Why should I use a pointer rather than the object itself?

http://stackoverflow.com/questions/22146094/why-should-i-use-a-pointer-rather-than-the-object-itself?newsletter=1&nlcode=47931%7c9e27

by olouv • 12 years ago

72 78 12 years ago

stackoverflow.com

GuiA• 12 years ago

Related Stack Overflow question: "C++: Why should `new` be used as little as possible?"

https://stackoverflow.com/questions/6500313/c-why-should-new...

gd15• 12 years ago

There are some very good answers to that question. Related to that question, I still fail to see any advantage to using shared_ptr, weak_ptr, unique_ptr, auto_ptr, etc. If you are doing C++ right, nearly everything should be allocated on the stack, and the things that aren't should be encapsulated in a class, RAII style. If you can't design your program so that the ownership and lifetime of your objects are clear, then you're using the wrong language, and shared_ptr won't save you.

dllthomas7• 12 years ago

The various pointer types are about keeping the ownership and lifetime of your objects clear.

Suncho3• 12 years ago

The point of the smart pointers is that they are RAII classes. If you have an object that manages more than a single resource, you open yourself up to all kinds of exception safety and thread safety issues.

For example, let's say you have a class that news up an object in its constructor. Presumably, the corresponding delete lives in the destructor. But if that constructor throws an exception, the destructor will never be called. If it throws the exception after the "new," then you have a memory leak.

So basically, you need a class whose only job is to manage a single pointer. You could certainly roll your own smart pointer, but it would have to work pretty much exactly like one of the ones in the standard library.

srean2• 12 years ago

The word RAII by itself is too non-specific. People typically mean: use (i) a scoped/auto pointer or (ii) a shared pointer (thats C++ speak for reference counted pointers). These are good thumb rules to rely on, but in itself they are very inadequate.

First, auto or scoped pointer gets you a stack discipline. If your objects weren't very big, you might as well create them on the stack itself. More efficient and suitable for multithreading. If the lifetime does not follow the lexical stack then you are out of luck with this style of RAII.

Next, reference counted pointers: Cycles are clearly a problem. The traditional mantra is: use weak pointers to break cycles, but then you are back to error prone manual management (of cycles). If I am to be trusted with using weak pointers correctly, I wouldn't be too bad with manual memory management either. Pure functional languages with eager semantics do not allow cycles, so are ref counts a good idea in such cases ?

Although, many do not think about it that way, reference counted system is a _garbage_collector_. Just a particularly simple one. Just because one understands how it works, does not disqualify it from being a garbage collection system. It is just not a very efficient garbage collector for cases where running time is sensitive to cache locality, which is true fairly often.

I am quite excited about Rust's regions. I dont think programming with regions is easy and you would want to have a garbage collector anyway. I hope this style of programming sees more mainstream visibility. Something should come out of all the MLton work on this. EDIT Indeed meant MLKit but it draws enough from MLton that I thought HNers would be able to relate.

There is also D's scoped guards, but I am not yet familiar with what they do.

nsmnsf2• 12 years ago

Can someone explain RAII to me? In the way that it's commonly used, it seems like it really means "stack variables that go out of scope are deallocated", which seems kind of, well, duh? I don't really understand why it's treated like a big deal.

(I mostly write C + Obj-C)

julian37• 12 years ago

Stack variables that go out of scope are deallocated, that's one of the points of RAII (an advantage over dynamic allocation, especially because scope is also terminated by exceptions.)

But crucially, their destructor is called (and any destructors of their member variables, etc.) The destructor might perform cleanup work beyond freeing up memory. For example, std::istream will close the file handle when it goes out of scope, and boost::lock_guard will release the lock.

RAII isn't rocket science, but there are some advantages over plain C where all cleanup has to be explicit and it's fairly easy to get resource deallocation wrong if you don't know what you're doing. And as the Wikipedia article says, RAII is vital in writing exception-safe code in C++ if you don't want to litter try/catch blocks all over your code.

http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initial...

dllthomas7• 12 years ago

RAII is "Resource Acquisition Is Initialization", and it means "tie the lifetime of a resource to the lifetime of an object" because we have ways of making sure objects are released when they are no longer wanted (of which stack allocation is the easiest, when it's applicable).

DerKommissar• 12 years ago

  mutex.lock();
  doStuff(); /*throws an exception, mutex is never unlocked*/
  mutex.unlock();

  ScopedLock lock(mutex);
  doStuff(); /*mutex unlocked by destructor in all cases*/

It guarantees that resources are freed no matter how the scope exits.

coherentpony• 12 years ago

https://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initia...

masklinn4• 12 years ago

it seems like it really means "stack variables that go out of scope are deallocated"

And the destructor, if any, is called. That means you can bundle automatic cleanup action in the destructor, no need for goto: cleanup, @try/@finally or some other sort of "manual" cleanup, the cleanup is implicitly set up by, well, the setup.

The ability itself is by no means unique to C++ (even amongst oldies, Smalltalk has BlockClosure#ensure: and Common Lisp has unwind-protect) but a surprising number of languages still don't have such a capability, and it's a pretty neat consequence of C++'s memory model.

nsmnsf2• 12 years ago

And the destructor, if any, is called

Right, that's implied by a C++ object being deallocated. I understand how it works - it's just that it's really obvious if you know anything at all about how the stack works (and C++ destructors).

detrino8• 12 years ago

To better understand the importance of RAII, you might consider what some other languages offer to solve similar problems.

Consider a C example:

    int f()
    {
        int ret = -1;

        handle * a = acquire_handle(0);
        if (a == NULL) goto fail1;
        handle * b = acquire_handle(1);
        if (b == NULL) goto fail2;
        handle * c = acquire_handle(2);
        if (c == NULL) goto fail3;

        // use a, b, c
        ret = 0;

        release_handle(c);
        fail3:
        release_handle(b);
        fail2:
        release_handle(a);
        fail1:
        return ret;
    }

Consider a C# example:

    void f()
    {
        using (handle a = new handle(0))
        using (handle b = new handle(1))
        using (handle c = new handle(2))
        {
            // use a, b, c
        }
    }

Now a C++ example using RAII:

    void f()
    {
        handle a{0};
        handle b{1};
        handle c{2};
        // use a, b, c
    }

These examples are mostly equivalent (Although the C#/C++ assume exceptions instead of error codes).

The C#/C++ examples are far more structured and less error prone than the C example.

The advantage of C++'s RAII over C#'s using statement is that cleanup is tied to the object rather than a statement. This means that RAII is both composable and non-leaky as an abstraction. You cannot forget to destruct an object in C++, and you don't have to care that its destructor frees resources. When you have an IDisposable member in C# you must manually mark your class as IDisposable and then implement the Dispose method yourself. Clients must also be aware that your class is IDisposable in order to use it correctly.

voidlogic• 12 years ago

Go:

  func f() error {
    var a, b, c *handle
    var err error	
    if a, err = acquireHandle(0); err != nil {
  	return err
    }
    defer a.Close()
    if a, err = acquireHandle(1); err != nil {
  	return err
    }
    defer b.Close()
    if a, err = acquireHandle(2); err != nil {
  	return err
    }
    defer c.Close()
    //use a, b, c
    return nil		
  }

ygra2• 12 years ago

Stylistic side note for C#: If you're nesting using blocks like that you can leave out the braces in all but the deepest instance which reads a little nicer, imho:

        using (handle a = new handle(0))
        using (handle b = new handle(1))
        using (handle c = new handle(2)) {
            // use a, b, c
        }

dllthomas7• 12 years ago

It's perhaps worth knowing, although it will hopefully never be relevant, that at least in GCC a longjmp will not run destructors.

penguindev6• 12 years ago

The worst problem is that the destructor should not throw an exception (AFAIK). Thus, c++ RIAA is not perfect - can't use it for things that could fail. edit: some people use it for closing/flushing files, bad idea.

edit 2: I judge a language in part by seeing if they actually can define nested / chained exceptions well. If they don't, they're probably not too serious about exception handling...

detrino8• 12 years ago

Perhaps its more fair to C to consider handle to be an opaque type like C#/C++. In that case the acquire block can be rewritten:

    handle a, b, c;
    if (!aquire_handle(&a, 0)) goto fail1;
    if (!aquire_handle(&b, 1)) goto fail2;
    if (!aquire_handle(&c, 2)) goto fail3;

Nursie• 12 years ago

To my mind it's like stack objects - not only do you not have to worry about freeing them, the destructor gets called and so do subordinate destructors for memeber objects. It was a bit of a revelation when I did some 'real' c++ for the first time a couple of years back.

In some ways it is just another of the myriad of implicit behaviours in C++ that make it hard to work with, though.

lutorm• 12 years ago

The crucial idea is that what's pointed to by those variables is also cleaned up. When a POD pointer goes out of scope, it doesn't help you that the pointer is deallocated, you need what's pointed to to be deallocated.

Beyond pointers, it applies to many things that have a "C-style" API. Without RAII, anything that needs to be explicitly cleaned up needs to be monitored. For example, if you declare a pthread mutex, it doesn't help you that the mutex variable itself goes out of scope, because it's just a pointer and that will not clean up the actual mutex.

And as people have said, without RAII it's impossible to write exception-safe code.

dllthomas7• 12 years ago

"And as people have said, without RAII it's impossible to write exception-safe code."

Assuming that's hyperbole, I agree with the sentiment. It's possible to write exception safe code with try & catch - it's just almost as ugly as checking exit statuses and easier to miss something.

thought_alarm• 12 years ago

"stack variables that go out of scope are deallocated"

That's all it is.

But, it guarantees that a given piece of code (an object's destructor) is always executed when execution leaves the current scope, either through normal execution, by calling `return`, or by throwing an exception.

In fact, RAII is the thing that allows exceptions to be remotely usable in C++; the lack of RAII in Obj-C is the reason why exceptions are not common in Obj-C.

It's used for regular memory management, but it's also used for managing access to other resources, such as files, mutexes, etc. E.g., you can use it ensure that a file handle is automatically closed when you leave scope.

joe_the_user2• 12 years ago

One interesting thing is that the QT framework prefers the use of pointers over reference variables. There's actually some reasonable logic here. Pointer syntax clearly shows you are acting on something in a different way than you act on an "ordinary" object. Reference object syntax is the same as ordinary object syntax but you are making changes that will go beyond the local scope and that is thus less obvious.

With this approach, you can still allocate objects on the stack - you then take their address and pass that to a subroutine.

acchow• 12 years ago

Presumably, this pointer vs. reference debate is only relevant in C and no longer has a place in C++ with the use of unique_ptr, shared_ptr, std::move, etc?

ori_b• 12 years ago

C has no references -- 'type& param' -- so there is no debate. This is a topic exclusive to C++, and isn't affected by smart pointers, which overload all the pointer operators in order to act like regular pointers, only with some sort of lifetime management.

EpicEng2• 12 years ago

Considering that C has no notion of references, no, I cannot agree...

joe_the_user2• 12 years ago

In the Qt idiom, these smart pointers are irrelevant (as mentioned in the other poster). Here, when you are passing a pointer, it's going to be a pointer to an object on the stack which is always going to be good - the pointer argument shows you that you are dealing with something outside the functions local scope, the variable allows you pass information back, etc.

Qt also has used its own smart pointers for a while but these generally are declared within some container class, so the idiom basically allows you to do allocation entirely within containers and so never use the new operator directly.

Now, there may be situations where you have a complex chain of multiple connections that don't fit to an existing container. The sloppy way then is to use new and the good way to write a custom container. The idiom can also when dealing external libraries that allocate memory directly or somehow demand you use the new operator.

stormbrew4• 12 years ago

note: in java, everything(except basic types) is a pointer. so you rather should ask the opposite: why do I need simple objects?

I think this is by far the most important piece of advice in that thread. It gets right to the heart of one of the most basic areas of confusion that programmers coming from garbage collected environments to ones with manual memory management have.

They need to understand that they have NOT been working in an environment without pointers, as they are often told to believe, but one in which nearly everything is a pointer. Understanding this is fundamental to understanding these differences and a 10,000 word essay about C++ allocation policies will not make it any clearer without dealing with that first.

malisper• 12 years ago

But the fact that they are programming in an environment where everything is a pointer is hidden to the them. In C++ one needs to explicitly use pointers whereas in Java all of the pointers are handled behind the scenes.

abengoam• 12 years ago

Not really. In Java it's a leaky abstraction - it's true that the JVM does a lot of micromanagement for you, but if you are careless you start getting exceptions (best case scenario) or start to get mysterious changes in your data (worst case scenario).

I agree with grandparent that understanding what a pointer is and that almost everything in a Java program is a pointer it's critical for a Java programmer.

banachtarski2• 12 years ago

I'm amazed at how many new programmers enter the workforce without a basic understanding of the stack and the heap.

IsTom• 12 years ago

I've met an 'architect' who didn't know either. Apparently Java world is governed by a different set of rules.

greatdox• 12 years ago

Most of them are recent high school and college dropouts. What did you expect them to know?

They learned programming by reading blogs and pirated PDF files on BitTorrent. Of course they don't got a level of understanding most hackers here have.

You want a good programmer you need 15+ years of experience plus a good understanding of computer science, recent high school and college dropouts don't have that.

You hire a cheaper labor source, you won't get that level of quality a more expensive labor source will give you and you get what you pay for!

stusmall• 12 years ago

I think that might be a bit too broad of a take away. I hate to be a pedant but C# allows user defined value types and I'm sure there are other GC'd languages that aren't as reference type heavy as Java. I think it would be a more fair statement if you replaced "garbage collected environments" with Java. I say "more fair" because I don't think I know any serious Java devs who don't understand the difference between reference and value types. I could just be lucky with who have crossed my path though.

EDIT: After thinking about it a little, most of the Java people I know have at least some embedded or native background. This could be a reason. I don't know what the market over all is like.

stormbrew4• 12 years ago

Consider the other direction from Java, into the fully dynamic languages. Many of those give you even less ability to control heap vs. stack allocations than Java does and often present an everything-is-a-reference facade even on things that are often optimized to stack objects (Fixnum in Ruby for eg).

But yes, a java programmer with embedded or native background would have a better understanding of this than people who learned Java first, which is what I'm really addressing here. And in the OP on SE you can clearly see that this is a big part of that person's confusion.

Suncho3• 12 years ago

I find it interesting that nobody mentions iterators anywhere in this discussion. One of the main uses of pointers is to iterate through data. Pointers are the most basic form of iterators in C++. STL iterators are based on, and often implemented in terms of, pointers.

EGreg2• 12 years ago

One is allocating on the stack and one on the heap.

Allocating on the heap means you have to delete it yourself in C++.

It's pretty staightforward, no?

orionblastar• 12 years ago

I audited a few MOOC classes.

They don't even bother to teach pointers, the stack, or heap. It just confuses the students they claim. Also teach students to store the value of PI into an Integer. Use DRJava and Java 6 and some bloated JAR library that makes Windows crash.

When I studied computer science in 1986, the very first things we learned where how memory and binary worked, plus what a pointer was and why it should be used and how to manage memory so it would not crash and lock up. We learned how to debug as well.

Most comp sci graduates that I talk to know almost nothing of how a pointer works, or the stack or heap, and binary might as well be Tagalog to them.

I think that over the years as the demand for IT workers increased, they relaxed the standards they used in teaching computer science and programming.

Any good MOOC should teach:

Pointers

The Stack

The Heap

Binary

Hexadecimal

How to debug code

Assembly Language instructions the code gets converted into and how to debug that

Jonathan_Swift• 12 years ago

It's not always obvious. Quite commonly one is used when the other would be better, even by otherwise good coders.

To beat the subject completely to death:

Pointers, References and Values

Passing Parameters, Returning Results, and Storing Member Variables with Musings on Good C++ Style

http://www.warplife.com/tips/code/c++/memory-management/para...

Why should I use a pointer rather than the object itself?

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