std::enable_shared_from_this

template< class T > class enable_shared_from_this;

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.

Member functions

Member objects

Notes

A common(until C++17)standard(since C++17) 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.

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

gp2.use_count() = 2
bp2.use_count() = 1
Bad::~Bad() called
Bad::~Bad() called
*** glibc detected *** ./test: double free or corruption

See also