Related
What is a smart pointer and when should I use one?
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_ptrs 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.
Here's a simple answer for these days of modern C++ (C++11 and later):
"What is a smart pointer?"
It's a type whose values can be used like pointers, but which provides the additional feature of automatic memory management: When a smart pointer is no longer in use, the memory it points to is deallocated (see also the more detailed definition on Wikipedia).
"When should I use one?"
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.
"But which smart pointer should I use in which of those cases?"
Use std::unique_ptr when you want your object to live just as long as a single owning reference to it lives. For example, use it for a pointer to memory which gets allocated on entering some scope and de-allocated on exiting the scope.
Use std::shared_ptr when you do want to refer to your object from multiple places - and do not want your object to be de-allocated until all these references are themselves gone.
Use 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).
There is a proposal to add hazard pointers to C++26, but for now you don't have them.
Don't use the boost:: smart pointers or std::auto_ptr except in special cases which you can read up on if you must.
"Hey, I didn't ask which one to use!"
Ah, but you really wanted to, admit it.
"So when should I use regular pointers then?"
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 nor de-allocate, and do not store a copy of the pointer which outlasts their execution.
UPDATE:
This answer is outdated concerning C++ types which were used in the past.
std::auto_ptr is deprecated and removed in new standards.
Instead of boost::shared_ptr the std::shared_ptr should be used which is part of the standard.
The links to the concepts behind the rationale of smart pointers still mostly relevant.
Modern C++ has the following smart pointer types and doesn't require boost smart pointers:
std::shared_ptr
std::weak_ptr
std::unique_ptr
There is also 2-nd edition of the book mentioned in the answer: C++ Templates: The Complete Guide 2nd Edition by David Vandevoorde Nicolai, M. Josuttis, Douglas Gregor
OLD ANSWER:
A smart pointer is a pointer-like type with some additional functionality, e.g. automatic memory deallocation, reference counting etc.
A small intro is available on the page Smart Pointers - What, Why, Which?.
One of the simple smart-pointer types is std::auto_ptr (chapter 20.4.5 of C++ standard), which allows one 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 the object remains. This helps avoiding memory leaks and is easy to use to implement RAII.
The 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:
Protecting Against Exceptions
Holders, (note, std::auto_ptr is implementation of such type of smart pointer)
Resource Acquisition Is Initialization (This is frequently used for exception-safe resource management in C++)
Holder Limitations
Reference Counting
Concurrent Counter Access
Destruction and Deallocation
Definitions provided by Chris, Sergdev and Llyod are correct. I prefer a simpler definition though, just to keep my life simple:
A smart pointer is simply a class that overloads the -> and * operators. Which means that your object semantically looks like a pointer but you can make it do way cooler things, including reference counting, automatic destruction etc.
shared_ptr and auto_ptr are sufficient in most cases, but come along with their own set of small idiosyncrasies.
A smart pointer is like a regular (typed) pointer, like "char*", except when the pointer itself goes out of scope then what it points to is deleted as well. You can use it like you would a regular pointer, by using "->", but not if you need an actual pointer to the data. For that, you can use "&*ptr".
It is useful for:
Objects that must be allocated with new, but that you'd like to have the same lifetime as something on that stack. If the object is assigned to a smart pointer, then they will be deleted when the program exits that function/block.
Data members of classes, so that when the object is deleted all the owned data is deleted as well, without any special code in the destructor (you will need to be sure the destructor is virtual, which is almost always a good thing to do).
You may not want to use a smart pointer when:
... the pointer shouldn't actually own the data... i.e., when you are just using the data, but you want it to survive the function where you are referencing it.
... the smart pointer isn't itself going to be destroyed at some point. You don't want it to sit in memory that never gets destroyed (such as in an object that is dynamically allocated but won't be explicitly deleted).
... two smart pointers might point to the same data. (There are, however, even smarter pointers that will handle that... that is called reference counting.)
See also:
garbage collection.
This stack overflow question regarding data ownership
A smart pointer is an object that acts like a pointer, but additionally provides control on construction, destruction, copying, moving and dereferencing.
One can implement one's own smart pointer, but many libraries also provide smart pointer implementations each with different advantages and drawbacks.
For example, Boost provides the following smart pointer implementations:
shared_ptr<T> is a pointer to T using a reference count to determine when the object is no longer needed.
scoped_ptr<T> is a pointer automatically deleted when it goes out of scope. No assignment is possible.
intrusive_ptr<T> is another reference counting pointer. It provides better performance than shared_ptr, but requires the type T to provide its own reference counting mechanism.
weak_ptr<T> is a weak pointer, working in conjunction with shared_ptr to avoid circular references.
shared_array<T> is like shared_ptr, but for arrays of T.
scoped_array<T> is like scoped_ptr, but for arrays of T.
These are just one linear descriptions of each and can be used as per need, for further detail and examples one can look at the documentation of Boost.
Additionally, the C++ standard library provides three smart pointers; std::unique_ptr for unique ownership, std::shared_ptr for shared ownership and std::weak_ptr. std::auto_ptr existed in C++03 but is now deprecated.
Most kinds of smart pointers handle disposing of the pointer-to object for you. It's very handy because you don't have to think about disposing of objects manually anymore.
The most commonly-used smart pointers are std::tr1::shared_ptr (or boost::shared_ptr), and, less commonly, std::auto_ptr. I recommend regular use of shared_ptr.
shared_ptr is very versatile and deals with a large variety of disposal scenarios, including cases where objects need to be "passed across DLL boundaries" (the common nightmare case if different libcs are used between your code and the DLLs).
Here is the Link for similar answers : http://sickprogrammersarea.blogspot.in/2014/03/technical-interview-questions-on-c_6.html
A smart pointer is an object that acts, looks and feels like a normal pointer but offers more functionality. In C++, smart pointers are implemented as template classes that encapsulate a pointer and override standard pointer operators. They have a number of advantages over regular pointers. They are guaranteed to be initialized as either null pointers or pointers to a heap object. Indirection through a null pointer is checked. No delete is ever necessary. Objects are automatically freed when the last pointer to them has gone away. One significant problem with these smart pointers is that unlike regular pointers, they don't respect inheritance. Smart pointers are unattractive for polymorphic code. Given below is an example for the implementation of smart pointers.
Example:
template <class X>
class smart_pointer
{
public:
smart_pointer(); // makes a null pointer
smart_pointer(const X& x) // makes pointer to copy of x
X& operator *( );
const X& operator*( ) const;
X* operator->() const;
smart_pointer(const smart_pointer <X> &);
const smart_pointer <X> & operator =(const smart_pointer<X>&);
~smart_pointer();
private:
//...
};
This class implement a smart pointer to an object of type X. The object itself is located on the heap. Here is how to use it:
smart_pointer <employee> p= employee("Harris",1333);
Like other overloaded operators, p will behave like a regular pointer,
cout<<*p;
p->raise_salary(0.5);
Let T be a class in this tutorial
Pointers in C++ can be divided into 3 types :
1) Raw pointers :
T a;
T * _ptr = &a;
They hold a memory address to a location in memory. Use with caution , as programs become complex hard to keep track.
Pointers with const data or address { Read backwards }
T a ;
const T * ptr1 = &a ;
T const * ptr1 = &a ;
Pointer to a data type T which is a const. Meaning you cannot change the data type using the pointer. ie *ptr1 = 19 ; will not work. But you can move the pointer. ie ptr1++ , ptr1-- ; etc will work.
Read backwards : pointer to type T which is const
T * const ptr2 ;
A const pointer to a data type T . Meaning you cannot move the pointer but you can change the value pointed to by the pointer. ie *ptr2 = 19 will work but ptr2++ ; ptr2-- etc will not work. Read backwards : const pointer to a type T
const T * const ptr3 ;
A const pointer to a const data type T . Meaning you cannot either move the pointer nor can you change the data type pointer to be the pointer. ie . ptr3-- ; ptr3++ ; *ptr3 = 19; will not work
3) Smart Pointers : { #include <memory> }
Shared Pointer:
T a ;
//shared_ptr<T> shptr(new T) ; not recommended but works
shared_ptr<T> shptr = make_shared<T>(); // faster + exception safe
std::cout << shptr.use_count() ; // 1 // gives the number of "
things " pointing to it.
T * temp = shptr.get(); // gives a pointer to object
// shared_pointer used like a regular pointer to call member functions
shptr->memFn();
(*shptr).memFn();
//
shptr.reset() ; // frees the object pointed to be the ptr
shptr = nullptr ; // frees the object
shptr = make_shared<T>() ; // frees the original object and points to new object
Implemented using reference counting to keep track of how many " things " point to the object pointed to by the pointer. When this count goes to 0 , the object is automatically deleted , ie objected is deleted when all the share_ptr pointing to the object goes out of scope.
This gets rid of the headache of having to delete objects which you have allocated using new.
Weak Pointer :
Helps deal with cyclic reference which arises when using Shared Pointer
If you have two objects pointed to by two shared pointers and there is an internal shared pointer pointing to each others shared pointer then there will be a cyclic reference and the object will not be deleted when shared pointers go out of scope. To solve this , change the internal member from a shared_ptr to weak_ptr. Note : To access the element pointed to by a weak pointer use lock() , this returns a weak_ptr.
T a ;
shared_ptr<T> shr = make_shared<T>() ;
weak_ptr<T> wk = shr ; // initialize a weak_ptr from a shared_ptr
wk.lock()->memFn() ; // use lock to get a shared_ptr
// ^^^ Can lead to exception if the shared ptr has gone out of scope
if(!wk.expired()) wk.lock()->memFn() ;
// Check if shared ptr has gone out of scope before access
See : When is std::weak_ptr useful?
Unique Pointer :
Light weight smart pointer with exclusive ownership. Use when pointer points to unique objects without sharing the objects between the pointers.
unique_ptr<T> uptr(new T);
uptr->memFn();
//T * ptr = uptr.release(); // uptr becomes null and object is pointed to by ptr
uptr.reset() ; // deletes the object pointed to by uptr
To change the object pointed to by the unique ptr , use move semantics
unique_ptr<T> uptr1(new T);
unique_ptr<T> uptr2(new T);
uptr2 = std::move(uptr1);
// object pointed by uptr2 is deleted and
// object pointed by uptr1 is pointed to by uptr2
// uptr1 becomes null
References :
They can essentially be though of as const pointers, ie a pointer which is const and cannot be moved with better syntax.
See : What are the differences between a pointer variable and a reference variable in C++?
r-value reference : reference to a temporary object
l-value reference : reference to an object whose address can be obtained
const reference : reference to a data type which is const and cannot be modified
Reference :
https://www.youtube.com/channel/UCEOGtxYTB6vo6MQ-WQ9W_nQ
Thanks to Andre for pointing out this question.
http://en.wikipedia.org/wiki/Smart_pointer
In computer science, a smart pointer
is an abstract data type that
simulates a pointer while providing
additional features, such as automatic
garbage collection or bounds checking.
These additional features are intended
to reduce bugs caused by the misuse of
pointers while retaining efficiency.
Smart pointers typically keep track of
the objects that point to them for the
purpose of memory management. The
misuse of pointers is a major source
of bugs: the constant allocation,
deallocation and referencing that must
be performed by a program written
using pointers makes it very likely
that some memory leaks will occur.
Smart pointers try to prevent memory
leaks by making the resource
deallocation automatic: when the
pointer to an object (or the last in a
series of pointers) is destroyed, for
example because it goes out of scope,
the pointed object is destroyed too.
A smart pointer is a class, a wrapper of a normal pointer. Unlike normal pointers, smart point’s life circle is based on a reference count (how many time the smart pointer object is assigned). So whenever a smart pointer is assigned to another one, the internal reference count plus plus. And whenever the object goes out of scope, the reference count minus minus.
Automatic pointer, though looks similar, is totally different from smart pointer. It is a convenient class that deallocates the resource whenever an automatic pointer object goes out of variable scope. To some extent, it makes a pointer (to dynamically allocated memory) works similar to a stack variable (statically allocated in compiling time).
What is a smart pointer.
Long version, In principle:
https://web.stanford.edu/class/archive/cs/cs106l/cs106l.1192/lectures/lecture15/15_RAII.pdf
A modern C++ idiom:
RAII: Resource Acquisition Is Initialization.
● When you initialize an object, it should already have
acquired any resources it needs (in the constructor).
● When an object goes out of scope, it should release every
resource it is using (using the destructor).
key point:
● There should never be a half-ready or half-dead object.
● When an object is created, it should be in a ready state.
● When an object goes out of scope, it should release its resources.
● The user shouldn’t have to do anything more.
Raw Pointers violate RAII: It need user to delete manually when the pointers go out of scope.
RAII solution is:
Have a smart pointer class:
● Allocates the memory when initialized
● Frees the memory when destructor is called
● Allows access to underlying pointer
For smart pointer need copy and share, use shared_ptr:
● use another memory to store Reference counting and shared.
● increment when copy, decrement when destructor.
● delete memory when Reference counting is 0.
also delete memory that store Reference counting.
for smart pointer not own the raw pointer, use weak_ptr:
● not change Reference counting.
shared_ptr usage:
correct way:
std::shared_ptr<T> t1 = std::make_shared<T>(TArgs);
std::shared_ptr<T> t2 = std::shared_ptr<T>(new T(Targs));
wrong way:
T* pt = new T(TArgs); // never exposure the raw pointer
shared_ptr<T> t1 = shared_ptr<T>(pt);
shared_ptr<T> t2 = shared_ptr<T>(pt);
Always avoid using raw pointer.
For scenario that have to use raw pointer:
https://stackoverflow.com/a/19432062/2482283
For raw pointer that not nullptr, use reference instead.
not use T*
use T&
For optional reference which maybe nullptr, use raw pointer, and which means:
T* pt; is optional reference and maybe nullptr.
Not own the raw pointer,
Raw pointer is managed by some one else.
I only know that the caller is sure it is not released now.
Smart Pointers are those where you don't have to worry about Memory De-Allocation, Resource Sharing and Transfer.
You can very well use these pointer in the similar way as any allocation works in Java. In java Garbage Collector does the trick, while in Smart Pointers, the trick is done by Destructors.
The existing answers are good but don't cover what to do when a smart pointer is not the (complete) answer to the problem you are trying to solve.
Among other things (explained well in other answers) using a smart pointer is a possible solution to How do we use a abstract class as a function return type? which has been marked as a duplicate of this question. However, the first question to ask if tempted to specify an abstract (or in fact, any) base class as a return type in C++ is "what do you really mean?". There is a good discussion (with further references) of idiomatic object oriented programming in C++ (and how this is different to other languages) in the documentation of the boost pointer container library. In summary, in C++ you have to think about ownership. Which smart pointers help you with, but are not the only solution, or always a complete solution (they don't give you polymorphic copy) and are not always a solution you want to expose in your interface (and a function return sounds an awful lot like an interface). It might be sufficient to return a reference, for example. But in all of these cases (smart pointer, pointer container or simply returning a reference) you have changed the return from a value to some form of reference. If you really needed copy you may need to add more boilerplate "idiom" or move beyond idiomatic (or otherwise) OOP in C++ to more generic polymorphism using libraries like Adobe Poly or Boost.TypeErasure.
What happens if a C/C++ API returns a raw pointer from an internally used shared_ptr, then the shared_ptr gets "deleted"? Is the raw pointer still valid? How can the API developer clean up the raw pointer when it's not in their control anymore?
As an example:
MyClass* thisReturnsAPtr()
{
std::shared_ptr<MyClass> aSharedPtr = std::make_shared<MyClass>(MyClass);
return aSharedPtr.get();
}
If there are no other shared_ptr around anymore that still hold a reference to the object and, thus, keep the object alive, then the object will be destroyed, the memory freed, and any still existing pointer that pointed to that object will become a dangling pointer.
In your example above, the pointer returned by the function thisReturnsAPtr is guaranteed to be an invalid pointer…
It can sometimes be useful to think of smart pointers as a delayed delete mechanism. What I mean by that is that if you allocate a class and give it to a smart pointer you are basically asking the smart pointer class to delete that pointer sometime in the future. For a shared_ptr that time in the future is when no more shared_ptrs exist for it.
Applying this to your code, you create a shared_ptr. You then return the raw pointer. Then your function ends and the only shared_ptr to that pointer gets destroyed and so the underlying class is deleted.
This makes your question "If I new a class and delete it then return the original pointer what happens". Which is a lot easier to answer and the answer to that is "badness". There are many answers to this question on stack overflow - such as C++ delete - It deletes my objects but I can still access the data?
"What happens to a raw pointer if it's shared_ptr is deleted?" - Nothing. It just becomes invalid to subsequently use that pointer.
"a C/C++ API returns a raw pointer from an internally used shared_ptr, then the shared_ptr gets "deleted"? Is the raw pointer still valid?"
No. It is not valid to then use the raw pointer. And the raw pointer does not change (to nullptr) to indicate that the pointed-to object has been deleted.
It is your responsibility to ensure, that if something like that happens, you do not use the raw pointer after it has been invalidated. How you do that is up to you - the language doesn't care/help you.
Using the raw pointer after the object it points to has been deleted, is Undefined Behaviour.
What is a smart pointer and when should I use one?
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_ptrs 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.
Here's a simple answer for these days of modern C++ (C++11 and later):
"What is a smart pointer?"
It's a type whose values can be used like pointers, but which provides the additional feature of automatic memory management: When a smart pointer is no longer in use, the memory it points to is deallocated (see also the more detailed definition on Wikipedia).
"When should I use one?"
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.
"But which smart pointer should I use in which of those cases?"
Use std::unique_ptr when you want your object to live just as long as a single owning reference to it lives. For example, use it for a pointer to memory which gets allocated on entering some scope and de-allocated on exiting the scope.
Use std::shared_ptr when you do want to refer to your object from multiple places - and do not want your object to be de-allocated until all these references are themselves gone.
Use 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).
There is a proposal to add hazard pointers to C++26, but for now you don't have them.
Don't use the boost:: smart pointers or std::auto_ptr except in special cases which you can read up on if you must.
"Hey, I didn't ask which one to use!"
Ah, but you really wanted to, admit it.
"So when should I use regular pointers then?"
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 nor de-allocate, and do not store a copy of the pointer which outlasts their execution.
UPDATE:
This answer is outdated concerning C++ types which were used in the past.
std::auto_ptr is deprecated and removed in new standards.
Instead of boost::shared_ptr the std::shared_ptr should be used which is part of the standard.
The links to the concepts behind the rationale of smart pointers still mostly relevant.
Modern C++ has the following smart pointer types and doesn't require boost smart pointers:
std::shared_ptr
std::weak_ptr
std::unique_ptr
There is also 2-nd edition of the book mentioned in the answer: C++ Templates: The Complete Guide 2nd Edition by David Vandevoorde Nicolai, M. Josuttis, Douglas Gregor
OLD ANSWER:
A smart pointer is a pointer-like type with some additional functionality, e.g. automatic memory deallocation, reference counting etc.
A small intro is available on the page Smart Pointers - What, Why, Which?.
One of the simple smart-pointer types is std::auto_ptr (chapter 20.4.5 of C++ standard), which allows one 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 the object remains. This helps avoiding memory leaks and is easy to use to implement RAII.
The 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:
Protecting Against Exceptions
Holders, (note, std::auto_ptr is implementation of such type of smart pointer)
Resource Acquisition Is Initialization (This is frequently used for exception-safe resource management in C++)
Holder Limitations
Reference Counting
Concurrent Counter Access
Destruction and Deallocation
Definitions provided by Chris, Sergdev and Llyod are correct. I prefer a simpler definition though, just to keep my life simple:
A smart pointer is simply a class that overloads the -> and * operators. Which means that your object semantically looks like a pointer but you can make it do way cooler things, including reference counting, automatic destruction etc.
shared_ptr and auto_ptr are sufficient in most cases, but come along with their own set of small idiosyncrasies.
A smart pointer is like a regular (typed) pointer, like "char*", except when the pointer itself goes out of scope then what it points to is deleted as well. You can use it like you would a regular pointer, by using "->", but not if you need an actual pointer to the data. For that, you can use "&*ptr".
It is useful for:
Objects that must be allocated with new, but that you'd like to have the same lifetime as something on that stack. If the object is assigned to a smart pointer, then they will be deleted when the program exits that function/block.
Data members of classes, so that when the object is deleted all the owned data is deleted as well, without any special code in the destructor (you will need to be sure the destructor is virtual, which is almost always a good thing to do).
You may not want to use a smart pointer when:
... the pointer shouldn't actually own the data... i.e., when you are just using the data, but you want it to survive the function where you are referencing it.
... the smart pointer isn't itself going to be destroyed at some point. You don't want it to sit in memory that never gets destroyed (such as in an object that is dynamically allocated but won't be explicitly deleted).
... two smart pointers might point to the same data. (There are, however, even smarter pointers that will handle that... that is called reference counting.)
See also:
garbage collection.
This stack overflow question regarding data ownership
A smart pointer is an object that acts like a pointer, but additionally provides control on construction, destruction, copying, moving and dereferencing.
One can implement one's own smart pointer, but many libraries also provide smart pointer implementations each with different advantages and drawbacks.
For example, Boost provides the following smart pointer implementations:
shared_ptr<T> is a pointer to T using a reference count to determine when the object is no longer needed.
scoped_ptr<T> is a pointer automatically deleted when it goes out of scope. No assignment is possible.
intrusive_ptr<T> is another reference counting pointer. It provides better performance than shared_ptr, but requires the type T to provide its own reference counting mechanism.
weak_ptr<T> is a weak pointer, working in conjunction with shared_ptr to avoid circular references.
shared_array<T> is like shared_ptr, but for arrays of T.
scoped_array<T> is like scoped_ptr, but for arrays of T.
These are just one linear descriptions of each and can be used as per need, for further detail and examples one can look at the documentation of Boost.
Additionally, the C++ standard library provides three smart pointers; std::unique_ptr for unique ownership, std::shared_ptr for shared ownership and std::weak_ptr. std::auto_ptr existed in C++03 but is now deprecated.
Most kinds of smart pointers handle disposing of the pointer-to object for you. It's very handy because you don't have to think about disposing of objects manually anymore.
The most commonly-used smart pointers are std::tr1::shared_ptr (or boost::shared_ptr), and, less commonly, std::auto_ptr. I recommend regular use of shared_ptr.
shared_ptr is very versatile and deals with a large variety of disposal scenarios, including cases where objects need to be "passed across DLL boundaries" (the common nightmare case if different libcs are used between your code and the DLLs).
Here is the Link for similar answers : http://sickprogrammersarea.blogspot.in/2014/03/technical-interview-questions-on-c_6.html
A smart pointer is an object that acts, looks and feels like a normal pointer but offers more functionality. In C++, smart pointers are implemented as template classes that encapsulate a pointer and override standard pointer operators. They have a number of advantages over regular pointers. They are guaranteed to be initialized as either null pointers or pointers to a heap object. Indirection through a null pointer is checked. No delete is ever necessary. Objects are automatically freed when the last pointer to them has gone away. One significant problem with these smart pointers is that unlike regular pointers, they don't respect inheritance. Smart pointers are unattractive for polymorphic code. Given below is an example for the implementation of smart pointers.
Example:
template <class X>
class smart_pointer
{
public:
smart_pointer(); // makes a null pointer
smart_pointer(const X& x) // makes pointer to copy of x
X& operator *( );
const X& operator*( ) const;
X* operator->() const;
smart_pointer(const smart_pointer <X> &);
const smart_pointer <X> & operator =(const smart_pointer<X>&);
~smart_pointer();
private:
//...
};
This class implement a smart pointer to an object of type X. The object itself is located on the heap. Here is how to use it:
smart_pointer <employee> p= employee("Harris",1333);
Like other overloaded operators, p will behave like a regular pointer,
cout<<*p;
p->raise_salary(0.5);
Let T be a class in this tutorial
Pointers in C++ can be divided into 3 types :
1) Raw pointers :
T a;
T * _ptr = &a;
They hold a memory address to a location in memory. Use with caution , as programs become complex hard to keep track.
Pointers with const data or address { Read backwards }
T a ;
const T * ptr1 = &a ;
T const * ptr1 = &a ;
Pointer to a data type T which is a const. Meaning you cannot change the data type using the pointer. ie *ptr1 = 19 ; will not work. But you can move the pointer. ie ptr1++ , ptr1-- ; etc will work.
Read backwards : pointer to type T which is const
T * const ptr2 ;
A const pointer to a data type T . Meaning you cannot move the pointer but you can change the value pointed to by the pointer. ie *ptr2 = 19 will work but ptr2++ ; ptr2-- etc will not work. Read backwards : const pointer to a type T
const T * const ptr3 ;
A const pointer to a const data type T . Meaning you cannot either move the pointer nor can you change the data type pointer to be the pointer. ie . ptr3-- ; ptr3++ ; *ptr3 = 19; will not work
3) Smart Pointers : { #include <memory> }
Shared Pointer:
T a ;
//shared_ptr<T> shptr(new T) ; not recommended but works
shared_ptr<T> shptr = make_shared<T>(); // faster + exception safe
std::cout << shptr.use_count() ; // 1 // gives the number of "
things " pointing to it.
T * temp = shptr.get(); // gives a pointer to object
// shared_pointer used like a regular pointer to call member functions
shptr->memFn();
(*shptr).memFn();
//
shptr.reset() ; // frees the object pointed to be the ptr
shptr = nullptr ; // frees the object
shptr = make_shared<T>() ; // frees the original object and points to new object
Implemented using reference counting to keep track of how many " things " point to the object pointed to by the pointer. When this count goes to 0 , the object is automatically deleted , ie objected is deleted when all the share_ptr pointing to the object goes out of scope.
This gets rid of the headache of having to delete objects which you have allocated using new.
Weak Pointer :
Helps deal with cyclic reference which arises when using Shared Pointer
If you have two objects pointed to by two shared pointers and there is an internal shared pointer pointing to each others shared pointer then there will be a cyclic reference and the object will not be deleted when shared pointers go out of scope. To solve this , change the internal member from a shared_ptr to weak_ptr. Note : To access the element pointed to by a weak pointer use lock() , this returns a weak_ptr.
T a ;
shared_ptr<T> shr = make_shared<T>() ;
weak_ptr<T> wk = shr ; // initialize a weak_ptr from a shared_ptr
wk.lock()->memFn() ; // use lock to get a shared_ptr
// ^^^ Can lead to exception if the shared ptr has gone out of scope
if(!wk.expired()) wk.lock()->memFn() ;
// Check if shared ptr has gone out of scope before access
See : When is std::weak_ptr useful?
Unique Pointer :
Light weight smart pointer with exclusive ownership. Use when pointer points to unique objects without sharing the objects between the pointers.
unique_ptr<T> uptr(new T);
uptr->memFn();
//T * ptr = uptr.release(); // uptr becomes null and object is pointed to by ptr
uptr.reset() ; // deletes the object pointed to by uptr
To change the object pointed to by the unique ptr , use move semantics
unique_ptr<T> uptr1(new T);
unique_ptr<T> uptr2(new T);
uptr2 = std::move(uptr1);
// object pointed by uptr2 is deleted and
// object pointed by uptr1 is pointed to by uptr2
// uptr1 becomes null
References :
They can essentially be though of as const pointers, ie a pointer which is const and cannot be moved with better syntax.
See : What are the differences between a pointer variable and a reference variable in C++?
r-value reference : reference to a temporary object
l-value reference : reference to an object whose address can be obtained
const reference : reference to a data type which is const and cannot be modified
Reference :
https://www.youtube.com/channel/UCEOGtxYTB6vo6MQ-WQ9W_nQ
Thanks to Andre for pointing out this question.
http://en.wikipedia.org/wiki/Smart_pointer
In computer science, a smart pointer
is an abstract data type that
simulates a pointer while providing
additional features, such as automatic
garbage collection or bounds checking.
These additional features are intended
to reduce bugs caused by the misuse of
pointers while retaining efficiency.
Smart pointers typically keep track of
the objects that point to them for the
purpose of memory management. The
misuse of pointers is a major source
of bugs: the constant allocation,
deallocation and referencing that must
be performed by a program written
using pointers makes it very likely
that some memory leaks will occur.
Smart pointers try to prevent memory
leaks by making the resource
deallocation automatic: when the
pointer to an object (or the last in a
series of pointers) is destroyed, for
example because it goes out of scope,
the pointed object is destroyed too.
A smart pointer is a class, a wrapper of a normal pointer. Unlike normal pointers, smart point’s life circle is based on a reference count (how many time the smart pointer object is assigned). So whenever a smart pointer is assigned to another one, the internal reference count plus plus. And whenever the object goes out of scope, the reference count minus minus.
Automatic pointer, though looks similar, is totally different from smart pointer. It is a convenient class that deallocates the resource whenever an automatic pointer object goes out of variable scope. To some extent, it makes a pointer (to dynamically allocated memory) works similar to a stack variable (statically allocated in compiling time).
What is a smart pointer.
Long version, In principle:
https://web.stanford.edu/class/archive/cs/cs106l/cs106l.1192/lectures/lecture15/15_RAII.pdf
A modern C++ idiom:
RAII: Resource Acquisition Is Initialization.
● When you initialize an object, it should already have
acquired any resources it needs (in the constructor).
● When an object goes out of scope, it should release every
resource it is using (using the destructor).
key point:
● There should never be a half-ready or half-dead object.
● When an object is created, it should be in a ready state.
● When an object goes out of scope, it should release its resources.
● The user shouldn’t have to do anything more.
Raw Pointers violate RAII: It need user to delete manually when the pointers go out of scope.
RAII solution is:
Have a smart pointer class:
● Allocates the memory when initialized
● Frees the memory when destructor is called
● Allows access to underlying pointer
For smart pointer need copy and share, use shared_ptr:
● use another memory to store Reference counting and shared.
● increment when copy, decrement when destructor.
● delete memory when Reference counting is 0.
also delete memory that store Reference counting.
for smart pointer not own the raw pointer, use weak_ptr:
● not change Reference counting.
shared_ptr usage:
correct way:
std::shared_ptr<T> t1 = std::make_shared<T>(TArgs);
std::shared_ptr<T> t2 = std::shared_ptr<T>(new T(Targs));
wrong way:
T* pt = new T(TArgs); // never exposure the raw pointer
shared_ptr<T> t1 = shared_ptr<T>(pt);
shared_ptr<T> t2 = shared_ptr<T>(pt);
Always avoid using raw pointer.
For scenario that have to use raw pointer:
https://stackoverflow.com/a/19432062/2482283
For raw pointer that not nullptr, use reference instead.
not use T*
use T&
For optional reference which maybe nullptr, use raw pointer, and which means:
T* pt; is optional reference and maybe nullptr.
Not own the raw pointer,
Raw pointer is managed by some one else.
I only know that the caller is sure it is not released now.
Smart Pointers are those where you don't have to worry about Memory De-Allocation, Resource Sharing and Transfer.
You can very well use these pointer in the similar way as any allocation works in Java. In java Garbage Collector does the trick, while in Smart Pointers, the trick is done by Destructors.
The existing answers are good but don't cover what to do when a smart pointer is not the (complete) answer to the problem you are trying to solve.
Among other things (explained well in other answers) using a smart pointer is a possible solution to How do we use a abstract class as a function return type? which has been marked as a duplicate of this question. However, the first question to ask if tempted to specify an abstract (or in fact, any) base class as a return type in C++ is "what do you really mean?". There is a good discussion (with further references) of idiomatic object oriented programming in C++ (and how this is different to other languages) in the documentation of the boost pointer container library. In summary, in C++ you have to think about ownership. Which smart pointers help you with, but are not the only solution, or always a complete solution (they don't give you polymorphic copy) and are not always a solution you want to expose in your interface (and a function return sounds an awful lot like an interface). It might be sufficient to return a reference, for example. But in all of these cases (smart pointer, pointer container or simply returning a reference) you have changed the return from a value to some form of reference. If you really needed copy you may need to add more boilerplate "idiom" or move beyond idiomatic (or otherwise) OOP in C++ to more generic polymorphism using libraries like Adobe Poly or Boost.TypeErasure.
Example code:
#include<memory>
#include<iostream>
int main()
{
std::unique_ptr<int> intPtr{new int(3)};
int* myPtr = intPtr.get();
*myPtr = 4;
std::cout<<"New result for intPtr: "<< *intPtr.get()<<std::endl;
}
Doesn't this defeat the whole purpose behind std::unique_ptr ? Why is this allowed?
Edit: I thought the whole purpose behind std::unique_ptr was to have sole ownership of an object. But this example, the object can be modified through another pointer. Doesn't this defeat std::unique_ptr's purpose?
While Smart pointers manage the lifetime of an object being pointed to, it is often still useful to have access to the underlying raw pointer.
In fact if we read Herb Sutter's GotW #91 Solution: Smart Pointer Parameters he recommends passing parameters by pointer or reference when the function is agnostic to the lifetime of the parameter, he says:
Pass by * or & to accept a widget independently of how the caller is
managing its lifetime. Most of the time, we don’t want to commit to a
lifetime policy in the parameter type, such as requiring the object be
held by a specific smart pointer, because this is usually needlessly
restrictive.
and we should pass by unique_ptr when the function is a sink:
Passing a unique_ptr by value is only possible by moving the object
and its unique ownership from the caller to the callee. Any function
like (c) takes ownership of the object away from the caller, and
either destroys it or moves it onward to somewhere else.
and finally pass a unique_ptr by reference when we can potentially modify it to refer to a different object:
This should only be used to accept an in/out unique_ptr, when the
function is supposed to actually accept an existing unique_ptr and
potentially modify it to refer to a different object. It is a bad way
to just accept a widget, because it is restricted to a particular
lifetime strategy in the caller.
Of course, we are required to get the underlying pointer if we have to interface with C libraries that take pointers.
In your specific example:
int* myPtr = intPtr.get();
There is no transfer of ownership to another smart pointer so there are no problems as long as you don't attempt to delete the pointer via myPtr. You can transfer ownership to another unique_ptr by moving it:
std::unique_ptr<int> intPtr2( std::move( intPtr ) ) ;
The example you have shown does not defeat the purpose of std::unique_ptr, which is to maintain ownership of memory allocation and deallocation. The line std::unique_ptr<int> intPtr{new int(3)} allocates a new int and takes ownership of it.
The line *myPtr = 4 does not change that ownership, it is merely assigning a value to the content of the int, via a pointer retrieved by std::unique_ptr::get(). The std::unique_ptr still owns the allocated memory for the int.
This doesn't defeat the purpose of unique_ptr at all. The type of smart pointer determines the ownership semantics, but there may be a case where you need to use the underlying object. For example you have an object owned by a unique_ptr but need to call a method that expect the object by reference. You can then dereference the .get() pointer to get the reference object to pass to the method.
What you shouldn't do is store the obtained pointer in a way that acquires ownership. This is one of the cases where the language gives you tools to work properly and if you take the raw pointer and store a second ownership to it then that's a problem with the use of the tool.
Get just allows you to obtain the raw pointer. Otherwise, you wouldn't be able to use a pointer-based API when you have a unique_ptr, as the only alternative is release, which would make the unique pointer useless. So you get get, which allows using pointer-based APIs without forcing everyone to use an unique_ptr. Without get, you could only call -> and * on the pointer, but if you have a primitive type, that would kind of defeat of having an unique_ptr in the first place.
I'd like to wrap raw pointer member to some smart pointer to prevent deleting inside a developing class. Owner of the object under pointer is outside of class. So, looks like boost::shared_ptr and std::auto_ptr does not fit. The following is a reduced example:
class Foo {
boost::weak_ptr<Bar> m_bar;
public:
void setBar(const Bar *bar) { // bar created on heap
m_bar = bar; // naturally compilation error
}
};
Of course, it induces compilation error. What is a correct way to initialize weak_ptr from a raw pointer (if exist)?
You can't do that, you can only create a weak_ptr out of a shared_ptr or another weak_ptr.
So the idea would be that the owner of the pointer hold a shared_ptr instead of a raw pointer, and everything should be ok.
The only way to do it is by getting hold of a shared_ptr or weak_ptr that owns the pointer, otherwise the weak_ptr has no way to find the existing owner in order to share ownership with it.
The only way to get a shared_ptr from a raw pointer that is already owned by another shared_ptr is if Bar derives from enable_shared_from_this<Bar>, then you can do
m_bar = bar->shared_from_this();
The purpose of a weak pointer is to be unable to use the raw pointer if it had been deleted. However, if you have a raw pointer, the weak pointer has no way to know that it was deleted. Instead, you must have a shared_ptr somewhere which "owns" the raw pointer. Then you can create a weak_ptr which references the shared_ptr.
When the shared_ptr goes out of scope and is the last "strong" smart pointer, it will automatically delete the raw pointer. Then, when you try to lock the weak_ptr, it will see that there is no "strong" pointer remaining and therefore the object does not exist.
Everything you've said seems perfectly reasonable, except for one thing:
void setBar(const Bar *bar)
This shouldn't take a raw pointer. It should take a weak_ptr ideally, or possibly a shared_ptr if you have some compelling argument.
The actual owner of the object in question should construct the weak_ptr and than call setBar with that. This preserves the ownership semantics. It looks like what you're doing is having the owning object take a raw pointer and pass that to setBar. This creates a semantic gap in the ownership of the object.
Pass shared pointer instead of raw pointer, and create your weak pointer from that shared pointer. This is really the only way if owner of the pointer is outside of a class.