Smart pointers usage

I have a project and I want make smart pointers usage better.

The main idea is to use them when returning new object from function. The question is what smart pointer to use? auto_ptr or shared_ptr from boost? As I know, auto_ptr is slower but it can fall back to the 'pure' pointer.

And if I'll use smart pointer in place where I don't need it, would it make the perfomance slower?

Answers


What makes you think auto_ptr is slower than shared_ptr? Typically I would expect the reverse to be true, since shared_ptr needs to update the reference count.

As for which you should use, different smart pointers imply different ownership semantics. Ownership implies the responsibility to delete the object when it is no longer needed.

  • A raw pointer implies no ownership; a program that uses smart pointers correctly may still make use of raw pointers in a lot of places where ownership is not intended (for example, if you need to pass an optional reference to an object into a function, you would often use a raw pointer).
  • scoped_ptr implies single (ie, non-shared), non-transferable ownership.
  • auto_ptr implies single (ie, non-shared) transferable ownership. This is the smart pointer I would use to return a newly constructed object from a function (the function is transferring the object to its caller). auto_ptr suffers from the disadvantage that due to limitations of the language when auto_ptr was defined, it is difficult to use correctly (this has given it a very bad reputation, though the intended purpose of a smart pointer with single, transferable ownership semantics was and is both valid and useful).
  • unique_ptr has the same semantics as auto_ptr, but uses new C++0x features (rvalue references) to make it a lot safer (less prone to incorrect use) than auto_ptr. If you are developing on a platform where unique_ptr is available, then you should use it instead of auto_ptr.
  • shared_ptr implies shared ownership. In my opinion this is over-used. It does have many valid uses, but it should not simply be used as a default option.

I would also add that shared_ptr is often used with STL containers because the other smart pointer classes do not achieve their intended function in that context (due to copying of values internally within the container). This often leads to use of shared_ptr where shared ownership is not really the intended meaning. In these cases, I suggest (where possible) using the boost pointer-container classes (ptr_vector, ptr_map and so on), which provide the (commonly desired) semantics of a container with transferable, but single (non-shared) ownership.

You should always think about the ownership of your objects: in most cases, with a clean system design, each object has one obvious owner, and ownership does not need to be shared. This has the advantage that it is easy to see exactly where and when objects will be freed, and no reference counting overhead is needed.

[edited to note the new unique_ptr]


You probably should use shared_ptr<>. It's hard to be more specific without knowing what exactly you want to do. Best read its documentation and see if it does what you need.

The performance difference will most likely be negligible. Only in extreme cases there might me an noticeable impact, like when copying these pointers many million times each second.


I prefer shared_ptr, auto_ptr can cause a lot of trouble and its use is not too intuitive. If you expect this object to be inserted on a STL container, so you certainly want to use shared_ptr.

Abot the performance you have a cost, but this is minimal and you can ignore it most of the time.


Depends. Shared pointers have a better use than auto_ptr which have the unusual characteristic of changing ownership on assignments. Also auto_ptr can not be used in containers. Also you can not use auto_ptr as return values if you do not want to transfer ownership. Shared pointers have all the benefits of smart pointers, have overloaded the relevant operators to act like a pointer and can be used in containers. Having said that, they are not cheap to use. You must analyze your needs to decide if you actually gain something by avoiding the shared_pointer implementation overheads


Use only shared_ptr. With auto_ptr you can have ONLY ONE reference to your object. Also auto_ptr isn't slower it must work faster than shared_ptr.

To not ask such questions you need to know, how this smart pointers work.

auto_ptr just storing pointer to your object and destroying it in it's destructor.

The problem of auto_ptr is that when you are trying to copy it it's stopping to point to your object.

For example

auto_ptr a_ptr(new someClass);

auto_ptr another_ptr=aptr;// after this another_ptr is pointing to your class, but a_ptr isn't pointing to it anymore!

This is why I don't recomend you to use auto_ptr.

Shared pointer counting how much smart pointers are pointing to your object and destroying your object when there is no more pointers to it. That's why you can have more than 1 pointer pointing to your object.

But Shared pointer also isn't perfect. And if in your program you have cyclic graph (when you have classes A and B and A have a member shared_ptr wich points to B and B have or B's member objects have shared_ptr pointing to A) than A and B will never deleted and you will have memory lick.

To write correct code with shared_ptr you need to be careful and also use weak_ptr. For more information look at here http://www.boost.org/doc/libs/1_45_0/libs/smart_ptr/smart_ptr.htm


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