What is a smart pointer and when should I use one?
什么是智能指针，我应该什么时候使用它？
A smart pointer is a class that wraps a 'raw' (or 'bare') C++ pointer, to manage the lifetime of the object being pointed to. There is no single smart pointer type, but all of them try to abstract a raw pointer in a practical way.
Smart pointers should be preferred over raw pointers. If you feel you need to use pointers (first consider if you really do), you would normally want to use a smart pointer as this can alleviate many of the problems with raw pointers, mainly forgetting to delete the object and leaking memory.
With raw pointers, the programmer has to explicitly destroy the object when it is no longer useful.
// Need to create the object to achieve some goal
MyObject* ptr = new MyObject();
ptr->DoSomething(); // Use the object in some way
delete ptr; // Destroy the object. Done with it.
// Wait, what if DoSomething() raises an exception...?
A smart pointer by comparison defines a policy as to when the object is destroyed. You still have to create the object, but you no longer have to worry about destroying it.
SomeSmartPtr<MyObject> ptr(new MyObject());
ptr->DoSomething(); // Use the object in some way.
// Destruction of the object happens, depending
// on the policy the smart pointer class uses.
// Destruction would happen even if DoSomething()
// raises an exception
The simplest policy in use involves the scope of the smart pointer wrapper object, such as implemented by boost::scoped_ptr or std::unique_ptr .
void f()
{
{
boost::scoped_ptr<MyObject> ptr(new MyObject());
ptr->DoSomethingUseful();
} // boost::scopted_ptr goes out of scope --
// the MyObject is automatically destroyed.
// ptr->Oops(); // Compile error: "ptr" not defined
// since it is no longer in scope.
}
Note that scoped_ptr instances cannot be copied. This prevents the pointer from being deleted multiple times (incorrectly). You can, however, pass references to it around to other functions you call.
Scoped pointers are useful when you want to tie the lifetime of the object to a particular block of code, or if you embedded it as member data inside another object, the lifetime of that other object. The object exists until the containing block of code is exited, or until the containing object is itself destroyed.
A more complex smart pointer policy involves reference counting the pointer. This does allow the pointer to be copied. When the last "reference" to the object is destroyed, the object is deleted. This policy is implemented by boost::shared_ptr and std::shared_ptr .
void f()
{
typedef std::shared_ptr<MyObject> MyObjectPtr; // nice short alias
MyObjectPtr p1; // Empty
{
MyObjectPtr p2(new MyObject());
// There is now one "reference" to the created object
p1 = p2; // Copy the pointer.
// There are now two references to the object.
} // p2 is destroyed, leaving one reference to the object.
} // p1 is destroyed, leaving a reference count of zero.
// The object is deleted.
Reference counted pointers are very useful when the lifetime of your object is much more complicated, and is not tied directly to a particular section of code or to another object.
There is one drawback to reference counted pointers — the possibility of creating a dangling reference:
// Create the smart pointer on the heap
MyObjectPtr* pp = new MyObjectPtr(new MyObject())
// Hmm, we forgot to destroy the smart pointer,
// because of that, the object is never destroyed!
Another possibility is creating circular references:
struct Owner {
boost::shared_ptr<Owner> other;
};
boost::shared_ptr<Owner> p1 (new Owner());
boost::shared_ptr<Owner> p2 (new Owner());
p1->other = p2; // p1 references p2
p2->other = p1; // p2 references p1
// Oops, the reference count of of p1 and p2 never goes to zero!
// The objects are never destroyed!
To work around this problem, both Boost and C++11 have defined a weak_ptr to define a weak (uncounted) reference to a shared_ptr .
UPDATE
This answer is rather old, and so describes what was 'good' at the time, which was smart pointers provided by the Boost library. Since C++11, the standard library has provided sufficient smart pointers types, and so you should favour the use of std::unique_ptr , std::shared_ptr and std::weak_ptr .
There is also std::auto_ptr . It is very much like a scoped pointer, except that it also has the "special" dangerous ability to be copied — which also unexpectedly transfers ownership! It is deprecated in the newest standards, so you shouldn't use it. Use the std::unique_ptr instead.
std::auto_ptr<MyObject> p1 (new MyObject());
std::auto_ptr<MyObject> p2 = p1; // Copy and transfer ownership.
// p1 gets set to empty!
p2->DoSomething(); // Works.
p1->DoSomething(); // Oh oh. Hopefully raises some NULL pointer exception.
Here's a simple answer for these days of modern C++:
It's a type of value which can be used like a pointer, but provides the additional feature of automatic memory management: When the pointer is no longer in use, the memory it points to is deallocated (see also the more detailed definition on Wikipedia).
In code which involves tracking the ownership of a piece of memory, allocating or de-allocating; the smart pointer often saves you the need to do these things explicitly.
std::unique_ptr when you don't intend to hold multiple references to the same object. For example, use it for a pointer to memory which gets allocated on entering some scope and de-allocated on exiting the scope. std::shared_ptr when you do want to refer to your object from multiple places - and do not want it to be de-allocated until all these references are themselves gone. std::weak_ptr when you do want to refer to your object from multiple places - for those references for which it's ok to ignore and deallocate (so they'll just note the object is gone when you try to dereference). boost:: smart pointers or std::auto_ptr except in special cases which you can read up on if you must. Ah, but you really wanted to, admit it.
Mostly in code that is oblivious to memory ownership. This would typically be in functions which get a pointer from someplace else and do not allocate, de-allocate or store a copy of the pointer which outlasts their execution.
Smart pointer is a pointer-like type with some additional functionality, eg automatic memory deallocation, reference counting etc.
Small intro is available on page Smart Pointers - What, Why, Which?.
One of the simple smart-pointer type is std::auto_ptr (chapter 20.4.5 of C++ standard), which allows to deallocate memory automatically when it out of scope and which is more robust than simple pointer usage when exceptions are thrown, although less flexible.
Another convenient type is boost::shared_ptr which implements reference counting and automatically deallocates memory when no references to object remains. This helps avoiding memory leaks and is easy to use to implement RAII.
Subject is covered in depth in book "C++ Templates: The Complete Guide" by David Vandevoorde, Nicolai M. Josuttis, chapter Chapter 20. Smart Pointers. Some topics covered:
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