Letting go of auto_ptr

Question

Occasionally, for fleeting moments, I think auto_ptr is cool. But most of the time I recognize that there are much simpler techniques that make it irrelevant. For example, if I want to have an object freed automatically, even if an exception is thrown, I could new up the object and assign to an auto_ptr. Very cool! But I could have more easily created my object as a local variable, and let the stack take care of it (duh!).

Thus I was not too surprised when I found google C++ coding standards banning the use of auto_ptr. Google states that scoped_ptr should be used instead (if a smart pointer is needed).

I'd like to know if anyone, contrary to my experience, can give a solid reason(s) of when auto_ptr is the best or simplest thing to use. If not, then I suppose I will ban using it myself (following google's lead).

update: For those who expressed concern, no I am not adopting google standards. For example, against google advice, I agree exception-handling should be activated. I also like using preprocessor macros, such as the printable enum I made. It is just the auto_ptr topic that struck me.

update2: It turns out my answer comes from two of the responders below, and a note from Wikipedia. First, Herb Sutter did show a valid use (source-sink idiom and lifetime-linked object composition). Second, there are shops where TR1 and boost are not available or banned and only C++03 is allowed. Third, according to Wikipedia, the C++0x spec is deprecating auto_ptr and replacing it with unique_ptr. So my answer is: use unique_ptr if available to me (on all platforms in consideration) else use auto_ptr for the cases that Sutter depicts.

Answer 1

It's the simplest thing to use when you need a scoped or unique pointer and you are working in a strict C++03 environment with no access to a tr1 implementation or boost.

Answer 2

Well one reason would be that scoped_ptr is non-copyable, so it's safer to use and harder to make mistakes with. auto_ptr allows transfer of ownership (eg. by passing it another auto_ptr as a constructor parameter). If you need to think things like transferring the ownership, the chances are you're better off with a smart pointer like shared_ptr.

Answer 3

Herb Sutter can help you out on this one: http://www.drdobbs.com/184403837

Answer 4

std::auto_ptr still has pointer semantics, so automatic (non-pointer) variables aren't a substitute. In particular, std::auto_ptr supports polymorphism and re-assignment. With stack variables you can use references for polymorphism, but references don't allow for re-assignment.

Sometimes std::auto_ptr will do just fine. For example, for implementing a pimpl. True, in the vast majority of cases boost' smart pointer library offers better choices for a smart pointer. But right now std::auto_ptr is a standard solution, whereas boost's smart pointers aren't.

Answer 5

Using auto_ptr as function return value you will enjoy no copiyng overhead and never have memory leak. std::auto_ptr<obj> foo() can be safely called in { foo(); } while obj *foo() cannot. boost::shared_ptr can solve this, but with higher overhead.

Also, some objects can't be created on stack because of memory constraints: thread stacks are relatively small. But boost::scoped_ptr is better in this case since it can't be accidentally released.

Answer 6

While banning auto_ptr seems attractive, but there is one issue:

template <typename T>
some_smart_ptr<T> create();

What will you replace the some_smart_ptr placeholder with ?

The generic answer, shared_ptr, is only worth it if the ownership is truly shared, if the function grants the caller exclusive ownership of the resources, it's misleading at best (and a typical case of premature pessimization as far as I am concerned).

On the other hand, in C++03, no other form of smart pointer can deliver: it's impossible, without move semantics, to provide what we'd like here. auto_ptr or a naked pointer are the two logical contenders. But then a naked pointer exposes you to the risk of leaks if the caller is careless.

With C++0x, unique_ptr advantageously replace auto_ptr in every situation.