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# std::enable_shared_from_this

From cppreference.com
< cpp‎ | memory

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`.

### [edit] 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)
weak_from_this (C++17) | 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 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.

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).

`enable_shared_from_this` provides the safe 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 (see example below)

### [edit] Example

Run this code

    #include <memory>
    #include <iostream>

    struct Good: std::enable_shared_from_this<Good> // note: public inheritance
    {
        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 = std::make_shared<Good>();
        std::shared_ptr<Good> gp2 = gp1->getptr();
        std::cout << "gp2.use_count() = " << gp2.use_count() << '\n';

        // Bad: shared_from_this is called without having std::shared_ptr owning the caller
        try {
            Good not_so_good;
            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';
        }

        // Bad, each shared_ptr thinks it's the only owner of the object
        std::shared_ptr<Bad> bp1 = std::make_shared<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
    bad_weak_ptr
    bp2.use_count() = 1
    Bad::~Bad() called
    Bad::~Bad() called
    *** glibc detected *** ./test: double free or corruption

### [edit] See also

shared_ptr (C++11) | smart pointer with shared object ownership semantics (class template) [edit]

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