Namespaces
Variants
Views
Actions

std::enable_shared_from_this

From cppreference.com
< cpp‎ | memory
 
 
 
Dynamic memory management
Uninitialized storage
(C++17)
(deprecated since c++17)
(deprecated since c++17)
(deprecated since c++17)
Garbage collection support
Miscellaneous
(C++11)
(C++11)
C Library
Low level memory management
 
 
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.

Publicly 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)
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)
returns the weak_ptr which shares ownership of *this
(public member function)

[edit] Member objects

Member name Definition
weak_this (private)(C++17) std::weak_ptr object tracking the control block of the first shared owner of *this. Exposition only

[edit] 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.

[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] Additional Notes

It is permitted to call shared_from_this only on a previously shared object, i.e. on an object managed by std::shared_ptr<T>. Otherwise the behavior is undefined (until C++17)std::bad_weak_ptr is thrown (by the shared_ptr constructor from a default-constructed weak_this) (since C++17).

[edit] Wrong Usage Example

#include <memory>
#include <iostream>
 
struct Good: std::enable_shared_from_this<Good>
{
    std::shared_ptr<Good> getptr() {
        return shared_from_this();
    }
};
 
int main()
{
    try {
        Good not_so_good;
        // shared_from_this is called without having std::shared_ptr owning the caller
        std::shared_ptr<Good> gp1 = not_so_good.getptr();
    } catch(std::bad_weak_ptr& e) {
        // undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
        std::cout << e.what() << '\n';    
    }
    // Same problem as above, no owning std::shared_ptr for the caller
    try {
        std::unique_ptr<Good> not_so_good_p = std::unique_ptr<Good>(new Good);
        std::shared_ptr<Good> gp2 = not_so_good_p->getptr();
    } catch(std::bad_weak_ptr& e) {
        // undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)
        std::cout << e.what() << '\n';    
    }
}

Output:

bad_weak_ptr
bad_weak_ptr


[edit] See also

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