std::enable_shared_from_this - cppreference.com (original) (raw)

std::enable_shared_from_this

| | | | | ------------------------------------------------------ | | ------------- | | 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 etc. 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.

[edit] Data members

[edit] Member functions

[edit] Notes

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 _weakthis_ 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 _weakthis_ 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 std::shared_ptr constructor from a default-constructed _weakthis_ ).

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).

[edit] Example

#include #include   class Good : public std::enable_shared_from_this { public: std::shared_ptr getptr() { return shared_from_this(); } };   class Best : public std::enable_shared_from_this { 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 create() { return std::make_shared(Private()); }   std::shared_ptr getptr() { return shared_from_this(); } };   struct Bad { std::shared_ptr getptr() { return std::shared_ptr(this); } Bad() { std::cout << "Bad::Bad() called\n"; } };   void testGood() { // Good: the two shared_ptr's share the same object std::shared_ptr good0 = std::make_shared(); std::shared_ptr 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 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 cannot stack-allocate it: std::shared_ptr best0 = Best::create(); std::shared_ptr 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 is the only owner of the object std::shared_ptr bad0 = std::make_shared(); std::shared_ptr 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

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also