std::enable_shared_from_this
| Defined in header <memory>
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||
| 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.
Inheriting from std::enable_shared_from_this<T> provides a 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.
Note that prior to calling shared_from_this on an object t, there must be a std::shared_ptr that owns t.
Also note that enable_shared_from_this provides an 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 eachother.
Contents |
[edit] Member functions
| constructs an enabled_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 shared_ptr which shares ownership of *this (public member function) | |
[edit] 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 can detect presence of a enable_shared_from_this base and share ownership with the existing std::shared_ptrs, instead of assuming the pointer is not managed by anyone else.
[edit] Example
#include <memory> #include <iostream> struct Good: std::enable_shared_from_this<Good> { 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(new Good); std::shared_ptr<Good> gp2 = gp1->getptr(); std::cout << "gp2.use_count() = " << gp2.use_count() << '\n'; // Bad, each shared_ptr thinks it's the only owner of the object std::shared_ptr<Bad> bp1(new Bad); std::shared_ptr<Bad> bp2 = bp1->getptr(); std::cout << "bp2.use_count() = " << bp2.use_count() << '\n'; } // UB: double-delete of Bad
Output:
gp2.use_count() = 2 bp2.use_count() = 1 Bad::~Bad() called Bad::~Bad() called *** glibc detected *** ./test: double free or corruption
[edit] See also
| (C++11) |
smart pointer with shared object ownership semantics (class template) |
