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std::enable_shared_from_this

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< cpp‎ | memory
Revision as of 18:49, 10 July 2013 by SuperBoi45 (Talk | contribs)

 
 
 
 
 
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.

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 each other.

Contents

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)

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.

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

See also

smart pointer with shared object ownership semantics
(class template) [edit]