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 shared_ptr which shares ownership of *this
                    (public member function)
   weak_from_this returns the weak_ptr which shares ownership of *this
   (C++17) (public member function)

Member objects
   Member name Definition
   weak_this (private)(C++17) std::weak_ptr object tracking the control block of the
                        first shared owner of *this. Exposition only

Notes
   A common implementation for enable_shared_from_this is to hold a weak reference
   (such as std::weak_ptr) to this. The constructors of std::shared_ptr detect the
   presence of an
   unambiguous and accessible (ie. public inheritance is mandatory)
   (since C++17) enable_shared_from_this base and assign the newly created
   std::shared_ptr to the internally stored weak reference
   if not already owned by a live std::shared_ptr
   (since C++17). Constructing a std::shared_ptr for an object that is already managed
   by another std::shared_ptr will not consult the internally stored weak reference 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
   the behavior is undefined
   (until C++17)
   std::bad_weak_ptr is thrown (by the shared_ptr constructor from a
   default-constructed weak_this)
   (since C++17).
   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).
   Feature-test macro: __cpp_lib_enable_shared_from_this

Example
// Run this code
 #include <memory>
 #include <iostream>
 struct Good : std::enable_shared_from_this<Good> // note: public inheritance
 {
     std::shared_ptr<Good> getptr() {
         return shared_from_this();
     }
 };
 struct Best : std::enable_shared_from_this<Best> // note: public inheritance
 {
     std::shared_ptr<Best> getptr() {
         return shared_from_this();
     }
     // No public constructor, only a factory function,
     // so there's no way to have getptr return nullptr.
     [[nodiscard]] static std::shared_ptr<Best> create() {
         // Not using std::make_shared<Best> because the c'tor is private.
         return std::shared_ptr<Best>(new Best());
     }
 private:
     Best() = default;
 };
 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

See also
   shared_ptr smart pointer with shared object ownership semantics
   (C++11) (class template)