std::enable_shared_from_this (3) - Linux Man Pages

std::enable_shared_from_this: std::enable_shared_from_this

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


                 constructs an enable_shared_from_this object
constructor (protected member function)
                 destroys an enable_shared_from_this object
destructor (protected member function)
                 returns a reference to this
operator= (protected member function)
                 returns a shared_ptr which shares ownership of *this
shared_from_this (public member function)


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

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
(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)

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 Bad
  {
      std::shared_ptr<Bad> getptr() {
          return std::shared_ptr<Bad>(this);
      }
      ~Bad() { std::cout << "Bad::~Bad() called\n"; }
  };


  int main()
  {
      // Good: the two shared_ptr's share the same object
      std::shared_ptr<Good> gp1 = std::make_shared<Good>();
      std::shared_ptr<Good> gp2 = gp1->getptr();
      std::cout << "gp2.use_count() = " << gp2.use_count() << '\n';


      // 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';
      }


      // Bad, each shared_ptr thinks it's the only owner of the object
      std::shared_ptr<Bad> bp1 = std::make_shared<Bad>();
      std::shared_ptr<Bad> bp2 = bp1->getptr();
      std::cout << "bp2.use_count() = " << bp2.use_count() << '\n';
  } // UB: double-delete of Bad

Possible output:


  gp2.use_count() = 2
  bad_weak_ptr
  bp2.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
           (class template)
(C++11)