec. Technically, it is entirely valid for a compliant C implementation to implement pointers as "fat" objects that store some handle to the object (or array) from which the pointer was created, and the index within; and complain loudly about any operations that are U.B. The fact that pointers are plain memory addresses in most C implementations out there is also strictly an implementation detail. Excellent point. (Though an interesting thing, while we're nitpicking, is that saying "you cannot do this because it is undefined behaviour" is logically inconsistent. If you cannot do it then that's because the compiler is stopping you from doing it. Undefined behaviours only happen when you can do something that leads to the undefined behaviour. Really I think you meant to make the moral statement "you should not because it is undefined behaviour".) But I fully agree with your larger point. There is a difference between the memory model handed to you by most implementations of C and the memory model defined by the language spec.
Anonymous
February 17, 2009
I entirely agree with the main premise of the post. My problem is that the example I find easiest to explain the difference between objects and references involves the word "address". I like to use the example of a house - you can write its address on a piece of paper, then copy it and give it to someone else, and the house itself isn't copied. If you give someone the address of your house and they go and move the furniture, when you get home you can see the furniture has been moved too. It works in various ways. It's not surprising that this is the case, because "address" means more than "location in memory" - the computing term was chosen to mirror the real-world term, rather than vice versa. We use "address" for many things: email address, web address etc. I'm still looking for an equally good example which doesn't use the word "address".
Anonymous
February 17, 2009
There are so many explanations and definitions to references, pointers and handles that have come out over the years, but there seems to be no general consensus of what are the correct definitions and comprehensive definitions. References have been defined as being "addresses". Pointers have been defined as being "volatile references to memory locations", references have been defined as being constrained pointers, handles as references to resources or opaque pointers, now references are defined as being opaque handles. Pointers have been defined as being indexes into the memory array, arrays have been defined as higher abstractions over pointers. There are clearly some circles here ... after all, all these at the base just are numbers of well defined lengths in binary format. It matters how you treat those numbers as values or as indirections to values. To a newcomer "a reference is an address to an object" is more clear than "an opaque handle", what is a handle to a newcomer? Does he know what that is? does he know what opaque really means in the context of memory management or object management ? To a experienced programmer a "opaque handle" is a better definition, because it says allot more about the behaviors involved, but to a newcomer explaining references as handles is very confusing, you can't reference an abstract concept (handle) to explain a reference :). ... my 2 cents.
Anonymous
February 17, 2009
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Anonymous
February 17, 2009
Perhaps, at least once in a lifetime, it is useful not to shoe-horn the entire managed mentality into what something is or is not. Same applies to pushing managed idioms as the only correct solution to world hunger (while it leaks so much memory for few TextBoxes, Buttons and Tabs especially... ) It is a well known fact for few decades that references are more optimisable btw.
Anonymous
February 17, 2009
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Anonymous
February 17, 2009
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Anonymous
February 17, 2009
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Anonymous
February 17, 2009
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Anonymous
February 17, 2009
For developers coming into C# fresh, or from scripting or another managed language, "address" doesn't have the meaning or the implications that it does for a C programmer. Address when understood in the sense of postal, email or web is a reasonable analogy for a reference in a managed runtime. To reach the widest audience, and keep things simple for novices, I think the original statement is fine, but should be accompanied by a warning: "a variable of reference type stores the address of the object (C programmers take note: not literally the address in memory; references are not pointers)."
Anonymous
February 18, 2009
@Pop, maybe I wasn't clear enough. My point was that a "reference" is a way to find/handle the object (correct me if I'm wrong), and the address is not the only way to find the house (Jon asked for a way to explain it without "address"). There are other possible implementations. I can write "the house at coordinates 150,95" on the paper instead of "the house at 27, Evergreen Terrace, Greendale". Different implementations of reference:

Address: based on an index (the streets registry of the Unites States, or whatever)
Coordinates: double index on the whole world map
Polar Coordinates: algorithm based on a common start point

Anonymous
February 18, 2009
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Anonymous
February 18, 2009
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Anonymous
February 18, 2009
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Anonymous
February 18, 2009
References are directions to a thing. That an acceptable implementation of directions (the most obvious and simplest) is the absolute position relative to a well defined origin does not mean that you should assume that that's how it works. In a GC environment the directions idea is even better because things move around in unpredictable ways but so long as you have a reference the GC promises it will keep the directions up to date all the time. If for some reason, say to talk to an older program that doesn't know how to read anything but the absolute location sort of directions, you can ask the GC to give you a copy of those absolute directions which is a pointer. However older programs also don't expect directions to change, and the GC probably couldn't change the pointer where it is going if it tried. So to be helpful (who likes getting lost in seg fault land) it can promise not to move the thing for a while, since you normally wish to do both at the same time fixed exists.
Anonymous
February 19, 2009
Regarding your examples in the response to my comment: the first one ("you can have a pointer to a pointer to a pointer to a pointer to an int if you want") is not an example of something you can do with pointers and can't do with reference - it's an example of a syntax (int****) that doesn't exist with references - but what can you do with that syntax? As for the second example - I never thought of that! Are there any more limitations in reference types? (There are a lot with value types - you can't keep two references to the same int as fields, for example) Regarding your response to Jonathan: There are more abstraction layers in between, right? I'd like to know the low-level details; what is actually kept in the reference. If I thought about it in a C# world (if the CLR was implemented in C#) it'd probably be a reference type with a pointer inside it, and the GC would change the pointer when relocating data (thus changing the pointer for everyone). But what is actually kept there? A pointer to a pointer? An index to a really big "All reference types data" array? An imp that tells the runtime where to look?
Anonymous
February 19, 2009
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Anonymous
February 19, 2009
Aren't we missing the really important point? We should define references by the semantic differences between reference types and value types. C# has variables Variables have a type Types are classified as either references types or value types Variables support assignment Value type variables copy state on assignment Reference type variables share state on assignment This means mutating operations on value type variables affect only that variable while mutating operations on reference type variables affect all variables that share that reference. For example consider the different semantics of the following operations; Person x = new Person(28); Person y = x; x.HappyBirthday(); int a = 28; b = a; a++; Then extend the assignment rules to passing parameters and how ref/out parameters can further change the semantics of variables.
Anonymous
February 20, 2009
> Value type variables copy state on assignment > Reference type variables share state on assignment > This means mutating operations on value type variables affect only that variable while mutating operations on reference type variables affect all variables that share that reference. I think this is a wrong way to go about it. When a variable is of a reference type, the value of that variable is the reference - so reference type variables also copy state on assignment, it's just that the state happens to be the reference! With your approach, you'll have to answer all sorts of inconvenient questions, such as "what state is copied from where to where when assigning a null value to a variable". In contrast, ref arguments can be described this way: there's no "ref value" - a ref is an alias for a location, and that's it. The reason is that, in C#, there are no operations on refs, so wherever you deal with them, you're always dealing with the aliased location, never with the "address". This is in contrast to reference types, for which you do deal with actual "references as values" - when comparing them, for example. This is really very similar to the difference between pointers and references in C++, which is probably why it's so tempting for people with C++ background to describe them that way.
Anonymous
February 22, 2009
I agree with Eamon's comment above. There's no need to put an equal sign between the terms "address" and "memory address". In fact, most C/C++ books explicitly say that a pointer variable stores a memory address.
Anonymous
February 24, 2009
c++ already has the "reference" type, it's just alias of some type, so I think this word in C# should have the same meaning, it's just an alias,used for convenience. In the c# language specification,the author never says a reference equals a pointer; pointers can do everything you needed to manipulate the memory,but pointers are also sources of bugs,so C# designed many features to free us from that
Anonymous
February 25, 2009
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Anonymous
February 26, 2009
Alex G., I believe that the Mr. Sharp example illustrates Eric's point exactly. Look at the modified quote below that does not contain "address". "Class types, such as Circle (described in Chapter 7), are handled differently. When you declare a Circle variable, the compiler does not generate code that allocates a block of memory big enough to hold Circle; all it does is allot a small piece of memory that can potentially hold a reference to another block of memory containing Circle. The memory for the actual Circle object is allocated only when the new keyword is used to create the object. A class is an example of a reference type. Reference types hold references to blocks of memory." Isn't this more clear than the original? The original adds confusion when it brings in the term "address" and tries to define it. I personally am confused whether Mr. Sharp is trying to say "address" and "reference" are the same or whether he is expicitly trying to say they are different. Keep it SIMPLE!
Anonymous
February 26, 2009
"In most modern cars, the accelerator pedal is just an input device of a computer that actually controls the engine (fuel injection); in a very similar way, a reference lets the programmer control the object whose lifecycle is actually managed by the runtime. A use of a pointer, in this context, would be tantamount to messing with the fuel valves directly, bypassing the computer." That's how I would write my beginner's book in the counterfactual world. :-) I like simple, vivid illustartions: I believe they are a viable alternative to both "inaccurate oversimplifications" and "accurate digressions", not to mention always good fun to read. Another example: here in Australia a police psychologist, when asked to explain, in plain English, the difference between a schizophrenic and an ordinary man with delusions, said that "men with delusions simply build castles in the sky; schizophrenics actually move in and live there." :-)
Anonymous
February 26, 2009
(In response to DRBlaise) Perhaps I am in the minority, but I truly feel that Mr. Sharp's explanation is clearer without removing the word "address." In my humble opinion, using the word "reference" to explain the the term "reference" is more confusing. But, alas, perhaps I really am the only one who feels this way.
Anonymous
February 27, 2009
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Anonymous
February 27, 2009
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Anonymous
February 28, 2009
"You're passing enough information to allow the callee to find the referenced thing" Isn't that a address? Can't set of information used to find something referenced be called a "address", maybe in C land a address is usually a memory address, but I don't know if that has to be true for C# land (or that it matters if it's a memory address or not) ...
Anonymous
March 02, 2009
I think the issue here is really just the english language, and pointer is very concrete while reference isn't, and then you go on to explain that they're 'same same but different' which is confusing. Pointer - it points to something. Reference - it refers to something, and ultimately it's a pointer somewhere. Pointer is concrete. It points to a specific memory address (location in memory). You can shoot your foot off because you can set a pointer to anything and attempt to dereference it as anything else. You can load data from a file directly into memory and set a pointer to the start and index it like an array if you wish. The reality is your program is essentially one big array of bits and is free to interpret (or misinterpret) at will. Many bugs are associated with the incorrect interpretation of the contents of memory. For a reference though, it's abstract because it's very similar to a pointer, but we all have to pretend it's not. We know it also points to something, and while our referenced object doesn't change, the pointer held by the reference is free to change without our knowing. So I haven't liked any of the metaphors so far. It's not like an address written on a piece of paper because a street address never changes in the normal sense. In explaining things to beginners it behooves us to create sensible metaphors and not geeky ones and without complex contortions of the metaphor to make it work. I really dislike metaphors .. but ... consider this. Given a city, there is a car. It's your car - a Red Ford Mustang convertable with fresh pine scent and cool new wheels. There are millions of cars, and millions of Ford Mustangs, and each car can be moved around. Now, you're not allowed to hold a pointer to the car because the car can be moved without your knowledge. e.g. If the car is towed or your brother borrows it he might not re-park it in the same place. Trying to hold a pointer will get you in trouble. You're mad so myRedFord.Kick() could end up kicking someone else's car, or you might kick empty air, or you might kick a building. But I do have a car, and I always know exactly which one it is when I go and get it. So if we want to know what a reference is, it's the licence plate (registration) of the car. Any number of things can refer to the car via it's registration token, and the registration token can be used to locate the car at a specific point in time, but the pointer itself is only useful for very short time slices. Knowledge of pointers is essential to understanding a computer. And I don't think you need to handwave. You just need to say a reference is the thing (object) for all intents and purposes, and the reference probably contains the pointer but you never get to see the real pointer because it doesn't matter to the program you write. As you say, it's an implementation detail. The reference is all the book-keeping that goes on, and the reference is, in the end, not owned by the programmer, it's owned by the memory manager.
Anonymous
March 17, 2009
Timely, accurate, and unbiased. Great article.
Anonymous
March 18, 2009
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Anonymous
March 18, 2009
For a novice OO programmer it suffices to say and it is TRUTH to say: A pass by reference is passing the thing itself. (Original) A pass by value is creating and passing a copy of the thing. (Copy) This started by talking about educating novice (C#) OO programmers. If we are going to educate OO programmers then let's not confuse them with talk about physical memory and addresses. We old school guys who started by coding hex or octal, moving on to assembler, and eventually to compiled and interpreted languages may need to think of the world in that way, but they don’t and we should not corrupt them. C and C++ are baroque tools. Much like assembler in that they are philosophically closely tied to an understanding of the hardware of the machine. These old school languages are powerful, expressive, and capable of astonishing efficiency in the right circumstances, but they also produce brittle, costly to support, and expensive solutions. This required understanding of the physical implementation details in these languages is in fact one of great drivers behind the advent of “modern” languages like C#. We need to raise our eyes up from the circuit board and focus more on modeling the world, that’s what OO is about. We don’t teach new surgeons how to smelt iron to make a scalpel, so why teach a novice OO programmer about memory locations. We need both metallurgist and surgeons but we don’t train them the same way.
Anonymous
March 18, 2009
A good post, with some interesting comments. As I am in the process of posting some related material would it be OK to include a pointer to this post as a reference? <grin> My biggest complaint is the "We dont need to teach novices...(fill in the blank)". As a person with over 32 years of professional development experience (including 25 as the Chief Architect and CEO of my own company), I have reapeatedly found that programmers who do not understand the internals and fundamentals have severe problems that manifest in many ways. Moderns technologies mean that we do not have to really think about the lowest levels on a regular basis, but without a deep understanding of what happens at the actual hardware level, the quality of work does significantly suffer.
Anonymous
March 19, 2009
I applaud your ability to call out the vague idea of a reference, while still giving an explanation that is vague unto itself (and rightly so). Great article.
Anonymous
March 19, 2009
" 'a reference is actually implemented as a small chunk of data which contains information used by the CLR to determine precisely which object is being referred to by the reference'. That's both vague and accurate without implying more than is wise...." Not to be factious, but that explanation is not only vague, it ranks with many similar phrases that I have seen make novices go sobbing into the night a throw said book, containing said “reference” right out their bedroom window. Try a gentler approach: such as “A reference merely refers to a chunk of data that can be retrieved on-demand by the developer. How said data is stored and maintained is entirely up to the CLR and is beyond the scope of this discussion.” Then and only then would I footnote an external source for a more specific detailed discussion, specifically warning the novice that to go there might cause them to become pre-maturely grey and could possibly result in an encounter with the infamous “Stray Pointer Dragon”!
Anonymous
March 20, 2009
A reference in C# is a "full service" pointer. Just pull up to the gas station and the attendant gives you want you want.
Anonymous
March 21, 2009
I notice you made no use of the word "abstraction" to describe references.
Anonymous
March 21, 2009
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Anonymous
March 21, 2009
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Anonymous
March 24, 2009
"a reference is actually implemented as a small chunk of data which contains information used by the CLR to determine precisely which object is being referred to by the reference" Yuck. What useful information does this definition impart to the novice programmer? The fact is that OOP is an abstraction on top of many more abstractions, going all the way down to assembler, registers, the MMU, etc. As with any abstraction, there are leaks, which eventually need to be understood to avoid certain problems or maximize efficiency. But the value of the abstraction is in delaying the explanation of the gory details until after the fundamental concepts have been presented and understood. Once you've reached that point, you at least have a context in which to talk about the leaks, and you can choose how deep you want to go, but it's certainly not material for chapter one or two unless the title of your book is something like "Advanced C# Concepts for the Experienced Programmer." For someone just starting to learn about OOP, I would stick with diagrams and pictures. Draw arrows pointing from variables to stamped out object instances, and from object instances to other object instances, circle them in red, and say "That's a reference. It lets you <i>refer</i> to object instances." It might not be very technical or in-depth, but it effectively conveys the information that the reader needs to know at that particular point in time, without requiring them to understand 15 other concepts first.
Anonymous
March 25, 2009
I haven't these comments but I disagree with the original post. "References store the address of an object" means exactly what it says. It does not mean "references are pointers" or "references are addresses". You're munging the two texts. It would be an "implementation detail", documentation-wise, to say (correctly) that "references internally store the address of an object, although that detail is hidden from you, it only does this as an implementation detail to map the reference to the memory space that the garbage collector has allocated for it". But those are too many words; it is 100% truthful to shorten that down to simply say "references store the address of an object". If indeed the text said "references are addresses to an object", a description that comes closer to describing a C# pointer, I'd agree with your point. Also, I believe that even if a reference working with the garbage collector to retain the address to the memory space of an object detail is only an "implementation detail", I think it's a very important implementation detail to know. This sort of documentation usually comes up in the discussion of stacks vs. heaps, value types vs. reference types, and/or memory optimization in the CLR. It might not be all that interesting from a syntactical point of view (C# being a language), it is a very important thing to know from an architectural point of view (CLR being a runtime).
Anonymous
March 26, 2009
While it is a subtle distinction, I have always defined a reference and a pointer as follows: Pointer: A type of variable whose value represents the memory address of the instance of an object or type. Reference: A type of variable that acts as a synonym to represent an instance of an object or type. They sound the same as you read them, but I think the distinction for me is where they are significant. A reference to me is the developer telling the compiler that whenever he/she refers to X, it should interpret that is meaning Y. It's true that references are often implemented using memory address and often represent pointers, but it does not have to be. Perhaps though some clever use of Macros in languages like C, C++, you could implement something similar? I think it is hard to seperate the two concepts beacuse their implementations are almost always identical behind the scenes. But something that helped me understand that there was a difference is how they are treated with C++: A reference MUST be initialized to an object, while a pointer can be null. You can have a pointer exist that does not have a value (i.e. memory address). But you cannot have a reference exist that doesn't refer to another instance - because conceptually, a reference does not have a value. On the other hand, a pointer is and always will be a variable that contains a memory address of an object or type. You can rely on any implementation of a pointer always containing a memory address.
Anonymous
March 26, 2009
For beginner programmers, you need to stay away from big words like "memory". I've seen way too many people who struggle with the difference between data in RAM and data on the hard drive - not that virtual memory helps. Your best bet will always be a physical metaphor rather than using any other programmer-ese words. Unfortunately, words like "address" and "pointer" have already been co-opted by earlier languages and now have additional baggage meaning that no longer applies in C#. For Larry: A different real-world metaphor that might work is the dry-cleaner's claim ticket. Every object is like a binder notebook, with a piece of information on each page. Some notebooks are "value type" that you carry with you, but cannot be shared beyond making copies. Other notebooks are "reference type" and are always owned by the GC. In order to use or modify the information stored in those objects, you have to have the claim ticket information written in your notebook. You can then use that claim ticket to be allowed access to the referenced notebook, where you can copy or modify the values written there. Since other notebooks can contain a copy of your claim ticket, owners of those notebooks can also have access to the referenced notebook. Fortunately, the C# compiler hides all that extra work needed to use a referenced variable, so you don't have to understand it before you can use it. Unfortunately, the C# compiler hides all that extra work needed to use a referenced variable, so you can easily forget the distinction. Why is there a need for value types, other than optimization?
Anonymous
March 26, 2009
>>>For beginner programmers, you need to stay away from big words like "memory". I've seen way too many people who struggle with the difference between data in RAM and data on the hard drive - not that virtual memory helps. If someone struggles with the difference between data in RAM and data on hard drive then probably he is not a programmer (by any definition). I found the the article entertaining and author was able to clearly able to make distinctions between pointer and references. But for the oversimiplication of address is bettwe when it comes to understand how the references can be used in the programs. "Reference is address to the object" is simple enoguh to understand. To make it clear it can be added "But unlike pointer it doesn't allow you to do adress manipulation". OR how about "reference is its Readonly Address?"
Anonymous
March 26, 2009
Sorry for repeat posting...something didn't work properly last time.... >>>For beginner programmers, you need to stay away from big words like "memory". I've seen way too many people who struggle with the difference between data in RAM and data on the hard drive - not that virtual memory helps. If someone struggles with the difference between data in RAM and data on hard drive then probably he/she is not a programmer (by any definition). I found the the article entertaining and author was clearly able to make distinctions between pointer and references. But for me the oversimiplication of address is better when it comes to understand how references can be used in the programs. "Reference is address to the object" is simple enoguh to understand. To make it clear it can be added "But unlike pointer it doesn't allow adress manipulation". OR how about "reference is Readonly Address of an object?"
Anonymous
March 27, 2009
You can understand how the confusion comes around though. Checkout the .NET 2.0 Foundation MCTS training kit which is obviously created by or endorsed by Microsoft (has it's logo all over it) and under a big title: What Is a Reference Type: Reference types store the address of their data, also known as a pointer, on the stack. If we're taught this in the beginning by the company who develops the language, you can see why the confusion comes around!
Anonymous
March 27, 2009
This is a very interesting article, although I must respectfully disagree with the author on whether novice programmers should hear the word "address" or "memory" without knowing something about the memory model of the language. There are few concepts in programming more fundamental than a variable, and I can't think of a language in which a variable is anything but some place in memory holding a value. I also can't think of a language I must know in order to understand that concept, whether its in writing a chunk of VBA to automate Excel or delclaring an integer in ol' F77. if you aspire to program, but can't be expected to understand the concept of a variable representing an address, one might want to consider a different profession.
Anonymous
March 27, 2009
>> if you aspire to program, but can't be expected to understand the concept of a variable representing an address, one might want to consider a different profession. Except that there's a big difference between writing a program and being in the programming profession. There are plenty of people out there who are not programmers, but are able to write a little bit of code here and there to get things done. I'm thinking of office workers writing VBA Excel macros, primarily graphics oriented web developers writing a small amount of glue code or JavaScript, or any other kind of hobbyist programmer. These are people who have no interest in taking CS101, nor should they be expected to.
Anonymous
March 27, 2009
David W Wrote> "if you aspire to program, but can't be expected to understand the concept of a variable representing an address, one might want to consider a different profession." Totally agree with you!!!.... New programmers are happy codifying in this "abstract" or "opaque" concept. Like some says above..."you arrives to the store... and choose any product stored in it"... Maybe the use of delegates clarify more this concept: references vs pointers.... Excelent post and useful and intersting point of views... thank you partners....!!
Anonymous
March 28, 2009
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Anonymous
March 28, 2009
I am a Teacher, and I usualy explain my student at first that a reference type is crreated on the heap and the address of the place where it was created will be saved on the stack. Then I make sure that they understand that they can not manipulate this address in any way, because garbage collector is owning this address, and can move it in order to compact the memory. At this stage in their career doesn't metter for them the unsafe code part of the .NET. Later on, when they will get some more experience, they will get the difference anyway.
Anonymous
March 28, 2009
c,c++对于一个初学计算机的人来说有一定的难度,希望Microsoft有关于visual basic方面的教程.
Anonymous
March 30, 2009
For novices, "a reference is a address of a variable in process space" is a good & quick way to understanding a reference, although the address is not static in C#.
Anonymous
March 31, 2009
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Anonymous
April 01, 2009
Growing up I studied both VB and C and always maintained a mental analog between ByRef in VB and pointer parameters in C. From a level of indirection standpoint these two mechanisms are equivalent (nevermind all the more powerful things I could do with pointers that I never wanted to do). Maybe this is a flaw in my own mental model (having never really used C/C++ only studied them). And surely there is a lot of nuance that is either left out or worse implied by mixing the semantics of pointers, smart pointers, const pointers, C++ references, TypedReferences, Handles, etc, etc. And URLs and URIs are really different things. When I moved to VB.NET the analogy of reference types to Pointers made it pretty easy for me to understand them on some level versus value types. In this instance I think that inaccurate simplification is more useful than harmful though I can empathize with your desire to not muddle the facts. Perhaps it is better to say it backwards - that Reference is the highest level abstraction and that pointers are merely an address-based implementation of the concept and that .NET references are likewise an implementation of the pattern of referencing - that way you're defining a .NET concretization in terms of a common abstraction, rather than by another sibling concretization. As an aside, at this stage of the game I wonder if the .NET team made the right design choice in coupling val/ref stack/heap semantics to classes and structures and whether C++/CLI is more spot on by decoupling the kind of object from its storage semantics.
Anonymous
April 05, 2009
glad you got that off your chest? :)
Anonymous
April 06, 2009
I think what it all boils down to is this: A pointer, in every C and C++ implementation I've ever seen, is a memory address in the process memory space. I suspect that pointers (unsafe as they may be) are implemented in exactly the same way in C#. Well, I certainly have seen such implementations. What about implementations of C that target 16 bit Windows running on x86? Do they have pointers which are "memory address in process memory space"? What does "process memory space" even mean in an operating system that doesn't have processes? And what does it mean when pointers come in different sizes?

Remember "near" vs "far" pointers, segmented architectures, selectors and offsets? Pointers used to be weird, man. -- Eric A reference is a data structure (in many implementations, a simple pointer) that contains all the information necessary for the code to find the object it refers to. in the case of C# and other CLR languages, the garbage collector may move the object, but it updates the reference to continue to describe the location of the same object, regardless of its actual location in memory. It's essentially like a shortcut on your desktop. file that the shortcut refers to can be moved, but as long as the shortcut is updated with the new location, you don't need to know where the actual file is, so long as you can click the shortcut on the desktop.
Anonymous
April 10, 2009
Great article, with one important objection. Since this is a discussion on semantics, I would argue about the semantic of "address". The wod "address" would drive only some programmers to think of a memory address. Others might think of a URI, for example. Most novices will probably just think of a street address. Take this definition in support: http://wordnetweb.princeton.edu/perl/webwn?s=address I do agree, the statement "references are memory addresses" is wrong. But, in general, "references are addresses" is not that inaccurate.
Anonymous
April 12, 2009
I think your questioning of C is incorrect. There is no question to why C doesn't have references. C is a low-level language that doesn't need the overhead of references. You manage your own memory. It's difficult but not horrible. If you don't understand it, you shouldn't be using it! There are plenty of other languages that will solve your problem most likely.
Anonymous
April 22, 2009
If we use the house analogy, a reference would be like saying "Tom's house". It's not an address, it's only known amongst the circle of friends, the "program" in which it is used. The actual address - 123 Elm Street - would be more like a pointer. So if you know Tom, you understand the reference but it doesn't give you the exact address because it's not needed. And if Tom moves, no problem for the reference. No analogy is perfect but for a beginner, this helps to keep things in context to know that a reference is not a pointer but it still, nonetheless points to an address. Thanks for the article.
Anonymous
April 30, 2009
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Anonymous
April 30, 2009
This is all well and good - and everyone sems happy with the numerous descriptions of pointers and addresses and dogs on leads etc... (good one that!) - but this confuses my address based understanding of pInvoke calls where you import an implementation using pointers and marshal it using ref... i'd always thought of references as just that, a reference, which is nice and easy and fits in with your original post of not cosidering them as anyhting other than a reference (don't need to know? why worry about it then?). However when dealing with C++ implementations you're trying to pInvoke to you often come across this marshalling grief - and that's where all the stuff above leaves me confused. what exactly is going on when you use ref in your c# definition of something which in the original c++ implementation was a pointer? is the type marshaller doing clever stuff on your behalf, getting the real address (what ever that means!! not wanting to open a can of wroms there) from the GC and substituting it at runtime? Or is this functionality dependant on the implementation of the GC using addresses as the references? i'm normally quite happy not knowing this stuff, but with pInvoke situations i've often found myself needing to know more than i want to!
Anonymous
May 09, 2009
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Anonymous
May 09, 2009
@Mike Birt Marshalling does indeed do loads of clever stuff on your behalf. If the call signature of the function you invoke includes pointer parameters then the referenced values might potentially be changed. If you fail to specify ref then your parameter value will probably make it to the invoked function but any changes won't make it back to the caller. This would cause problems with API functions that expect you to pass them a struct to populate.
Anonymous
May 11, 2009
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Anonymous
May 13, 2009
This is an excellent description! This is the kind of reading us techies live for. Wise choice of words! If people like this author still work for MS, then I will change my rating on MS stock to "buy" :)
Anonymous
August 18, 2009
Its good that such things gets under questions, gives better understanding to underlying logic of programming languages. I think in this post defenition of reference is also a very vague. Pointer adds even more confusion. From technical point of view there can be tons of things hapening beyond the scene, but from logical point of view its pretty simple. Reference itself is an intension and extension of it is the object it referes to. Relation or function or all handwaving describing the way binding between intension and extension happenens is a function(relation or whatever you like it to call) translating(mapping) intension of reference for the curent state into its extension (in current state). Thanks for article.
Anonymous
August 18, 2009
An excellent post (and followup), but one inconsistancy (quotations reversed in order to make post easier...) "Pointers are strictly "more powerful" than references; anything you can do with references you can do with pointers, but not vice versa. I imagine that's why there are no references in C -- it's a deliberately austere and powerful language. " "The inventor of the C programming language, oddly enough, chose to not have the concept of references at all. Rather, Ritchie chose to have "pointers" be first-class entities in the language" You explain very well WHY there are no references with the first quote (which occurs later in your original post). Provided there was "one way to skin a cat" many other "features" were eliminated. Remember the C was originally designed to run on a DEC PDP-11 mini-computer. The maximum addressable space (directly at one time by aq single process) was [actually still is - I have a few of them as well as a PDP-8 and Vaxen] 32KW....sooo many aspects of C derive from this processors architecture....
Anonymous
October 31, 2012
Very very good post, I have seen so far for pointer and references

共用方式為

References are not addresses

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