Does C++11 have something equivalent to boost::intrusive_ptr?
My problem is that I have a C-style interface over my C++ code. Both sides of the interface can use C++, but exposing the C interface is required for compatibility reasons. I cannot use std::shared_ptr because I have to manage the object through two (or more) smart pointers. I am unable to figure out a solution with something like boost::intrusive_ptr.
Does c++11 have something equivalent to boost::intrusive_ptr?
No.
It does have std::make_shared which means std::shared_ptr is almost (see note below) as efficient as an intrusive smart pointer, because the reference counts will be stored adjacent in memory to the object itself, improving locality of reference and cache usage. It also provides std::enable_shared_from_this which allows you to retrieve a std::shared_ptr when you only have a built-in pointer to an object owned by a shared_ptr, but that doesn't allow you to manage the object using different smart pointer types.
shared_ptr expects to be entirely responsible for managing the object. A different smart pointer type might only manage the "strong" refcount and not the "weak" refcount, which would allow the counts to get out of sync and break the invariants of shared_ptr.
Note: Using make_shared allows shared_ptr to be almost as efficient as an intrusive pointer. The object and the reference counting info can be allocated in a single chunk of memory when make_shared is used, but there will still be two reference counts (for the "strong" and "weak" counts) which isn't the case for intrusive pointers as they don't support a weak_ptr. Also, the shared_ptr object itself always has to store two pointers (the one that will be returned by shared_ptr::get() and another pointer to the "control block" that contains the reference counts and knows the dynamic type of the owned object) so has a larger footprint than an intrusive pointer.
Related
Comparisons, Pros, Cons, and When to Use?
This is a spin-off from a garbage collection thread where what I thought was a simple answer generated a lot of comments about some specific smart pointer implementations so it seemed worth starting a new post.
Ultimately the question is what are the various implementations of smart pointers in C++ out there and how do they compare? Just simple pros and cons or exceptions and gotchas to something you might otherwise think should work.
I've posted some implementations that I've used or at least glossed over and considered using as an answer below and my understanding of their differences and similarities which may not be 100% accurate so feel free to fact check or correct me as needed.
The goal is to learn about some new objects and libraries or correct my usage and understanding of existing implementations already widely in use and end up with a decent reference for others.
C++03
std::auto_ptr - Perhaps one of the originals it suffered from first draft syndrome only providing limited garbage collection facilities. The first downside being that it calls delete upon destruction making them unacceptable for holding array allocated objects (new[]). It takes ownership of the pointer so two auto pointers shouldn't contain the same object. Assignment will transfer ownership and reset the rvalue auto pointer to a null pointer. Which leads to perhaps the worst drawback; they can't be used within STL containers due to the aforementioned inability to be copied. The final blow to any use case is they are slated to be deprecated in the next standard of C++.
std::auto_ptr_ref - This is not a smart pointer it's actually a design detail used in conjunction with std::auto_ptr to allow copying and assignment in certain situations. Specifically it can be used to convert a non-const std::auto_ptr to an lvalue using the Colvin-Gibbons trick also known as a move constructor to transfer ownership.
On the contrary perhaps std::auto_ptr wasn't really intended to be used as a general purpose smart pointer for automatic garbage collection. Most of my limited understanding and assumptions are based on Herb Sutter's Effective Use of auto_ptr and I do use it regularly although not always in the most optimized way.
C++11
std::unique_ptr - This is our friend who will be replacing std::auto_ptr it will be quite similar except with the key improvements to correct the weaknesses of std::auto_ptr like working with arrays, lvalue protection via private copy constructor, being usable with STL containers and algorithms, etc. Since it's performance overhead and memory footprint are limited this is an ideal candidate for replacing, or perhaps more aptly described as owning, raw pointers. As the "unique" implies there is only one owner of the pointer just like the previous std::auto_ptr.
std::shared_ptr - I believe this is based off TR1 and boost::shared_ptr but improved to include aliasing and pointer arithmetic as well. In short it wraps a reference counted smart pointer around a dynamically allocated object. As the "shared" implies the pointer can be owned by more than one shared pointer when the last reference of the last shared pointer goes out of scope then the object will be deleted appropriately. These are also thread safe and can handle incomplete types in most cases. std::make_shared can be used to efficiently construct a std::shared_ptr with one heap allocation using the default allocator.
std::weak_ptr - Likewise based off TR1 and boost::weak_ptr. This is a reference to an object owned by a std::shared_ptr and will therefore not prevent the deletion of the object if the std::shared_ptr reference count drops to zero. In order to get access to the raw pointer you'll first need to access the std::shared_ptr by calling lock which will return an empty std::shared_ptr if the owned pointer has expired and been destroyed already. This is primarily useful to avoid indefinite hanging reference counts when using multiple smart pointers.
Boost
boost::shared_ptr - Probably the easiest to use in the most varying scenarios (STL, PIMPL, RAII, etc) this is a shared referenced counted smart pointer. I've heard a few complaints about performance and overhead in some situations but I must have ignored them because I can't remember what the argument was. Apparently it was popular enough to become a pending standard C++ object and no drawbacks over the norm regarding smart pointers come to mind.
boost::weak_ptr - Much like previous description of std::weak_ptr, based on this implementation, this allows a non-owning reference to a boost::shared_ptr. You not surprisingly call lock() to access the "strong" shared pointer and must check to make sure it's valid as it could have already been destroyed. Just make sure not to store the shared pointer returned and let it go out of scope as soon as you're done with it otherwise you're right back to the cyclic reference problem where your reference counts will hang and objects will not be destroyed.
boost::scoped_ptr - This is a simple smart pointer class with little overhead probably designed for a better performing alternative to boost::shared_ptr when usable. It's comparable to std::auto_ptr especially in the fact that it can't be safely used as an element of a STL container or with multiple pointers to the same object.
boost::intrusive_ptr - I've never used this but from my understanding it's designed to be used when creating your own smart pointer compatible classes. You need to implement the reference counting yourself, you'll also need to implement a few methods if you want your class to be generic, furthermore you'd have to implement your own thread safety. On the plus side this probably gives you the most custom way of picking and choosing exactly how much or how little "smartness" you want. intrusive_ptr is typically more efficient than shared_ptr since it allows you to have a single heap allocation per object. (thanks Arvid)
boost::shared_array - This is a boost::shared_ptr for arrays. Basically new [], operator[], and of course delete [] are baked in. This can be used in STL containers and as far as I know does everything boost:shared_ptr does although you can't use boost::weak_ptr with these. You could however alternatively use a boost::shared_ptr<std::vector<>> for similar functionality and to regain the ability to use boost::weak_ptr for references.
boost::scoped_array - This is a boost::scoped_ptr for arrays. As with boost::shared_array all the necessary array goodness is baked in. This one is non-copyable and so can't be used in STL containers. I've found almost anywhere you find yourself wanting to use this you probably could just use std::vector. I've never determined which is actually faster or has less overhead but this scoped array seems far less involved than a STL vector. When you want to keep allocation on the stack consider boost::array instead.
Qt
QPointer - Introduced in Qt 4.0 this is a "weak" smart pointer which only works with QObject and derived classes, which in the Qt framework is almost everything so that's not really a limitation. However there are limitations namely that it doesn't supply a "strong" pointer and although you can check if the underlying object is valid with isNull() you could find your object being destroyed right after you pass that check especially in multi-threaded environments. Qt people consider this deprecated I believe.
QSharedDataPointer - This is a "strong" smart pointer potentially comparable to boost::intrusive_ptr although it has some built in thread safety but it does require you to include reference counting methods (ref and deref) which you can do by subclassing QSharedData. As with much of Qt the objects are best used through ample inheritance and subclassing everything seems to be the intended design.
QExplicitlySharedDataPointer - Very similar to QSharedDataPointer except it doesn't implicitly call detach(). I'd call this version 2.0 of QSharedDataPointer as that slight increase in control as to exactly when to detach after the reference count drops to zero isn't particularly worth a whole new object.
QSharedPointer - Atomic reference counting, thread safe, sharable pointer, custom deletes (array support), sounds like everything a smart pointer should be. This is what I primarily use as a smart pointer in Qt and I find it comparable with boost:shared_ptr although probably significantly more overhead like many objects in Qt.
QWeakPointer - Do you sense a reoccurring pattern? Just as std::weak_ptr and boost::weak_ptr this is used in conjunction with QSharedPointer when you need references between two smart pointers that would otherwise cause your objects to never be deleted.
QScopedPointer - This name should also look familiar and actually was in fact based on boost::scoped_ptr unlike the Qt versions of shared and weak pointers. It functions to provide a single owner smart pointer without the overhead of QSharedPointer which makes it more suitable for compatibility, exception safe code, and all the things you might use std::auto_ptr or boost::scoped_ptr for.
There is also Loki which implements policy-based smart pointers.
Other references on policy-based smart pointers, addressing the problem of the poor support of the empty base optimization along with multiple inheritance by many compilers:
Smart Pointers Reloaded
A Proposal to Add a Policy-Based Smart Pointer Framework to the Standard Library
In addition to the ones given, there are some safety oriented ones too:
SaferCPlusPlus
mse::TRefCountingPointer is a reference counting smart pointer like std::shared_ptr. The difference being that mse::TRefCountingPointer is safer, smaller and faster, but does not have any thread safety mechanism. And it comes in "not null" and "fixed" (non-retargetable) versions that can be safely assumed to always be pointing to a validly allocated object. So basically, if your target object is shared among asynchronous threads then use std::shared_ptr, otherwise mse::TRefCountingPointer is more optimal.
mse::TScopeOwnerPointer is similar to boost::scoped_ptr, but works in conjunction with mse::TScopeFixedPointer in a "strong-weak" pointer relationship kind of like std::shared_ptr and std::weak_ptr.
mse::TScopeFixedPointer points to objects that are allocated on the stack, or whose "owning" pointer is allocated on the stack. It is (intentionally) limited in it's functionality to enhance compile-time safety with no runtime cost. The point of "scope" pointers is essentially to identify a set of circumstances that are simple and deterministic enough that no (runtime) safety mechanisms are necessary.
mse::TRegisteredPointer behaves like a raw pointer, except that its value is automatically set to null_ptr when the target object is destroyed. It can be used as a general replacement for raw pointers in most situations. Like a raw pointer, it does not have any intrinsic thread safety. But in exchange it has no problem targeting objects allocated on the stack (and obtaining the corresponding performance benefit). When run-time checks are enabled, this pointer is safe from accessing invalid memory. Because mse::TRegisteredPointer has the same behavior as a raw pointer when pointing to valid objects, it can be "disabled" (automatically replaced with the corresponding raw pointer) with a compile-time directive, allowing it to be used to help catch bugs in debug/test/beta modes while incurring no overhead cost in release mode.
Here is an article describing why and how to use them. (Note, shameless plug.)
Ok, so the last time I wrote C++ for a living, std::auto_ptr was all the std lib had available, and boost::shared_ptr was all the rage. I never really looked into the other smart pointer types boost provided. I understand that C++11 now provides some of the types boost came up with, but not all of them.
So does someone have a simple algorithm to determine when to use which smart pointer? Preferably including advice regarding dumb pointers (raw pointers like T*) and the rest of the boost smart pointers. (Something like this would be great).
Shared ownership:
The shared_ptr and weak_ptr the standard adopted are pretty much the same as their Boost counterparts. Use them when you need to share a resource and don't know which one will be the last to be alive. Use weak_ptr to observe the shared resource without influencing its lifetime, not to break cycles. Cycles with shared_ptr shouldn't normally happen - two resources can't own each other.
Note that Boost additionally offers shared_array, which might be a suitable alternative to shared_ptr<std::vector<T> const>.
Next, Boost offers intrusive_ptr, which are a lightweight solution if your resource offers reference-counted management already and you want to adopt it to the RAII principle. This one was not adopted by the standard.
Unique ownership:
Boost also has a scoped_ptr, which is not copyable and for which you can not specify a deleter. std::unique_ptr is boost::scoped_ptr on steroids and should be your default choice when you need a smart pointer. It allows you to specify a deleter in its template arguments and is movable, unlike boost::scoped_ptr. It is also fully usable in STL containers as long as you don't use operations that need copyable types (obviously).
Note again, that Boost has an array version: scoped_array, which the standard unified by requiring std::unique_ptr<T[]> partial specialization that will delete[] the pointer instead of deleteing it (with the default_deleter). std::unique_ptr<T[]> also offers operator[] instead of operator* and operator->.
Note that std::auto_ptr is still in the standard, but it is deprecated.
§D.10 [depr.auto.ptr]
The class template auto_ptr is deprecated. [ Note: The class template unique_ptr (20.7.1) provides a better solution. —end note ]
No ownership:
Use dumb pointers (raw pointers) or references for non-owning references to resources and when you know that the resource will outlive the referencing object / scope. Prefer references and use raw pointers when you need either nullability or resettability.
If you want a non-owning reference to a resource, but you don't know if the resource will outlive the object that references it, pack the resource in a shared_ptr and use a weak_ptr - you can test if the parent shared_ptr is alive with lock, which will return a shared_ptr that is non-null if the resource still exists. If want to test whether the resource is dead, use expired. The two may sound similar, but are very different in the face of concurrent execution, as expired only guarantees its return value for that single statement. A seemingly innocent test like
if(!wptr.expired())
something_assuming_the_resource_is_still_alive();
is a potential race condition.
Deciding what smart pointer to use is a question of ownership. When it comes to resource management, object A owns object B if it is in control of the lifetime of object B. For example, member variables are owned by their respective objects because the lifetime of member variables is tied to the lifetime of the object. You choose smart pointers based on how the object is owned.
Note that ownership in a software system is separate from ownership as we would think of it outside of software. For example, a person might "own" their home, but that doesn't necessarily mean that a Person object has control over the lifetime of a House object. Conflating these real world concepts with software concepts is a sure-fire way to program yourself into a hole.
If you have sole ownership of the object, use std::unique_ptr<T>.
If you have shared ownership of the object...
- If there are no cycles in ownership, use std::shared_ptr<T>.
- If there are cycles, define a "direction" and use std::shared_ptr<T> in one direction and std::weak_ptr<T> in the other.
If the object owns you, but there is potential of having no owner, use normal pointers T* (e.g. parent pointers).
If the object owns you (or otherwise has guaranteed existence), use references T&.
Caveat: Be aware of the costs of smart pointers. In memory or performance limited environments, it could be beneficial to just use normal pointers with a more manual scheme for managing memory.
The costs:
If you have a custom deleter (e.g. you use allocation pools) then this will incur overhead per pointer that may be easily avoided by manual deletion.
std::shared_ptr has the overhead of a reference count increment on copy, plus a decrement on destruction followed by a 0-count check with deletion of the held object. Depending on the implementation, this can bloat your code and cause performance issues.
Compile time. As with all templates, smart pointers contribute negatively to compile times.
Examples:
struct BinaryTree
{
Tree* m_parent;
std::unique_ptr<BinaryTree> m_children[2]; // or use std::array...
};
A binary tree does not own its parent, but the existence of a tree implies the existence of its parent (or nullptr for root), so that uses a normal pointer. A binary tree (with value semantics) has sole ownership of its children, so those are std::unique_ptr.
struct ListNode
{
std::shared_ptr<ListNode> m_next;
std::weak_ptr<ListNode> m_prev;
};
Here, the list node owns its next and previous lists, so we define a direction and use shared_ptr for next and weak_ptr for prev to break the cycle.
Use unique_ptr<T> all the time except when you need reference counting, in which case use shared_ptr<T> (and for very rare cases, weak_ptr<T> to prevent reference cycles). In almost every case, transferrable unique ownership is just fine.
Raw pointers: Good only if you need covariant returns, non-owning pointing which can happen. They're not terrifically useful otherwise.
Array pointers: unique_ptr has a specialization for T[] which automatically calls delete[] on the result, so you can safely do unique_ptr<int[]> p(new int[42]); for example. shared_ptr you'd still need a custom deleter, but you wouldn't need a specialized shared or unique array pointer. Of course, such things are usually best replaced by std::vector anyway. Unfortunately shared_ptr does not provide an array access function, so you'd still have to manually call get(), but unique_ptr<T[]> provides operator[] instead of operator* and operator->. In any case, you have to bounds check yourself. This makes shared_ptr slightly less user-friendly, although arguably the generic advantage and no Boost dependency makes unique_ptr and shared_ptr the winners again.
Scoped pointers: Made irrelevant by unique_ptr, just like auto_ptr.
There's really nothing more to it. In C++03 without move semantics this situation was very complicated, but in C++11 the advice is very simple.
There are still uses for other smart pointers, like intrusive_ptr or interprocess_ptr. However, they're very niche and completely unnecessary in the general case.
Cases of when to use unique_ptr:
Factory methods
Members that are pointers (pimpl included)
Storing pointers in stl containters (to avoid moves)
Use of large local dynamic objects
Cases of when to use shared_ptr:
Sharing objects across threads
Sharing objects in general
Cases of when to use weak_ptr:
Large map that acts as a general reference (ex. a map of all open sockets)
Feel free to edit and add more
Ok, so the last time I wrote C++ for a living, std::auto_ptr was all the std lib had available, and boost::shared_ptr was all the rage. I never really looked into the other smart pointer types boost provided. I understand that C++11 now provides some of the types boost came up with, but not all of them.
So does someone have a simple algorithm to determine when to use which smart pointer? Preferably including advice regarding dumb pointers (raw pointers like T*) and the rest of the boost smart pointers. (Something like this would be great).
Shared ownership:
The shared_ptr and weak_ptr the standard adopted are pretty much the same as their Boost counterparts. Use them when you need to share a resource and don't know which one will be the last to be alive. Use weak_ptr to observe the shared resource without influencing its lifetime, not to break cycles. Cycles with shared_ptr shouldn't normally happen - two resources can't own each other.
Note that Boost additionally offers shared_array, which might be a suitable alternative to shared_ptr<std::vector<T> const>.
Next, Boost offers intrusive_ptr, which are a lightweight solution if your resource offers reference-counted management already and you want to adopt it to the RAII principle. This one was not adopted by the standard.
Unique ownership:
Boost also has a scoped_ptr, which is not copyable and for which you can not specify a deleter. std::unique_ptr is boost::scoped_ptr on steroids and should be your default choice when you need a smart pointer. It allows you to specify a deleter in its template arguments and is movable, unlike boost::scoped_ptr. It is also fully usable in STL containers as long as you don't use operations that need copyable types (obviously).
Note again, that Boost has an array version: scoped_array, which the standard unified by requiring std::unique_ptr<T[]> partial specialization that will delete[] the pointer instead of deleteing it (with the default_deleter). std::unique_ptr<T[]> also offers operator[] instead of operator* and operator->.
Note that std::auto_ptr is still in the standard, but it is deprecated.
§D.10 [depr.auto.ptr]
The class template auto_ptr is deprecated. [ Note: The class template unique_ptr (20.7.1) provides a better solution. —end note ]
No ownership:
Use dumb pointers (raw pointers) or references for non-owning references to resources and when you know that the resource will outlive the referencing object / scope. Prefer references and use raw pointers when you need either nullability or resettability.
If you want a non-owning reference to a resource, but you don't know if the resource will outlive the object that references it, pack the resource in a shared_ptr and use a weak_ptr - you can test if the parent shared_ptr is alive with lock, which will return a shared_ptr that is non-null if the resource still exists. If want to test whether the resource is dead, use expired. The two may sound similar, but are very different in the face of concurrent execution, as expired only guarantees its return value for that single statement. A seemingly innocent test like
if(!wptr.expired())
something_assuming_the_resource_is_still_alive();
is a potential race condition.
Deciding what smart pointer to use is a question of ownership. When it comes to resource management, object A owns object B if it is in control of the lifetime of object B. For example, member variables are owned by their respective objects because the lifetime of member variables is tied to the lifetime of the object. You choose smart pointers based on how the object is owned.
Note that ownership in a software system is separate from ownership as we would think of it outside of software. For example, a person might "own" their home, but that doesn't necessarily mean that a Person object has control over the lifetime of a House object. Conflating these real world concepts with software concepts is a sure-fire way to program yourself into a hole.
If you have sole ownership of the object, use std::unique_ptr<T>.
If you have shared ownership of the object...
- If there are no cycles in ownership, use std::shared_ptr<T>.
- If there are cycles, define a "direction" and use std::shared_ptr<T> in one direction and std::weak_ptr<T> in the other.
If the object owns you, but there is potential of having no owner, use normal pointers T* (e.g. parent pointers).
If the object owns you (or otherwise has guaranteed existence), use references T&.
Caveat: Be aware of the costs of smart pointers. In memory or performance limited environments, it could be beneficial to just use normal pointers with a more manual scheme for managing memory.
The costs:
If you have a custom deleter (e.g. you use allocation pools) then this will incur overhead per pointer that may be easily avoided by manual deletion.
std::shared_ptr has the overhead of a reference count increment on copy, plus a decrement on destruction followed by a 0-count check with deletion of the held object. Depending on the implementation, this can bloat your code and cause performance issues.
Compile time. As with all templates, smart pointers contribute negatively to compile times.
Examples:
struct BinaryTree
{
Tree* m_parent;
std::unique_ptr<BinaryTree> m_children[2]; // or use std::array...
};
A binary tree does not own its parent, but the existence of a tree implies the existence of its parent (or nullptr for root), so that uses a normal pointer. A binary tree (with value semantics) has sole ownership of its children, so those are std::unique_ptr.
struct ListNode
{
std::shared_ptr<ListNode> m_next;
std::weak_ptr<ListNode> m_prev;
};
Here, the list node owns its next and previous lists, so we define a direction and use shared_ptr for next and weak_ptr for prev to break the cycle.
Use unique_ptr<T> all the time except when you need reference counting, in which case use shared_ptr<T> (and for very rare cases, weak_ptr<T> to prevent reference cycles). In almost every case, transferrable unique ownership is just fine.
Raw pointers: Good only if you need covariant returns, non-owning pointing which can happen. They're not terrifically useful otherwise.
Array pointers: unique_ptr has a specialization for T[] which automatically calls delete[] on the result, so you can safely do unique_ptr<int[]> p(new int[42]); for example. shared_ptr you'd still need a custom deleter, but you wouldn't need a specialized shared or unique array pointer. Of course, such things are usually best replaced by std::vector anyway. Unfortunately shared_ptr does not provide an array access function, so you'd still have to manually call get(), but unique_ptr<T[]> provides operator[] instead of operator* and operator->. In any case, you have to bounds check yourself. This makes shared_ptr slightly less user-friendly, although arguably the generic advantage and no Boost dependency makes unique_ptr and shared_ptr the winners again.
Scoped pointers: Made irrelevant by unique_ptr, just like auto_ptr.
There's really nothing more to it. In C++03 without move semantics this situation was very complicated, but in C++11 the advice is very simple.
There are still uses for other smart pointers, like intrusive_ptr or interprocess_ptr. However, they're very niche and completely unnecessary in the general case.
Cases of when to use unique_ptr:
Factory methods
Members that are pointers (pimpl included)
Storing pointers in stl containters (to avoid moves)
Use of large local dynamic objects
Cases of when to use shared_ptr:
Sharing objects across threads
Sharing objects in general
Cases of when to use weak_ptr:
Large map that acts as a general reference (ex. a map of all open sockets)
Feel free to edit and add more
I have heard that auto pointers own their object whereas shared pointers can have many objects pointing to them. Why dont we use shared pointers all the time.
In relation to this what are smart pointers, people use this term interchangeably with shared pointers. Are they the same?
std::auto_ptr is an outdated, deprecated implementation of exclusive pointer ownership. It's been replaced by std::unique_ptr in C++11. Exclusive ownership means that the pointer is owned by something, and the lifetime of the object is tied to the lifetime of the owner.
Shared pointers (std::shared_ptr) implement shared pointer ownership — they keep the object alive as long as there are alive references to it, because there is no single owner. It's usually done with reference counting, which means they have additional runtime overhead as opposed to unique pointers. Also reasoning about shared ownership is more difficult than reasoning about exclusive ownership — the point of destruction becomes less deterministic.
Smart pointer is a term that encompasses all types that behave like pointers, but with added (smart) semantics, as opposed to raw T*. Both unique_ptr and shared_ptr are smart pointers.
Shared pointers are slightly more costly as they hold a reference count.
In some case, if you have a complex structure with shared pointer at multiple recursive levels, one change can touch the reference count of many of those pointers.
Also in multiple CPU core architectures, the atomic update of a reference count might become not slightly costly at least, but actually really costly, if the multiple core are currently accessing the same memory area.
However shared pointers are simple and safe to use, whereas the assignment properties of auto pointers is confusing, and can become really dangerous.
Smart pointers usually is frequently used just as a synonym of shared pointer, but actually covers all the various pointers implementation in boost, including the one that's similar to shared pointers.
There can be many forms of smart pointers. Boost inspired shared_ptr which is now in C++11 is one of them. I suggest using shared_ptr at almost all the places when in doubt instead of auto_ptr which has many quirks.
In short, shared_ptr is just a reference counting implementation to share same object.
Refer:
http://www.gotw.ca/publications/using_auto_ptr_effectively.htm
http://en.cppreference.com/w/cpp/memory/shared_ptr
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?
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