enable_shared_from_this

std::enable_shared_from_this

 

C++

 

Utilities library

 

Dynamic memory management

 

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 astd::shared_ptr namedpt to safely generate additional std::shared_ptr instancespt1, pt2, ... that all share ownership of t with pt.

Inheriting from std::enable_shared_from_this<T> provides the typeT with a member function shared_from_this. If an objectt of type T is managed by a std::shared_ptr<T> named pt, then calling T::shared_from_this will return a newstd::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 astd::shared_ptr that ownst.

Also note that enable_shared_from_this provides an alternative to an expression likestd::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

(constructor)	constructs an enabled_shared_from_this object (protected member function)
(destructor)	destroys an enable_shared_from_this object (protected member function)
operator=	returns a reference to this (protected member function)
shared_from_this	returns a shared_ptr which shares ownership of*this (public member function)

Notes

A common implementation for enable_shared_from_this is to hold a weak reference (such asstd::weak_ptr) tothis. The constructors ofstd::shared_ptr detect the presence of anenable_shared_from_this base and assign the newly created std::shared_ptr to the internally stored weak reference. Constructing astd::shared_ptr for an object that is already managed by anotherstd::shared_ptr will not consult the internally stored weak reference and thus will lead to undefined behavior.

Example

Run this code

#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

See also