What is the difference between the following set of pointer? When do you use each pointer in a production code, if at all?
Examples would be appreciated!
1.scoped_ptr
2.shared_ptr
3.weak_ptr
4.intrusive_ptr
Edit#1
Do you guys use boost in production code?
I found a very detailed explanation with code samples for 3 of the pointer types here. I found another site that shows weak_ptr with code examples here.
Both of those seem like excellent, detailed resources.
scoped_ptr is the simplest. When it goes out of scope, it is destroyed. The following code is illegal (scoped_ptrs are non-copyable) but will illustrate a point:
std::vector< scoped_ptr<T> > tPtrVec;
{
scoped_ptr<T> tPtr(new T());
tPtrVec.push_back(tPtr);
// raw T* is freed
}
tPtrVec[0]->DoSomething(); // accessing freed memory
shared_ptr is reference counted. Every time a copy or assignment occurs, the reference count is incremented. Every time an instance's destructor is fired, the reference count for the raw T* is decremented. Once it is 0, the pointer is freed.
std::vector< shared_ptr<T> > tPtrVec;
{
shared_ptr<T> tPtr(new T());
// This copy to tPtrVec.push_back and ultimately to the vector storage
// causes the reference count to go from 1->2
tPtrVec.push_back(tPtr);
// num references to T goes from 2->1 on the destruction of tPtr
}
tPtrVec[0]->DoSomething(); // raw T* still exists, so this is safe
weak_ptr is a weak-reference to a shared pointer that requires you to check to see if the pointed-to shared_ptr is still around
std::vector< weak_ptr<T> > tPtrVec;
{
shared_ptr<T> tPtr(new T());
tPtrVec.push_back(tPtr);
// num references to T goes from 1->0
}
shared_ptr<T> tPtrAccessed = tPtrVec[0].lock();
if (tPtrAccessed[0].get() == 0)
{
cout << "Raw T* was freed, can't access it"
}
else
{
tPtrVec[0]->DoSomething(); // raw
}
intrusive_ptr is typically used when there is a 3rd party smart ptr you must use. It will call a free function to add and decrement the reference count.See the link to boost documentation for more info.
Yes we do.
I second the advice about looking at the documentation. It is not as scary as it seems. And few short hints:
scoped_ptr - a pointer automatically deleted when it goes out of scope. Note - no assignment possible, but introduces no overheadintrusive_ptr - reference counting pointer with no overhead of smart_ptr. However the object itself stores the reference countweak_ptr - works together with shared_ptr to deal with the situations resulting in circular dependencies (read the documentation, and search on google for nice picture ;)shared_ptr - the generic, most powerful (and heavyweight) of the smart pointers (from the ones offered by boost)auto_ptr, that ensures that the object to which it points gets destroyed automatically when control leaves a scope. However it has different copy semantics than the rest of the guys.unique_ptr - will come with C++0xResponse to edit: Yes
Don't overlook boost::ptr_container in any survey of boost smart pointers. They can be invaluable in situations where a e.g std::vector<boost::shared_ptr<T> > would be too slow.
It's easy when you have properties that you can assign each smart pointer. There are two important properties.
The first means that only one smart pointer ever can point to the same object at the same time. If the smart pointer is to be returned from functions, the ownership is transferred to the returned smart pointer, for example.
The second means that multiple smart pointers can point to the same object the same time. This is what applies to a raw pointer to: Two pointers can point to the same object. Thus, this is what one would use by default, if there is no strict need for another kind of smart pointer.
Some smart pointer support neither the first nor the second. They can therefor not be returned from functions or passed somewhere else. Most suitable for RAII purposes where the smart pointer is kept local and is just created so it free's an object after it goes out of scope.
Share of ownership can be implemented by having a copy constructor. This naturally copies a smart pointer and both the copy and the original will reference the same object. Transfer of ownership can not be really implemented in C++ currently, because there is no means to transfer something from one object to another supported by the language: If you try to return an object from a function, what is happening is that the object is copied. So a smart pointer that implements transfer of ownership has to use the copy constructor to implement that transfer of ownership. However, this in turn breaks its usage in containers, because requirements state a certain behavior of the copy constructor of elements of containers which is incompatible with this so-called "moving constructor" behavior of these smart pointers.
C++1x provides native support for transfer-of-ownership by introducing so-called "move constructors" and "move assignment operators". It also comes with such a transfer-of-ownership smart pointer called unique_ptr.
scoped_ptr is a smart pointer that is neither transferable nor sharable. It's just usable if you locally need to allocate memory, but be sure it's freed again when it goes out of scope. But it can still be swapped with another scoped_ptr, if you wish to do so.
shared_ptr is a smart pointer that shares ownership (second kind above). It is reference counted so it can see when the last copy of it goes out of scope and then it frees the object managed.
weak_ptr is not really a smart pointer itself, but it is used to reference a manged object (managed by a shared_ptr) without adding a reference count. Normally, you would need to get the raw pointer out of the shared_ptr and copy that around. But that would not be safe, as you would not have a way to check when the object was actually deleted. So, weak_ptr provides a mean by referencing an object managed by shared_ptr. If you need to access the object, you can lock the management of it (to avoid that in another thread a shared_ptr frees it while you use the object) and then use it. If the weak_ptr points to an object already deleted, it will notice you by throwing an exception. Using weak_ptr is most beneficial when you have a cyclic reference: Reference counting cannot easily cope with such a situation.
intrusive_ptr is like a shared_ptr but it does not keep the reference count in a shared_ptr but leaves incrementing/decrementing the count to some helper functions that need to be defined by the object that is managed. This has the advantage that an already referenced object (which has a reference count incremented by an external reference counting mechanism) can be stuffed into an intrusive_ptr - because the reference count is not anymore internal to the smart pointer, but the smart pointer uses an existing reference counting mechanism. Often this will also provide a speed enhancement, because the reference count can be stored in the managed object itself (thus the name intrusive) which provides better cache locality.
unique_ptr is a transfer of ownership pointer. You cannot copy it, but you can move it by using C++1x's move constructors:
unique_ptr<type> p(new type);
unique_ptr<type> q(p); // not legal!
unique_ptr<type> r(move(p)); // legal. p is now empty, but r owns the object
unique_ptr<type> s(function_returning_a_uique_ptr()); // legal!
This is the semantic that std::auto_ptr obeys, but because of missing native support for moving, it fails to provide them without pitfalls. unique_ptr will automatically steal resources from a temporary other unique_ptr which is one of the key features of move semantics. auto_ptr will be deprecated in the next C++ Standard release in favor of unique_ptr. C++1x will also allow stuffing objects that are only movable but not copyable into containers. So you can stuff unique_ptr's into a vector for example. I'll stop here and reference you to a fine article about this if you want to read more about this.