Question about smart pointers in C++

Question

Is auto_ptr good to work with only for local variables?
If I need to work with classes too, do i need copied pointer ?

What would be the purpose of auto_ptr for local variables? Why would you need any different behavior for classes? Your question leaves more questions. — Sion Sheevok
– Sion Sheevok, Commented Mar 1, 2011 at 23:05
well,if i use auto_ptr for a function variable,when i exit the function doesnt the destructor being called ? — Adrian
– Adrian, Commented Mar 1, 2011 at 23:08
@vBx, yes, that's the entire purpose of smart pointers in general (which is part of what makes your question confusing). Note, however, that in recent compilers, std::auto_ptr has been deprecated in favor of std::unique_ptr. — ildjarn
– ildjarn, Commented Mar 1, 2011 at 23:18
From your comment, maybe you need to review the concept of scopes. I believe if you understood scoping better you would have the answer to your question. — nathan
– nathan, Commented Mar 1, 2011 at 23:19

gregg · Accepted Answer · 2011-03-02 16:34:58Z

3

The auto_ptr destructor does deallocate the memory, so you are correct. Once you leave the auto_ptr's scope the thing you're pointing to will go away. You might be looking for something like a shared_ptr, which is a reference counted smart pointer. It will be part of the next C++ standard. Boost has an implementation of it, and it's also part of the TR1 library. If your compiler supports std::tr1 entities then you should have the shared_ptr.

Edit

As pointed out in comments, auto_ptr copy semantics cause a transfer of ownership that does not necessarily result in deletion of the object. So, an auto_ptr type variable can be assigned to another, and could be used as a function return value. The key with auto_ptr is that only one of them at a time can reference a particular entity.

I think I was assigning the traits of scoped_ptr to auto_ptr incorrectly and a little unfairly. My own bias is against auto_ptr because that transfer of ownership causes a side effect on the source object that is not normally associated with copying.

edited Mar 2, 2011 at 16:34

answered Mar 1, 2011 at 23:22

gregg

1,0275 silver badges6 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

ildjarn Over a year ago

Note that boost::scoped_ptr or std::unique_ptr are the apples-to-apples proper replacements for std::auto_ptr.

Matteo Italia Over a year ago

actually std::auto_ptr's copy constructor implements ownership transfer semantic, so if you return a local auto_ptr the ownership will be transferred to the returned instance of auto_ptr i.e. to the caller.

gregg Over a year ago

@matteo: I stand corrected. Thank you. Will edit to reflect that knowledge.

Max Lybbert · Accepted Answer · 2011-03-01 23:30:50Z

0

You aren't required to use pointers in C++ in many cases, so you may not need any kind of smart pointer:

struct Foo {
    int bar;
    int twice_bar()
    {
        return 2 * bar;
    }
};

int twice_x(int x)
{
    Foo f;
    f.bar = x;
    return f.twice_bar();
}

answered Mar 1, 2011 at 23:30

Max Lybbert

20.2k5 gold badges50 silver badges69 bronze badges

Collectives™ on Stack Overflow

Question about smart pointers in C++

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related