std::enable_shared_from_this(3)

NAME
std::enable_shared_from_this - std::enable_shared_from_this

Synopsis
   Defined in header <memory>
   template< class T > class enable_shared_from_this; (since C++11)
   std::enable_shared_from_this allows an object t that is currently managed by a
   std::shared_ptr named pt to safely generate additional std::shared_ptr instances
   pt1, pt2, ... that all share ownership of t with pt.
   Publicly inheriting from std::enable_shared_from_this<T> provides the type T with a
   member function shared_from_this. If an object t of type T is managed by a
   std::shared_ptr<T> named pt, then calling T::shared_from_this will return a new
   std::shared_ptr<T> that shares ownership of t with pt.

Member functions
   constructor constructs an enable_shared_from_this object
                    (protected member function)
   destructor destroys an enable_shared_from_this object
                    (protected member function)
   operator= returns a reference to *this
                    (protected member function)
   shared_from_this returns a std::shared_ptr which shares ownership of *this
                    (public member function)
   weak_from_this returns a std::weak_ptr which shares ownership of *this
   (C++17) (public member function)

Member objects
   Member name Definition
   weak-this (exposition only) std::weak_ptr object tracking the control block of the
                        first shared owner of *this.

Notes
   A common implementation for enable_shared_from_this is to hold a weak reference
   (such as std::weak_ptr) to *this. For the purpose of exposition, the weak reference
   is called weak-this and considered as a mutable std::weak_ptr member.
   The constructors of std::shared_ptr detect the presence of an unambiguous and
   accessible (i.e. public inheritance is mandatory) enable_shared_from_this base and
   assign the newly created std::shared_ptr to weak-this if not already owned by a live
   std::shared_ptr. Constructing a std::shared_ptr for an object that is already
   managed by another std::shared_ptr will not consult weak-this and thus will lead to
   undefined behavior.
   It is permitted to call shared_from_this only on a previously shared object, i.e. on
   an object managed by std::shared_ptr<T>. Otherwise, std::bad_weak_ptr is thrown (by
   the shared_ptr constructor from a default-constructed weak-this).
   enable_shared_from_this provides the safe alternative to an expression like
   std::shared_ptr<T>(this), which is likely to result in this being destructed more
   than once by multiple owners that are unaware of each other (see example below).

Example
// Run this code
 #include <iostream>
 #include <memory>
 class Good : public std::enable_shared_from_this<Good>
 {
 public:
     std::shared_ptr<Good> getptr()
     {
         return shared_from_this();
     }
 };
 class Best : public std::enable_shared_from_this<Best>
 {
     struct Private{ explicit Private() = default; };
 public:
     // Constructor is only usable by this class
     Best(Private) {}
     // Everyone else has to use this factory function
     // Hence all Best objects will be contained in shared_ptr
     static std::shared_ptr<Best> create()
     {
         return std::make_shared<Best>(Private());
     }
     std::shared_ptr<Best> getptr()
     {
         return shared_from_this();
     }
 };
 struct Bad
 {
     std::shared_ptr<Bad> getptr()
     {
         return std::shared_ptr<Bad>(this);
     }
     ~Bad() { std::cout << "Bad::~Bad() called\n"; }
 };
 void testGood()
 {
     // Good: the two shared_ptr's share the same object
     std::shared_ptr<Good> good0 = std::make_shared<Good>();
     std::shared_ptr<Good> good1 = good0->getptr();
     std::cout << "good1.use_count() = " << good1.use_count() << '\n';
 }
 void misuseGood()
 {
     // Bad: shared_from_this is called without having std::shared_ptr owning the caller
     try
     {
         Good not_so_good;
         std::shared_ptr<Good> gp1 = not_so_good.getptr();
     }
     catch (std::bad_weak_ptr& e)
     {
         // undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
         std::cout << e.what() << '\n';
     }
 }
 void testBest()
 {
     // Best: Same but can't stack-allocate it:
     std::shared_ptr<Best> best0 = Best::create();
     std::shared_ptr<Best> best1 = best0->getptr();
     std::cout << "best1.use_count() = " << best1.use_count() << '\n';
     // Best stackBest; // <- Will not compile because Best::Best() is private.
 }
 void testBad()
 {
     // Bad, each shared_ptr thinks it's the only owner of the object
     std::shared_ptr<Bad> bad0 = std::make_shared<Bad>();
     std::shared_ptr<Bad> bad1 = bad0->getptr();
     std::cout << "bad1.use_count() = " << bad1.use_count() << '\n';
 } // UB: double-delete of Bad
 int main()
 {
     testGood();
     misuseGood();
     testBest();
     testBad();
 }

Possible output:
 good1.use_count() = 2
 bad_weak_ptr
 best1.use_count() = 2
 bad1.use_count() = 1
 Bad::~Bad() called
 Bad::~Bad() called
 *** glibc detected *** ./test: double free or corruption
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2529 C++11 specification for enable_shared_from_this was clarified and
                       unclear and maybe unimplementable fixed

See also
   shared_ptr smart pointer with shared object ownership semantics
   (C++11) (class template)
   make_shared creates a shared pointer that manages a new object
   make_shared_for_overwrite (function template)
   (C++20)