What is a smart pointer and when should I use one?


What is a smart pointer and when should I use one?


Here's a simple answer for these days of C++11/C++14/C++17:

  • What is a smart pointer?
    It's a type 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).
  • When should I use one?
    In code which involves tracking the ownership of a piece of memory, allocating or de-allocating (and it 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're not planning multiple references to the same object. For example, for a pointer 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 it 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).
    • Don't use the boost:: 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 do I use regular pointers then?
    In code that is oblivious to memory ownership. So, usually in functions which get a pointer from someplace else and don't allocate, de-allocate or store a copy of it which outlasts their execution.
By : einpoklum

Smart pointer is a pointer-like type with some additional functionality, e.g. 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:

By : sergtk

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.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);

//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.

By : nnrales

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By: admin