std::enable_shared_from_this
Defined in header
<memory>
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template< class T > class enable_shared_from_this;
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(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 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
.
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 each other.
Contents |
[edit] Member functions
constructs an enabled_shared_from_this object (protected member function) |
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destroys an enable_shared_from_this object (protected member function) |
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returns a reference to this (protected member function) |
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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 detect the presence of an enable_shared_from_this
base and assign the newly created std::shared_ptr to the internally stored weak reference. 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.
[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)
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smart pointer with shared object ownership semantics (class template) |