std::enable_shared_from_this
| 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 (protected member function) | |
destroys an enable_shared_from_this object (protected member function) | |
| returns a reference to *this (protected member function) | |
| returns a std::shared_ptr which shares ownership of *this (public member function) | |
| (C++17) |
returns a std::weak_ptr which shares ownership of *this (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
#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> { public: 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
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 unclear and maybe unimplementable
|
clarified and fixed |
See also
| (C++11) |
smart pointer with shared object ownership semantics (class template) |
| creates a shared pointer that manages a new object (function template) |