Std::enable_shared_from_this (3) Linux Manual Page

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

std::enable_shared_from_this (3) Linux Manual Page