How to declare std unique ptr and what is the use of it

Question

I try to understand how std  unique ptr works and for that I found this document  The author starts from the following example   include  lt utility gt     declarations of unique ptr using std  unique ptr     default construction unique ptr lt int gt  up    creates an empty object    initialize with an argument unique ptr lt int gt  uptr  new int 3    double  pd  new double  unique ptr lt double gt  uptr2  pd      overloaded   and - gt   uptr2   23 5  unique ptr lt std  string gt  ups  new std  string  quot hello quot     int len ups- gt size     What is confusing to me is that in this line unique ptr lt int gt  uptr  new int 3     We use integer as an argument  between round brackets  and here unique ptr lt double gt  uptr2  pd    we used an pointer as an argument  Does it make any difference  What is also not clear to me  is how pointers  declared in this way will be different from the pointers declared in a  quot normal quot  way

User · Answer

Unique pointers are guaranteed to destroy the object they manage when they go out of scope. http://en.cppreference.com/w/cpp/memory/unique_ptr

In this case:

unique_ptr<double> uptr2 (pd);

pd will be destroyed when uptr2 goes out of scope. This facilitates memory management by automatic deletion.

The case of unique_ptr<int> uptr (new int(3)); is not different, except that the raw pointer is not assigned to any variable here.

User · Answer

There is no difference in working in both the concepts of assignment to unique ptr  int  intPtr   new int 3   unique ptr lt int gt  uptr  intPtr    is similar to unique ptr lt int gt  uptr  new int 3     Here unique ptr automatically deletes the space occupied by uptr    how pointers  declared in this way will be different from the pointers declared in a  quot normal quot  way   If you create an integer in heap space  using new keyword or malloc   then you will have to clear that memory on your own  using delete or free respectively   In the below code  int  heapInt   new int 5    initialize int in heap memory      use heapInt   delete heapInt   Here  you will have to delete heapInt  when it is done using  If it is not deleted  then memory leakage occurs  In order to avoid such memory leaks unique ptr is used  where unique ptr automatically deletes the space occupied by heapInt when it goes out of scope  So  you need not do delete or free for unique ptr

User · Answer

From cppreference  one of the std  unique ptr constructors is explicit unique ptr  pointer p   noexcept  So to create a new std  unique ptr is to pass a pointer to its constructor  unique ptr lt int gt  uptr  new int 3     Or it is the same as int  int ptr   new int 3   std  unique ptr lt int gt  uptr  int ptr    The different is you don t have to clean up after using it  If you don t use std  unique ptr  smart pointer   you will have to delete it like this delete int ptr   when you no longer need it or it will cause a memory leak

User · Answer

The constructor of unique ptr lt T gt  accepts a raw pointer to an object of type T  so  it accepts a T     In the first example   unique ptr lt int gt  uptr  new int 3      The pointer is the result of a new expression  while in the second example   unique ptr lt double gt  uptr2  pd     The pointer is stored in the pd variable   Conceptually  nothing changes  you are constructing a unique ptr from a raw pointer   but the second approach is potentially more dangerous  since it would allow you  for instance  to do   unique ptr lt double gt  uptr2  pd          unique ptr lt double gt  uptr3  pd     Thus having two unique pointers that effectively encapsulate the same object  thus violating the semantics of a unique pointer    This is why the first form for creating a unique pointer is better  when possible  Notice  that in C  14 we will be able to do   unique ptr lt int gt  p   make unique lt int gt  42     Which is both clearer and safer  Now concerning this doubt of yours      What is also not clear to me  is how pointers  declared in this way will be different from the pointers declared in a  normal  way    Smart pointers are supposed to model object ownership  and automatically take care of destroying the pointed object when the last  smart  owning  pointer to that object falls out of scope    This way you do not have to remember doing delete on objects allocated dynamically - the destructor of the smart pointer will do that for you - nor to worry about whether you won t dereference a  dangling  pointer to an object that has been destroyed already         unique ptr lt int gt  p   make unique lt int gt  42          Going out of scope         I did not leak my integer here  The destructor of unique ptr called delete   Now unique ptr is a smart pointer that models unique ownership  meaning that at any time in your program there shall be only one  owning  pointer to the pointed object - that s why unique ptr is non-copyable    As long as you use smart pointers in a way that does not break the implicit contract they require you to comply with  you will have the guarantee that no memory will be leaked  and the proper ownership policy for your object will be enforced  Raw pointers do not give you this guarantee

[c++] How to declare std::unique_ptr and what is the use of it?

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