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 was also std::auto_ptr
. It was very much like a scoped pointer, except that it also had the "special" dangerous ability to be copied — which also unexpectedly transfers ownership.
It was deprecated in C++11 and removed in C++17, so you shouldn't use it.
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.
OLD ANSWER
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()
{
{
std::unique_ptr<MyObject> ptr(new MyObject());
ptr->DoSomethingUseful();
} // 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 std::unique_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.
std::unique_ptr
s 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 {
std::shared_ptr<Owner> other;
};
std::shared_ptr<Owner> p1 (new Owner());
std::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
.
Best Answer
First, "ref-qualifiers for *this" is a just a "marketing statement". The type of
*this
never changes, see the bottom of this post. It's way easier to understand it with this wording though.Next, the following code chooses the function to be called based on the ref-qualifier of the "implicit object parameter" of the function†:
Output:
The whole thing is done to allow you to take advantage of the fact when the object the function is called on is an rvalue (unnamed temporary, for example). Take the following code as a further example:
This may be a bit contrived, but you should get the idea.
Note that you can combine the cv-qualifiers (
const
andvolatile
) and ref-qualifiers (&
and&&
).Note: Many standard quotes and overload resolution explanation after here!
† To understand how this works, and why @Nicol Bolas' answer is at least partly wrong, we have to dig in the C++ standard for a bit (the part explaining why @Nicol's answer is wrong is at the bottom, if you're only interested in that).
Which function is going to be called is determined by a process called overload resolution. This process is fairly complicated, so we'll only touch the bit that is important to us.
First, it's important to see how overload resolution for member functions works:
§13.3.1 [over.match.funcs]
Why do we even need to compare member and non-member functions? Operator overloading, that's why. Consider this:
You'd certainly want the following to call the free function, don't you?
That's why member and non-member functions are included in the so-called overload-set. To make the resolution less complicated, the bold part of the standard quote exists. Additionally, this is the important bit for us (same clause):
(The last bit just means that you can't cheat overload resolution based on implicit conversions of the object a member function (or operator) is called on.)
Let's take the first example at the top of this post. After the aforementioned transformation, the overload-set looks something like this:
Then the argument list, containing an implied object argument, is matched against the parameter-list of every function contained in the overload-set. In our case, the argument list will only contain that object argument. Let's see how that looks like:
If, after all overloads in the set are tested, only one remains, the overload resolution succeeded and the function linked to that transformed overload is called. The same goes for the second call to 'f':
Note however that, had we not provided any ref-qualifier (and as such not overloaded the function), that
f1
would match an rvalue (still§13.3.1
):Now, onto why @Nicol's answer is atleast partly wrong. He says:
That is wrong,
*this
is always an lvalue:§5.3.1 [expr.unary.op] p1
§9.3.2 [class.this] p1